Displays

Fluids can circulate in all levels of the crust, as veins, ore deposits and chemical alterations and isotopic shifts indicate. It is furthermore generally accepted that faults and fractures play a central role as preferred fluid conduits. Fluid flow is, however, not only passively reacting to the presence of faults and fractures, but actively play a role in their creation, (re-) activation and sealing by mineral precipitates. This means that the interaction between fluid flow and fracturing is a two-way process, which is further controlled by tectonic activity (stress field), fluid sources and fluxes, as well as the availability of alternative fluid conduits, such as matrix porosity. Here we explore the interaction between matrix permeability and dynamic fracturing on the spatial and temporal distribution of fluid flow for upward fluid fluxes. Envisaged fluid sources can be dehydration reactions, release of igneous fluids, or release of fluids due to decompression or heating.

Our 2D numerical cellular automaton-type simulations span the whole range from steady matrix-flow to highly dynamical flow through hydrofractures. Hydrofractures are initiated when matrix flow is insufficient to maintain fluid pressures below the failure threshold. When required fluid fluxes are high and/or matrix porosity low, flow is dominated by hydrofractures and the system exhibits self-organised critical phenomena. The size of fractures achieves a power-law distribution, as failure events may sometimes trigger avalanche-like amalgamation of hydrofractures. By far most hydrofracture events only lead to local fluid flow pulses within the source area. Conductive fracture networks do not develop if hydrofractures seal relatively quickly, which can be expected in deeper crustal levels. Only the larger events span the whole system and actually drain fluid from the system. We present the 10 square km hydrothermal Hidden Valley Mega-Breccia on the Paralana Fault System in South Australia as a possible example of large-scale fluid expulsion events. Although field evidence suggests that the breccia formed over a period of at least 150 Myrs, actual cumulative fluid duration may rather have been in the order of days only. This example illustrates the extreme dynamics that crustal-scale fluid flow in hydrofractures can achieve.

How to cite: Bons, P. D., de Riese, T., Gomez-Rivas, E., Naaman, I., and Sachau, T.: Hydrofractures and crustal-scale fluid flow, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-18248, https://doi.org/10.5194/egusphere-egu2020-18248, 2020.

In marine rift basins, rift-climax deep-water clastics in the hanging wall of rift- or basin-bounding fault systems are commonly juxtaposed against crystalline basement rocks in the footwall. Displacing highly permeable, unconsolidated sediments against low-permeable rock distinguishes these faults significantly from others displacing hard rock. Due to limited surface exposure of such fault zones, studies elucidating their structure and evolution are rare. Consequently, their impact on fluid circulation and in-fault, near-fault, and hanging wall sediment diagenesis are also poorly understood. Motivated by this, we here investigate a well-exposed strand of a major basin-bounding fault system in the East Greenland rift system, namely the Dombjerg Fault which bounds the Wollaston Forland Basin, NE Greenland. Here, Upper Jurassic and Lower Cretaceous syn-rift deep-water clastics are juxtaposed against Caledonian metamorphic basement.

Previously, a ~1 km-wide zone of increased calcite cementation of the hanging wall sediments along the Dombjerg fault core was identified (Kristensen et al., 2016). Now, based on U/Pb calcite dating, we are able to show that cementation and formation of this zone started during the rift climax in Berrisian/Valanginian times. Using clumped isotope analysis, we determined cement formation temperatures of ~30-70˚C. Temperatures likely do not relate to the normal geothermal gradient, but to elevated fluid temperatures of upward directed circulation along the fault.

Vein formation within the cementation zone clusters between ~125-100 Ma in the post-rift stage, indicating that fracturing in the hanging wall is not directly related to the main phase of activity of the adjacent Dombjerg Fault. Vein formation temperatures range between ~30-80˚C, signifying a shallow burial depth of the hanging wall deposits. Further, similar minor element concentrations of veins and adjacent cements argue for diffusional mass transfer, which in turn infers a subdued fluid circulation and low permeability of the fracture network. These results imply that the chemical alteration zone formed an impermeable barrier quickly after sediment deposition and maintained this state even after fracture formation.

We argue that the existence of such a cementation zone should be considered in any assessments that target basin-bounding fault systems for, e.g., hydrocarbon, groundwater, geothermal energy, and carbon storage exploration. Our study highlights that the understanding fluid flow properties as well as fault-controlled diagenesis affecting the fault itself and/or adjacent basinal clastics is of great fundamental and economic importance.

References:

Kristensen, T. B., Rotevatn, A., Peacock, D. C. P., Henstra, G. A., Midtkandal, I., and Grundvag, S. A. (2016). Structure and flow properties of syn-rift border faults: The interplay between fault damage and fault-related chemical alteration (Dombjerg Fault, Wollaston Forland, NE Greenland), J. Struct. Geol., 92, 99-115, doi:10.1016/j.jsg.2016.09.012.

How to cite: Salomon, E., Rotevatn, A., Kristensen, T. B., Grundvåg, S.-A., Henstra, G. A., Meckler, A. N., Gerdes, A., and Roper, R. A.: Fault-controlled diagenesis and fluid circulation along a major syn-rift border fault system – insights from the Dombjerg Fault, Wollaston Forland Basin, NE Greenland, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-14590, https://doi.org/10.5194/egusphere-egu2020-14590, 2020.

Comb-veins are mineral-filled fractures oriented perpendicular to fault surfaces, with their intersection with the fault surface generating lineations that are perpendicular to the downdip slip direction. Despite the large occurrence along normal faults within seismogenic extensional tectonic settings (i.e. Greece, Turkey, Italy), their origin, geochemical signature, and kinematics are still poorly constrained. Here we present the first multidisciplinary study, combining field to microscale observations (optical microscope and cathodoluminescence) with geochemical-geochronological analyses (U-Th dating, stable-clumped isotopes, Strontium isotopes, whole-rock geochemistry, and fluid inclusions), on calcite-filled comb-veins cutting through the principal surface of the seismogenic Val Roveto Fault in the central Apennines, Italy. We show that comb-veins precipitated in Late Pleistocene time (between 300 ky and 140 ky) below the present-day outcrop level at a maximum depth of ∼350 m and temperatures between 32 and 64°C from deep-seated fluids modified by reactions with crustal rocks and with a mantle contribution (up to ∼39%). The observed geochemical signature and temperatures are not compatible which those of cold meteoric water and/or shallow groundwater (maximum temperature of 12 °C) circulating within shallow aquifers (≤ 500 m depth) in the study region. Therefore, we propose that deep-seated crust/mantle-derived warm fluids were squeezed upward during earthquakes and were hence responsible for calcite precipitation at shallow depths in co-seismic comb fractures. As comb-veins are rather common, particularly along seismogenic normal faults, we suggest that further studies are necessary to test whether these veins are often of co-seismic origin. If so, they may become a unique and irreplaceable tool to unravel the seismic history and crustal-scale fluid circulation of active faults.

How to cite: Smeraglia, L., Bernasconi, S. M., Berra, F., Billi, A., Boschi, C., Caracausi, A., Carminati, E., Castorina, F., Doglioni, C., Italiano, F., Rizzo, A. L., Uysal, I. T., and Zhao, J.: Comb-veins as a marker for crustal-scale fluid circulation: insight from geochronological (U-Th dating), geochemical, and field to microstructural analyses along the seismogenic Val Roveto Fault (central Apennines, Italy), EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-660, https://doi.org/10.5194/egusphere-egu2020-660, 2020.

Zannone is a very important island, located in the Neogene-Quaternary extensional domain of the Tyrrhenian back-arc basin, as it is the unique spot where the Paleozoic (?) crystalline basement is hypotesized to be exposed in central Apennines. The exposure of such hypothetical basement in the Zannone Island is very problematic as it implies very large normal displacements (> 3 km) along surrounding faults. No such displacements are known along faults close to Zannone Island.

In this work, we study the hypothetical Paleozoic crystalline basement exposed in the Zannone Island with the main aim of understanding its geological nature and relationships with the surrounding rocks. We use a multidisciplinary approach including 1) interpretation of seismic reflection profile; 2) field survey; 3) petro-textural observations; 4) microthermometry on fluid inclusions; 5) geochemical analyses of stable and clumped isotopes; 6) Illite crystallinity and mineralogical analyses of clays and host rocks; 7) analyses of minor gaseous species (He, Ne, and Ar concentrations and isotope ratios) in fluid inclusions; 8) U-Pb geochronology of syn-tectonic calcite, and 9) K-Ar dating of syn-kinematic clay minerals.

Our results show that the hypothetical Paleozoic (?) crystalline basement exposed on the Zannone Island is, instead, represented by siliciclastic rocks of very low metamorphic grade. This is testified by the quartzarenites nature of the rocks, the presence of chloritoid and by the observed incipient foliation marked by fine-grained white micas and disposed parallel to the bedding. The contact between such siliciclastic rocks and the overlapping Triassic Dolostone is represented by a low-angle thrust cut by sets of high-angle normal faults with associated calcite mineralizations. K-Ar dating on clay minerals in fault gouge reveals that at least one event of authigenesis (i.e. fluid-assisted tectonic activity) occurred in Zannone Island <22 Ma ago. U-Pb dating on sin-tectonic calcite mineralizations allowed to constrain the compressional deformation and subsequent normal faulting in the study area at around 7 Ma. This result is consistent with the 1) described emplacement of imbricate thrust sheets onshore close to Zannone Island and 2) syn-tectonic sediments-filling basins observed by seismic reflection studies. Microthermometry on fluid inclusions and stable isotopes analyses on syn-tectonic mineralizations highlighted the involvement of two different fluids during tectonic processes. One characterized by low salinity (as NaCl equivalent; i.e. meteoric-derived fluids) and one by high salinity (as NaCl equivalent; i.e. deep crustal-derived fluids). Microthermometry on fluid inclusions allowed to constrain a wide range of P-T entrapment conditions. For this reason, we highlighted a transition from lithostatic toward hydrostatic pressure during precipitation of syn-tectonic mineralizations.

How to cite: Curzi, M., Billi, A., Carminati, E., Albert, R., Aldega, L., Bernasconi, S., Boschi, C., Caracausi, A., Cardello, L., Conti, A., Drivenes, K., Franchini, S., Gerdes, A., Rizzo, A. L., Rossetti, F., Smeraglia, L., Sørensen, B. E., Van der Lelij, R., Vignaroli, G., and Viola, G.: Deciphering orogenic and post-orogenic fluid-assisted deformations by coupling structural, mineralogical, geochemical, and geochronological investigation methods. An example from Zannone Island, Italy, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-970, https://doi.org/10.5194/egusphere-egu2020-970, 2020.

Active faults are characterized by creation/destruction of secondary (tectonic) permeability in response to a continuous interplay between deformation and fluid pressure fluctuations during the seismic cycle. The study of the paleofluid circulation in fault rocks can thus provide insights into the hydraulic and mechanical behavior of the seismogenic crust.

This work integrates data from field geology with geochemical and geochronological constraints to understand the spatio-temporal evolution of the paleofluid circulation in the Mount Morrone Fault System (MMFS), a ~25 km-long tectonic structure activated during the extensional Quaternary phase of the central Apennines (Italy). The MMFS cuts through a Mesozoic-Cenozoic multilayer carbonate succession for a cumulative stratigraphic offset of about 2 km. Fluvio-lacustrine and slope deposits (Middle-Late Pleistocene) occur at its hanging wall and are variably involved by faulting. The MMFS is currently classified as a silent seismic fault, with an estimated Mw= 6.5-7.0 potential magnitude and recurrence time at 2.4 ka for an expected earthquake.

The structural survey focused on the western strand of the MMFS cutting through a succession of Sinemurian dolomitized limestones. A composite network of NW-SE-striking, SW-dipping fault surfaces defines the structural architecture of the MMFS in the study outcrops, with high angle (dip > 55°) faults that systematically cut and displace medium-to-low angle (dip in the order of 30°-50°) faults. Both fault systems are characterized by dominant dip-slip movement and normal kinematics. Lenses of cm-thick cataclasites often occur along the slip surfaces. Cataclasites are made by sub-angular to sub-rounded carbonate clasts (up to 1 cm-wide) dispersed in a very fine-grained matrix. Layers of cm-thick carbonate concretions occur associated with the cataclasites, testifying for pulses of fluid discharge along the fault surface during the tectonic activity of the MMFS. Microstructural investigations document that: (i) carbonate concretions show an internal texture of fibrous vein having fiber growth direction roughly perpendicular to the vein wall, and (ii) the basal portions of the carbonate concretions are fractured and incorporated within the underlying cataclasites through the deposition of a new calcite cement. The geochemical (δ¹³C and δ¹⁸O stable isotope) analyses on selected samples attest for a progressive chemical shift of the mineralizing fluid from marine (in host rock and in cataclasites) to meteoric waters (in carbonate concretions). The U-Th dating of carbonate concretions and calcite slickenfibers constrains the fault-controlled fluid circulation to the Middle Pleistocene, with ages spanning from 270 to 180 ka. Significantly, the dating of carbonate concretions documents a 12-kyr cyclicity of the fluid infiltration in the fault zone.

The development of the secondary permeability in the MMFS thus corresponds to a combination of faulting and tensile fracturing, in response to a cyclic increasing of the shear stress and the pore pressure during the seismic cycle. The polyphasic deformation system of the MMFS constitutes a record of fault activation and reactivation episodes that could contribute to define the recurrence model of seismic events on regional-scale faults.

How to cite: Vignaroli, G., Argante, V., Rossetti, F., Petracchini, L., Soligo, M., Brilli, M., Yu, T.-L., and Shen, C.-C.: Cyclicity of paleofluid infiltration in the active Mount Morrone Fault System (central Apennines, Italy) constrained by carbonate concretions, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-2866, https://doi.org/10.5194/egusphere-egu2020-2866, 2020.

Vein-hosted fluid inclusions may represent remnants of subsurface paleo-fluids and therefore provide a valuable record of fracture-controlled fluid flow. Isotope data (δ²H and δ¹⁸O) of fluid inclusions are particularly useful for studying the provenance and type of paleo-fluids circulating in the subsurface. Although isotopic analysis of sub-microliter amounts of fluid inclusion water is not straightforward, major steps forward have been made over the past decade through the development of continuous-flow set-ups. These techniques make use of mechanical crushing at a relatively low-temperature (110˚C) and allow for on-line analysis of both δ²H and δ¹⁸O ratios of bulk fluid inclusion water. However, continuous-flow techniques have mostly been used in speleothem research, and have not yet found a widespread application on vein systems for hydrogeological reconstructions.

We used isotope data of fluid inclusions hosted in calcite vein cements to reconstruct regional fluid migration pathways in the Albanian foreland fold-and-thrust system. Tectonic forces during thrust emplacement typically instigate distinct phases of fracturing accompanied by complex fluid flow patterns. The studied calcite veins developed in a sequence of naturally fractured Cretaceous to Eocene carbonate rocks as a result of several fracturing events from the early stages of burial onward. Fluid inclusion isotope data of the veins reveal that fluids circulating in the carbonates were derived from an underlying reservoir, which consisted of a mixture of meteoric water and evolved marine fluids, probably derived from deep-seated evaporites. The meteoric fluids infiltrated in the hinterland before being driven outward into the foreland basin. The fluid inclusion isotope data furthermore show that meteoric water becomes increasingly dominant in the system through time as migration pathways shortened and marine formation fluids were progressively flushed out.

The diagenetic stability of fluid inclusions is of key interest in the study of their isotope ratios. Recrystallization, secondary fluid infiltration and isotope exchange processes could potentially drive alterations of fluid inclusion isotope signatures after entrapment. In this case, however, isotope signatures of fluid inclusions seem to have remained largely unaltered, despite the Cretaceous to Tertiary age of the vein system. Oxygen isotope exchange processes between the fluid inclusion water and host mineral could have been inhibited at the relatively low temperatures of vein formation (i.e. <80˚C). Although more research into the diagenetic stability of fluid inclusion isotope ratios is required, the fluid inclusion isotope record has potential as a powerful tool for fluid provenancing in subsurface fluid flow systems.

How to cite: de Graaf, S., Nooitgedacht, C., Vonhof, H., van der Lubbe, J., and Reijmer, J.: Isotope analysis of vein-hosted fluid inclusions: A case study on fracture-controlled fluid flow in the Albanian foreland fold-and-thrust belt, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-20149, https://doi.org/10.5194/egusphere-egu2020-20149, 2020.

Micro-cracks in fault damage zones can heal through diffusive mass transfer driven by differences in chemical potential, with rates controlled by temperature and pressure. The diffusion of pore fluid pressure in fault damage zones accelerates mass diffusion and assists healing processes. In this work, we use fluid flow model coupled with heat transfer and crack healing to investigate, through different scenarios, the role of subsurface warm fluid migration, along damage zones, in enhancing healing and re-shaping the fault permeability structure. Our results show that if the flow communication exists between the bed and only one side of the damage zone and not the other side, it leads to an asymmetric permeability structure caused by healing in the side circulated by fluids (ex: Rapolano geothermal area, Italy). Another scenario is when the damage zone adjacent to the fault core is not the interval with the highest permeability, as conventionally expected, which is the case of the Alpine Fault, New Zealand. As shown by our simulations, this can be due to healing by diffusive mass transfer, favored by the localized high geothermal gradients and the upward fluid migration through the fault relay structure.

How to cite: Yehya, A. and Rice, J. R.: Influence of fluid-assisted micro-crack healing on fault permeability structure, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-22180, https://doi.org/10.5194/egusphere-egu2020-22180, 2020.

Detailed fluid inclusion analyses of fracture cements in tightly cemented hydrocarbon-bearing sandstones and shales reveal that natural fractures tend to form under conditions approaching maximum burial, coinciding with hydrocarbon generation, and during incipient exhumation. Fluid inclusion analyses also reveal that these fractures form under abnormal (above-hydrostatic) pore fluid pressures. While compaction disequilibrium can account for elevated pore fluid pressures that promote fracture growth during early prograde burial, hydrocarbon maturation is likely the primary driver for fracture growth under peak burial conditions. Tectonic processes and thermal stresses provide secondary drivers. Thermal contraction with exhumation and cooling of the rock mass can promote fracture growth depending on the PVT properties of the fluid phase. The possible contribution of hydrocarbon generation after peak burial as a driver for fracture growth during incipient exhumation is discussed.

How to cite: Eichhubl, P.: Fracture growth during exhumation in low-permeability rock formations—the role of fluid PVT properties, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-11638, https://doi.org/10.5194/egusphere-egu2020-11638, 2020.

Fault and fracture networks can govern fluid flow patterns in the subsurface and predicting fluid flow on a regional scale is of interest in a variety of fields like groundwater management, mining engineering, energy, and mineral resources. Especially the pore fluid pressure can have a strong impact on the strength of fault zones and might be one of the drivers for fault reactivation. Reliable simulations of the transient changes in fluid pressure need to account for the generic architecture of fault zones that comprises strong permeability contrast between the fault core and damage zone.

Particularly, the distribution and connectivity of large-scale fault zones can have a strong impact on the flow field. Yet, modelling numerically such features in their full complexity remains challenging. Often faults zones are conceptualized as forming exclusively either barriers or conduits to fluid flow. However, a generic architecture of fault zones often comprises a discrete fault core surrounded by a diffuse damage zone and conceptualizing large scale discontinuities simply as a barrier or conduit is unlikely to capture the regional scale fluid flow dynamics. It is known that if the fault zone is hosted in low-permeability strata, such as clays or crystalline rocks, a transversal flow barrier can form along the fault core whereas the fracture-rich fault damage zone represents a longitudinal conduit. In more permeable host-rocks (i.e. sandstones or carbonates) the reverse situation can occur, and the permeability distributions in the damage zones can be governed by the abundance of low-permeability deformation features. A reliable numerical model needs to account for the difference and strong contrasts in fluid flow properties of the core and the damage zone, both transversally and longitudinally, in order to make prediction about the regional fluid flow pattern.

Here, we present a numerical method that accounts for the generic fault zone architecture as lower dimensional interfaces in conforming meshes during fluid flow simulations in fault networks. With this method we aim to decipher the impact of fault zone architecture on subsurface flow pattern and fluid pressure evolution in fractures and faulted porous media. The method is implemented in a finite element framework for Multiphysics simulations. We demonstrate the impact of considering the more generic geological structure of individual faults on the flow field by conceptualizing discontinuities either as barriers, conduits or as a conduit-barrier system and show were these conceptualizations are applicable in natural systems. We further show that a reliable regional scale fluid flow simulation in faulted porous media needs to account for the generic fault zone architecture. The approach is finally used to evaluate the fluid flow response of statistically parameterised faulted media, in order to investigate the impact and sensitivity of each variable parameter.

How to cite: Kelka, U., Poulet, T., and Peeters, L.: Permeability contrasts of fault zones - from conceptual model to numerical simulation, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-6520, https://doi.org/10.5194/egusphere-egu2020-6520, 2020.

In sedimentary basins highly overpressured formations and zones are observed worldwide. The high overpressures have been generated over millions of years due to sedimentation amount and rate, compaction, lateral fluid flow, diagenesis and other processes. The lateral fluid flow is often controlled by the fault pattern and sealing properties of the faults in the area, thus defining what is often termed pressure compartments. When high overpressures builds-up over time in such compartment, eventually natural hydraulic faulting and fracturing will start to develop to cease and relief the overpressure.

In this work we have aimed to simulate fracture generation, how they in an upscaled approach evolve and progress upwards, and how this will influence the water fluid flow and the pore pressure distribution. We use an in-house software (PressureAhead) to simulate three-dimensional water fluid pressure generation and dissipation over millions of years. Interpreted seismic horizons for the whole stratigraphy are back-stripped (decompaction) in order to provide the basin burial history as input to the forward simulator. Uplift and erosion events are included. For each timestep, the effect of pressure generation and dissipation is calculated. For the fault and failure development, the combined Griffith-Coulomb failure criteria are implemented to calculate when failure occurs, secondly, when the fracture has been formed and the cohesion is lost, the frictional sliding criteria is used. The fractures are in this approach working as a pressure valve, that will stay open as long as the pressure support is large enough. Compared to previous approach, the failure criteria is now evaluated for the whole stratigraphic column in 3D Using this approach, the effect of natural fracturing taking place in different parts of the basin at different geological events can be modelled.

The new simulation approach will be presented for a dataset from the deeper part of the Viking Graben, North Sea offshore Norway. The study area covers an NNE-SSW trending graben defined by large faults. Seventeen seismic horizons (resolution 50x50 m) from Middle Jurassic to seafloor have been used to set up the model. The modelling is carried out over the 150 My, with time steps of 250 000 years. Examples of varying key input parameters will be shown. Strength and weakness with such an upscaled modelling framework will be discussed.

How to cite: Lothe, A. E., Grøver, A., and Roli, O.-A.: Upscaled modelling of natural fracturing and leakage due to high overpressures , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-22014, https://doi.org/10.5194/egusphere-egu2020-22014, 2020.

It is important to understand the effects of fluid over-pressure in rocks because gradients in over-pressure can lead to failure of rocks and expulsion of fluids. Examples are hydro-fracturing in engineering as well as fluid generation during hydrocarbon maturation, metamorphic reactions or over-pressure below seals in sedimentary basins. In order to have an understanding of the complexity of effective stress fields, fracture, failure and fluid drainage the process was studied with a dynamic hydro-mechanical numerical model. The evolution of fluid pressure build up, fracturing and the dynamic interaction between solid and fluid is modeled. Three scenarios are studied: fluid pressure build up in a sedimentary basin, in a confined zone and in a horizontal layer that is offset by a fault. Results indicate that the geometry of the fluid-overpressure zone has a first order control on the patterns including porosity evolution and fracturing. If the over-pressure develops below a seal in a sedimentary basin, the effective differential and mean stress approach zero and the horizontal and vertical effective stresses flip in orientation leading to horizontal hydro-factures or breccia zones. If the over-pressure zone is confined vertically as well, the standard effective stress model develops with the effective mean stress decreasing while the differential stress remains mainly constant. This leads to semi-vertically aligned extensional and conjugate shear failure at much lower over-pressures than in the sedimentary basin. A perfectly aligned horizontal layer that increases in fluid pressure internally leads to a horizontal hydro-fracture within the layer. A faulted layer develops complex multi-directional failure with the fault itself being a location of early fracturing followed by brecciation of the layer itself. All simulations undergo a phase transition in porosity evolution with an initially random porosity reducing its symmetry and forming a static porosity wave with an internal dilation zone and the development of dynamic porosity channels within this zone that drain the over-pressure. Our results show that patterns of fractures, hence fluid release, that form due to high fluid overpressures can only be successfully predicted if the geometry of the geological system is known, including the fluid overpressure source and the position of seals and faults that offset source layers and seals.

How to cite: Koehn, D., Piazolo, S., Sachau, T., and Toussaint, R.: Porosity channeling and fracturing in fluid over-pressure zones , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-8644, https://doi.org/10.5194/egusphere-egu2020-8644, 2020.

The mechanics of fracture propagation and interaction influence the growth and permeability of developing fracture networks. A set of initial flaws grows quasi-statically in response to a remote tensile stress. A finite element, stress intensity factor-based approach grows these flaws into non-planar three-dimensional discrete fracture networks (GDFNs). Their extension and growth angle is a function of local stress intensity factors along a fracture tip. Stress concentration increase when proximal fractures are aligned, and decreases when they are sub-coplanar. These interactions can result in the reactivation of fractures that were initially inactive, and the arrest of fractures that become entrapped by proximal growing fractures. Interaction can cause growth away from an intersection front between two fractures, resulting in evolving fracture patterns that become non-uniform and non-planar, forming dense networks. These GDFNs provide representations of subsurface networks that numerically model the physical process of concurrent fracture growth. Permeability tensors of the geomechanical 3D networks are computed, assuming Darcy flow. Growth influences apertures, and in turn, the hydraulic properties of the network. GDFNs provide a promising way to model subsurface fracture networks, and their related hydro-mechanical processes, where fracture mechanics is the primary influence on the geometric and hydraulic properties of the networks.

How to cite: Paluszny, A., Thomas, R. N., and Zimmerman, R. W.: Permeability tensors of three-dimensional numerically grown geomechanical discrete fracture networks, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-5538, https://doi.org/10.5194/egusphere-egu2020-5538, 2020.

For understanding the formation of mountain belts it is necessary to gain quantitative insights on fault and fracture mechanics on multiple scales. In particular, for addressing the role of fluids on larger processes, it is inevitable to constrain fault and fracture geometries at depth, as well as gain insights on how fluids influence fault mechanics. At least partly, the future of such analyses lies in exploiting large data sets, as well as in multi- and interdisciplinary research.

In this talk I will present results from variety of geological settings, including dilatant faults at Mid-Ocean Ridges, the Oman Mountains, the Khao Kwang fold-trust belt in Thailand, and the European Alps. I will show how multi-scale studies and the use of large data sets helps constraining fluid migration in mountain belts, fault geometries, as well as possible feedbacks between fluid flow and strain localization. Results are then applied to discuss the role of mechanical stratigraphy on structural style in foreland fold-thrust belts.

How to cite: von Hagke, C.: From Faults and Fluids to Mountain Belt Dynamics, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-4688, https://doi.org/10.5194/egusphere-egu2020-4688, 2020.

Naturally fractured reservoirs represent one of the most challenging resource in the oil and gas industry. The understanding based on centimeter scale observations is upscaled and modeled at 100-meter scale.

In this paper, we will illustrate with case study examples of conceptual fracture model elaborated using static and dynamic data, the disconnect between the scale of observation and the scale of modelling. We will also discuss the potential disconnect between the detail of fundamental, but necessary, research work in universities against the coarse resolution of the models built in the oil industry, and how we can benefit of the differences in scales and approaches.

The appraisal and development of fractured reservoirs offer challenges due to the variations in reservoir quality and natural fracture distribution. Typically, the presence of open, connected fractures is one of the key elements to achieve a successful development. Fracture modelling studies are carried out routinely to support both appraisal and development strategies of these fractured reservoirs.

Overall fracture modelling workflow consists first of a fracture characterization phase concentrating on the understanding of the deformation history and the evaluation of the nature, type and distribution of the fractures; secondly of a fracture modelling part where fracture properties for the dynamic simulation are generated and calibrated against dynamic data. The pillar of the studies is the creation of 3D conceptual fracture diagrams/concepts which summarize both the understanding and the uncertainty of the fracture network of interest. These conceptual diagrams rely on detailed observations at the scale of the wellbore using core and borehole image data which are on contrasting scale compare to the 10’s of meters to 100’s of meter scale of the grid cells of the dynamic models used for the production history match and forecast. These contrasting scales will be the thread of the presentation.

How to cite: Richard, P. and Bazalgette, L.: Scale discrepancy paradox between observation and modelling in fractured reservoir models in oil and gas industry., EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-20161, https://doi.org/10.5194/egusphere-egu2020-20161, 2020.

It is of great importance in many fields to be able to forecast the likely propagation paths of fluid-driven factures. These include mineral veins, human-made hydraulic fractures, and dikes/inclined sheets. The physical principles that control the propagation of all fluid-driven fractures are the same. Here the focus is on dikes and inclined sheets where the selected path determines whether, where, and when a particular dike/sheet reaches the surface to erupt. Here we provide analytical and numerical models on dike/sheet paths in crustal segments (including volcanoes) that include layers of various types (lava flows, pyroclastic flows, tuff layers, soil layers, etc) as well as mechanically weak contacts and faults. The modelling results are then compared with, and tested on, actual data of two types. (a) Seismic data on the paths of dikes/sheets as well as human-made hydraulic fractures, and (b) field data on the actual propagation paths of dikes/sheets in layered and faulted rocks

The numerical results show that, particularly in stratovolcanoes, the paths are likely to be complex with common deflections along layer contacts, in agreement with field observations.  Also, some dikes/sheets may use existing faults as parts of their paths, primarily steeply dipping and recently active normal faults. The propagation path is thus not entirely in pure mode I but rather partly in a mixed mode. The energy required to propagate the dike/sheet is mainly the surface energy needed to rupture the rock, to form two new surfaces and move them apart as the fracture propagates. The energy available to drive the fracture is the stored elastic energy in the hosting crustal segment.

From its point of initiation in the magma-chamber roof, a dike/sheet can, theoretically, select any one of an infinite number of paths to follow to its point of arrest or eruption. It is shown that the eventual path selected is the one of least action, that is, the path along which the time integral of the difference between the kinetic and potential energies is an extremum (normally a minimum) relative to all other possible paths with the same endpoints. If the kinetic energy is omitted, and there are no constraints, then least action becomes the minimum potential energy, which was postulated as a basis for understanding dike propagation by Gudmundsson (1986). Here it is shown how this theoretical framework can help us make reliable forecasts of dike/sheet paths and associated volcanic eruptions.

Gudmundsson, A., 1986. Formation of dykes, feeder-dykes, and the intrusion of dykes from magma chambers. Bulletin of Volcanology, 47, 537-550.

Gudmundsson, A., 2020. Volcanotectonics: Understanding the Structure, Deformation, and Dynamics of Volcanoes. Cambridge University Press, Cambridge.

Drymoni, K., Browning, J. Gudmundsson, A., 2020. Dyke-arrest scenarios in extensional regimes: insights from field observations and numerical models, Santorini, Greece. Journal of Volcanology and Geothermal Research (in press).

How to cite: Gudmundsson, A., Drymoni, K., Bazargan, M., and Adeoye-Akinde, K.: Forecasting the propagation paths of fluid-driven fractures, particularly dikes and inclined sheets , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-20010, https://doi.org/10.5194/egusphere-egu2020-20010, 2020.

Fracture cementation is an important control on the recovery of prefailure levels of permeability and strength in faults and reservoir rock. The timescales of this process, however, are almost entirely unknown from direct analysis of the rock record. We report U‐Th dating results that quantify rates of fracture cementation in syntectonic calcite veins from the Loma Blanca fault, New Mexico, USA. Measured cementation rates vary from ~0.05 to 0.80 mm/ka and exhibit a power function correlation with minimum fracture apertures. We argue that this correlation is the result of crystal growth in a transport‐limited system, where cementation rates were proportional to rates of fluid flow in individual fractures. We argue that such transport‐limited growth necessarily leads to a heterogeneous distribution of cementation rates as fluids migrate through fracture networks of variable and changing aperture. For this reason, individual fractures are not expected to seal at monotonic rates through time but could instead experience order‐of‐magnitude increases or decreases in sealing rate depending on their geometric properties (e.g., aperture, length/width, and orientation) and position within a continually evolving fracture network. We further argue that such transport‐limited, flux‐dependent cementation necessarily leads to a heterogeneous distribution of permeability and strength recovery as fluids migrate through fault‐zone fracture networks. These heterogeneities may influence rupture propagations pathways and the continual development of fault‐zone architecture/complexity.

How to cite: Williams, R., Mozley, P., Sharp, W., and Goodwin, L.: U-Th Dating of Syntectonic Calcite Veins Reveals the Dynamic Nature of Fracture Cementation and Healing in Faults, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-12447, https://doi.org/10.5194/egusphere-egu2020-12447, 2020.

Fractures and faults act as important permeable pathways in the subsurface and are of great significance to the petroleum industry and for future Carbon Capture and Storage. Fractures allow fluid-flow through impermeable units such as mudrocks and can affect how these lithologies act as top seals, source rocks and/or unconventional reservoirs. Natural fractures within mudrocks can strongly influence top seal integrity, primary migration and the performance of unconventional (e.g. shale gas) reservoirs. This project studies the exhumed, early-mature, Jurassic mudrock succession of the Cleveland Basin, NE England, combining structural geology with isotope geochemistry and geochronology. The primary objective is to provide an absolute chronology of faulting and fracturing through novel U-Pb geochronology of fracture-fill calcite. The abundance of well-exposed, natural fractures with different orientations and failure modes provides an opportunity to investigate the properties of these fractures, and provide a basin-wide temporal and spatial framework of evolving deformation. The second objective is to use trace element, stable isotope, and clumped isotope analyses, to constrain fluid composition and temperature. In combination, these objectives will provide an integrated understanding of fracturing, faulting and fluid migration during burial and exhumation of a sedimentary basin.

Current fracture-fill dates from U-Pb geochronology provide intriguing insights into the history of the Cleveland Basin. We have identified and dated three phases of deformation and associated fluid-flow that have contrasting kinematics and fluid-flow regimes. The E-W trending Flamborough Head Fault Zone (FHFZ) bounds the basin to the south, and calcite preserved in one of the major extensional faults provides ages of 64-56 Ma. Calcite from N-S to NNW-SSE trending normal faults and associated fractures in the north of the Cleveland Basin provide ages of 44-25 Ma, revealing a previously unknown phase of Cenozoic faulting, which we speculatively relate to salt-related deformation. Structural and petrographic information suggest that the E-W and N-S trending faults have contrasting fracture-fluid-flow systems. Large (up to 30 cm), chalk hosted, vuggy calcite cements with geopetal sediment-fills in the E-W fault zone suggest it acted as an open fluid conduit with voluminous fluid-flow, linking the shallow sub-surface with deeper levels of the stratigraphy. In contrast, typically thin (<5 mm) vein fills with varying crack-seal-slip type textures in the N-S mudstone-hosted fractures of the Cleveland Basin provide evidence of episodic slip of variable displacement (44-40 Ma); these fracture openings may partly be controlled by pore fluid pressures and pre-date fault movement along the regional Peak Fault and smaller scales N-S faults (40-25 Ma) which are characterised by damage zone calcite mineralisation and extensional jog structures. Initial stable isotopic results are giving indications of fluid temperatures and sourcing which will be built on further by clumped isotope and fluid inclusions work.

How to cite: Lee, J., Roberts, N., Holdworth, R., Aplin, A., Haslam, R., and John, C.: Dating faults, fractures and fluids with U-Pb calcite geochronology: Application to contrasting fracture and fluid-flow modes of the Cleveland Basin, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-5387, https://doi.org/10.5194/egusphere-egu2020-5387, 2020.

The quantification of tectonic forces or, alternatively, stresses represents a significant step towards the understanding of the natural processes governing plate tectonics and deformation at all scales. However, paleostress reconstructions based on the observation and measurement of natural fractures are traditionally limited to the determination of four out of the six parameters of the stress tensor. In the present study, we attempt to reconstruct full paleostress tensors by extending the methodologies advanced by previous authors. We selected Panasqueira Mine, Central Portugal, as natural laboratory, and focused on the measurement of sub-horizontal quartz veins, which are favorably exposed in three dimensions in the underground galleries of the mine. Inversion of the vein data allowed for quantifying the respective orientations of the stress axes and the shape ratio of the stress ellipsoid. In order to reconstruct an additional stress parameter, namely pressure, we extensively sampled vein material and combined fluid inclusion analyses on quartz samples with geothermometric analyses on sulphide minerals. Finally, we adjusted the radius of the obtained Mohr circle with the help of rupture laws, and obtained the six parameters of the paleostress tensor that prevailed during vein formation. Our results suggests a NW-SE reverse stress regime with a shape ratio equal to ~0.6, lithostatic pore pressures of ~300 MPa and differential stress lower than ~20 MPa.

How to cite: Pascal, C., Jaques, L., and Yamaji, A.: Full paleostress tensor determination: case of the Panasqueira Mine, Portugal., EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-2192, https://doi.org/10.5194/egusphere-egu2020-2192, 2020.

The fact that inherited fault systems show strong variability in their 3D shape provides good reasons to consider the strength of the Earth’s brittle crust as variably anisotropic. In this work we quantify this strength anisotropy as a function of fault system complexity by combining 3D boundary element model, frictional slip theory and fast iterative computation method. This method allows to analyze together a very large number of scenarios of stress and fault mechanical properties variations through space and time. Using both synthetic and real fault system geometries we analyze a very large number of numerical simulations (125,000) to define for the first time macroscopic rupture envelopes for fault systems, referred to as “fault slip envelopes”. Fault slip envelopes are defined using variable friction, cohesion and stress state, and their shape is directly related to the fault system 3D geometry and the friction coefficient on fault surfaces. The obtained fault slip envelopes shows that very complex fault geometry implies low and isotropic strength of the fault system compared to geometry having limited fault orientations relative to the remote stresses, providing strong strength anisotropy. This technique is applied to the realistic geological conditions of the Olkiluoto high-level nuclear waste repository (Finland). The model results suggests that Olkiluoto fault system has a better probability to slip under the present day Andersonian thrust stress regime, than for the strike-slip and normal stress regimes expected in the future due to the probable presence of an ice sheet. This new tool allows to quantify the anisotropy of strength and probability of slip of 3D real fault networks as a function of a wide range of possible geological conditions an mechanical properties. This significantly helps to define the most conservative fault slip hazard case or to account for potential uncertainties in the input data for slip. This technique therefore applies to earthquakes hazard studies, geological storage, geothermal resources along faults and fault leaks/seals in geological reservoirs.

How to cite: Soliva, R., Maerten, F., Maerten, L., and Mattila, J.: Fault slip envelope: A new parametric investigation tool for fault system strength and slip, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-8688, https://doi.org/10.5194/egusphere-egu2020-8688, 2020.

The present study provides insights on the segmented nature of normal faults as a function of scale, and attempts to identify whether segmentation is scale invariant, scale dependent or hierarchical. This is a topic of critical importance for studies of fault development and in modelling exercises where one needs to extrapolate observations at one scale to other scales. 

Results are based on data observed in the Blue Lias in Somerset (UK), in Fumanya mine (Spain) and in a 3D seismic reflection survey in the Bonaparte Basin (Australia). Fault segmentation is investigated quantitatively based on previously established methodologies and we focus on neutral relay zones observed between fault segments along the strike of the normal faults.

We found that there are quantitative indications that the shape of the relay zones, the breaching of the relays and the degree of segmentation are all scale independent in Kilve and Fumanya. We propose that this is related to the low variability across scales in the geological parameters controlling segmentation, due to the relative homogeneity of the rock medium across the studied scales, the lack of influence of pre-existing faults or fractures, and the similar deformation histories for all studied faults. By contrast, faults show scale dependency in the Bonaparte Basin where large faults are under the influence of an oblique reactivation of pre-existing faults. Independently of the area, segmentation observed continuously through scale stresses the need to take into account resolution of observation in discussing fault development.

How to cite: Roche, V., Manzocchi, T., Camanni, G., Childs, C., and Papanikolaou, V.: Scale dependency of segmentation along the strike of normal faults., EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-18032, https://doi.org/10.5194/egusphere-egu2020-18032, 2020.

Fractures constitute the main pathway for fluids in fault damage zones hosted in low-porosity rocks. Understanding the factors controlling fracture distribution is hence fundamental to better assess fluids circulation in fault damage zones, with evident implications for fault mechanics, hydrogeology and hydrocarbon exploration. Being usually characterized by a strong damage and structural complexity, this is of particularly importance for relay zones.

We integrated classical and modern structural geology techniques to investigate the factors controlling fracture distribution within a portion of a relay ramp damage zone pertaining to the Tre Monti fault (Central Italy). The damage zone is hosted within peritidal carbonates and located at the footwall of the relay ramp front segment. We analysed the distribution of the fracture density in the outcrop through (1) scanlines measured in the field, (2) oriented rock samples, and (3) scan-areas performed on a virtual outcrop model obtained by aerial structure-from-motion.

Our results highlight structural and lithological control on fracture distribution. Scanlines and virtual scan-areas show that fracture density increases with the distance from the front segment of the relay ramp. Moreover, all the methods highlight that supratidal and intertidal carbonate facies exhibit higher fracture density than subtidal limestones.

This apparently anomalous trend of fracture density, that increases moving away from a main fault segment, has two main explanations. (1) The damage is associated with the relay ramp development: approaching the centre of the relay ramp (i.e., moving away from the front segment) an increase in the number of subsidiary faults with their associated damage zones promotes high fracture densities. (2) The increase in fracture density can be attributed to the increasing content in supratidal and intertidal carbonate facies that are more abundant in the centre of the relay ramp.

Our results provide important suggestions for factors controlling fracture distribution and fluid flow within relay ramps hosted by shallow water limestones. We show that the trend of fracture distribution with respect to a main fault is not easily predictable in presence of a relay ramp, because it can be modulated by the subsidiary faults formation and slip during the relay ramp development. Moreover, carbonate facies play a non-negligible role in fracture distribution within fault zones hosted in shallow-water carbonates.

How to cite: Mercuri, M., Carminati, E., Tartarello, M. C., Brandano, M., Mazzanti, P., Brunetti, A., McCaffrey, K. J. W., and Collettini, C.: Factors controlling fracture distribution within a carbonate-hosted relay ramp: insights from the Tre Monti fault (Central Apennines), EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-491, https://doi.org/10.5194/egusphere-egu2020-491, 2020.

Fracture systems develop at different stages of progressive deformation and are often genetically associated with folding. The frontal Main Boundary thrust (MBT) sheet is folded in a fault bend fold (Ahmed et al., 2018) and is exposed in Siang window in far eastern Arunachal Himalayan fold-thrust belt (FTB). The Buxa dolomite of the Lesser Himalayan sequence forms part of the MBT sheet and records four different sets of fractures (Basa et al., 2019). We present results from Discrete Fracture Network (DFN) model from the Buxa dolomite. Integrating fold test, cross-cutting, offset and abutting relationships, we have established that the low-angle fracture set (0°-20°) formed as a result of early layer parallel shortening.  These low-angle and the two sets of medium angle fractures (20°-60°) formed prior to the fault-bend folding. The late stage, high-angle fractures (60°-90°) developed synchronous to the fault-bend fold (Basa et al., 2019). We model the fractures formed before and during the folding event using 3D MOVE’s Fracture Modeling module to evaluate how the properties of secondary porosity and permeability, induced by fracture sets, fracture area/unit volume (P32) and overall connectivity are affected by the folding event. The input parameters of fracture orientation, intensity, length and aperture were measured from the field. For the aspect ratio, theoretical value of 1:2 (Olding, 1997; Olson, 2003) was considered.

Results from DFN analysis indicate that the average porosity increases from pre-folding (model-1) (~0.0028) to syn- to post-folding (model-2) (~0.0071). The permeability also increases from ~231 Darcy in model-1 to ~3988 Darcy in model-2. There is also a significant rise in P32 (~2.8m²/m³ to ~4.3m²/m³⁾ value from model-1 to model-2. The late high-angle fracture set led to increase in overall connectivity, including porosity, permeability and fracture intensity. This is also corroborated from the field results that reveal high-angle fractures are more conducive to vein formation (~41%) compared to the lower angle fracture-sets (~15 %).

How to cite: Ahmed, F. and Bhattacharyya, K.: Characterization of fracture networking and connectivity: Insights from Discrete Fracture Network Model and Multi-Scale analysis from the Buxa Dolomite of the Main Boundary thrust sheet, Siang widow, Arunachal Himalayan Fold thrust belt, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-924, https://doi.org/10.5194/egusphere-egu2020-924, 2020.

Fractures in rocks are sensitive cursers that may enhance porosity and permeability. This is particularly true in carbonates because background fractures might be ubiquitous after embrittlement at early burial (Lavenu & Lamarche, 2018). Barren fractures at depth are susceptible to chemical reactions with underground fluids and cementation that might totally or partially reduce porosity and permeability (Laubach et al., 2019; Aubert et al., 2019). Hence, early background fractures with long lasting tectonic history and structural diagenesis, in addition to fractures neo-formed at any time during burial, tectonic inversion and folding join the game of matrix/fracture permeability and porosity modification. To predict the fractures contribution to flow in Naturally Fractured Reservoirs, it is fundamental to know the fracture sequence and geometry resulting from the geological history in folded carbonates, from the host-rock embrittlement to the present-day situation. At any step, we intent quantifying the fracture geometry and estimating their contribution to the host reservoir properties.

The study is performed in Upper Jurassic to Lower Cretaceous carbonates (Oxfordian, Tithonian, Berriasian) formed in the South-Provençal Basin. From deposition to present-day, the platform carbonates underwent alternating subsidence, uplift, erosion and folding. We sampled a scan-line along a horizontal path across both flanks of the Mirabeau Anticline (SE France). We measured all tectonic and stratigraphic features crossed by the line, checked their nature and position. We deciphered their chronological relationships with respect to each other and to the bed tilting. We compiled all cross-cutting relationships into a coherent sequence of deformation of pre-, syn- and post-fold structures and correlated it to burial, uplift and folding of the host rock. At each brittle stage, the fracture pattern was characterized in terms of architecture, mechanical stratigraphy and reservoir properties in order to draw a time-path in a matrix versus fracture permeability and porosity table (Nelson Reservoir types) during 150My. After embrittlement, the host-rocks bear fractures, pressure-solution, faulting, folding and erosion. If it was a reservoir, its Nelson type would have evolved from IV to III during the burial and initial brittle deformation. The tectonic inversion and onset of multiple-scale brittle structures would have increased and decreased the fracture and matrix contribution respectively and the reservoir evolved to types II and I. During the 150My history, fracture porosity and permeability depends on their geometry (veins versus tension gashes) and cementation. This results in several switches from type II to I as a function of the fracture timing, geometry, connectivity and diagenesis.

Aubert I. et al. (2019). Imbricated structure and hydraulic path induced by strike-slip reactivation of a normal fault in carbonates. Fifth International Conference on Fault and Top Seals, 8-12 September 2019, Palermo, Italy.

Bestani L.et al. (2016) Reconstruction of the Provence Chain evolution, southeastern France., Tectonics Vol: 35, p.1506–1525

Laubach, S. E. et al. (2019) The role of chemistry in fracture pattern development and opportunities to advance interpretations of geological materials. Reviews Geophysics, 57.

Lavenu A.P.C., Lamarche J. (2018) What controls diffuse fractures in platform carbonates? Insights from Provence (France) and Apulia (Italy), JSG 108, p. 94-107

How to cite: Lamarche, J., Espurt, N., Kaci, T., Lionel, M., and P. Pascal, R.: Impact of multiphase fracture sequence in folded carbonates on the evolution of naturally fractured reservoir types. An outcrop case study from Mirabeau Anticline (SE France), EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-2195, https://doi.org/10.5194/egusphere-egu2020-2195, 2020.

Deformation bands and structurally-related diagenetic heterogeneities, here named Structural Diagenetic Heterogeneities (SDH), have been recognized to affect subsurface fluid flow on a range of scales and potentially promoting reservoir compartmentalization, altering flow paths, influencing flow buffering, and sealing during production. Their impact on reservoir hydraulic properties depends on many factors, such as their permeability contrast with respect to the undeformed reservoir rock, their anisotropy, thickness, geometry as well as their physical connectivity and arrangement in the subsurface. Deformation bands offsets (from a few mm to 20-40 mm) and diagenetic heterogeneities (carbonate nodules) dimensions (from 0.2 to 15 m in length; from 0.1 to 1.0 m in thickness) make them SDH below seismic resolution.

We used Ground Penetrating Radar (GPR) for detection and analysis of the assemblage “deformation bands - carbonate nodules”, in high-porosity arkose sandstone of the Northern Apennines (Italy). Petrophysical (air-permeability) and mechanical (uniaxial compressive strength) properties of host rock, deformation bands, and calcite-cement nodules were evaluated along a 30-meters thick stratigraphic log to characterize the permeability and strength variations of those features. 2D GPR surveys allowed the description of the SDH spatial organization, geometry, and continuity in the subsurface. The assemblage “deformation bands – nodules” decreases porosity and permeability and produces a strengthening effect of the rock volume, inducing a strong mechanical and petrophysical heterogeneity to the pristine rock. Different textural, petrophysical, and geomechanical properties of deformation bands, nodules, and host rock result in different GPR response (dielectric permittivity; instantaneous attributes). We show that GPR can be useful to characterize variations in petrophysical and geomechanical properties other than characterize the geometry and spatial distribution of flow baffles and small-scale flow barriers in the subsurface such as deformation bands and cement-nodules. GPR showed its worth as a high-resolution and non-invasive tool to extend outcrop information (petrophysical and geomechanical data) to 3D subsurface volumes in a way to reconstruct realistic and detailed outcrop analogues. Such potential could be critical in assisting and improving the characterization of SDH networks in the study of faulted aquifers and reservoirs in porous sandstones.

How to cite: Del Sole, L., Calafato, A., and Antonellini, M.: Combining Ground-Penetrating Radar profiles with geomechanical and petrophysical in situ measurements to characterize sub-seismic resolution structural and diagenetic heterogeneities in porous sandstones (Northern Apennines, Italy), EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-3118, https://doi.org/10.5194/egusphere-egu2020-3118, 2020.

Based on cores, image logs and thin sections, five sets of fractures are developed in the study area, where faults are developed. Most of fractures are open without fillings, and some fractures are filled with calcite, quartz, bitumen, pyrite and mud. Fractures are mainly controlled by lithology, mechanical stratigraphy and faults. Based on mutual crosscutting relationships of fractures, mineral filling sequence of fracture fillings, fluid inclusion and carbon-oxygen isotope analysis of calcite fillings in fractures, and quartz spintronic resonance analysis of quartz fillings in fractures, in combination with thermal and burial history, the formation sequence and time of fractures were analyzed. The results show that fractures mainly formed over three period, that is, the late Triassic, Middle to Late Jurassic, and Late Cretaceous to Paleogene. Then，combined with the paleostress evolution and fracture characteristics of the study area, the formation mechanism of fractures was discussed.

How to cite: Lyu, W., Zeng, L., Chen, S., Tang, L., and Zhang, Y.: Controlling factors and formation mechanism of fractures in the tight-gas sandstones of the Upper Triassic Xujiahe Formation, western Sichuan Basin, China, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-4123, https://doi.org/10.5194/egusphere-egu2020-4123, 2020.

Fracture network characterization is critical for many subsurface engineering problems in petroleum, mining, nuclear waste disposal and Enhanced Geothermal Systems (EGS). Due to limited exposure, direct measurement of fracture network properties at great depth is not possible and geophysical imaging techniques cannot resolve the fractures. Therefore, tomographic imaging techniques have been proposed and applied to reconstruct the structural discontinuities of rock mass. Stress-based tomography is a novel concept aiming at probabilistic imaging of the fracture network using the stress perturbations along deep boreholes. Currently, this approach has only been successfully tested on two-dimensional fracture networks. However, its great potential to unravel the heterogeneous structure of fractured rocks at great depth motivates further scientific effort. Here, we present the potential, open questions, current challenges and necessary future developments in order to apply this methodology to image three-dimensional multiscale structure of the rock mass in the field. Other tomographic approaches such as tracer and hydraulic tomography invert tracer breakthrough curves (BTCs) and pressure response in an observational well. We suggest a joint and comparative tomographic analysis in a Bayesian inversion framework to reconstruct Discrete Fracture Networks (DFN). This is expected to provide a new view of the strengths of each tomographic variant.

How to cite: Bayer, P., Afshari Moein, M. J., Somogyvári, M., Ringel, L., and Jalali, M.: Stress-based tomography: potential, open-questions and future developments, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-19401, https://doi.org/10.5194/egusphere-egu2020-19401, 2020.

Methane seepage on continental margins derived from shallow gas reserves and gas hydrate stores is significant globally, with initial observations most commonly derived from pockmark expressions on the seafloor. The processes driving fluid flow (liquids and gases) through upper (200m) marine sediments is not well understood. Pockmarks signify present or past seepage events, and are prominent across Vestnesa Ridge. Not all pockmarks are active (venting), suggesting that the mechanism behind fluid flow varies across the ridge.  The main structures observed in the seismic are gas chimneys, faults and fractures. Here we study the characteristics of the observed features through attribute analysis at three significant horizons (age estimates: <0.2Ma, ~0.2Ma and ~1.5Ma). We extract fault orientations through the generation of 3D fault attributes and analysis of fault detect volumes. Attribute extracts at horizons, using amplitude and edge detection methods, together with spectral decomposition and RGB blending, have revealed fine-scale (<10m) faults. High amplitude lineaments at multiple depths match fault trends and radial fracturing is observed around gas chimneys. Small faults propagate outwards from gas chimneys and feed into larger tectonically derived faults, suggesting horizontal inter-connectivity at specific depths.  Enhanced imaging of gas chimneys and small scale features, contribute to our understanding of how fluids migrate through the sediment column. We hypothesize that the connecting fractures, forming between main fault zones may suggest sediment overpressure and restricted flow through tectonically induced faults resulting in horizontal fluid transport.

How to cite: Cooke, F., Plaza-Faverola, A., Bünz, S., and Singhroha, S.: High-resolution 3D seismic investigation of fine scale faults and fractures along the Vestnesa Ridge, western Svalbard Margin , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-19487, https://doi.org/10.5194/egusphere-egu2020-19487, 2020.

Fault damage zones can be preferential conduits for geofluids, depending on the secondary permeability developed with fracturing. Large-scale outcrop analogues allow a complete characterization of fracture networks, that cannot be satisfactorily imaged in the subsurface (e.g. with seismics). In this project we mapped fractures in the damage zone of the Victoria Fault, a major normal fault crosscutting Miocene shallow-water carbonates of Malta, combining field analysis and a high resolution photogrammetric Digital Outcrop Model (DOM). This allowed characterizing (i) the damage zone width, (ii) its spatial organization, (iii) geometrical parameters of the fracture network and (iv) its connectivity, and (v) the variability of these parameters in different stratigraphic units.

How to cite: Losa, A., Martinelli, M., and Bistacchi, A.: Quantitative fracture characterization in the damage zone of the Victoria Fault, Malta, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-22583, https://doi.org/10.5194/egusphere-egu2020-22583, 2020.

The present contribution focuses on carbonates fault cores exposed in central and southern Italy, which crosscut Mesozoic limestones and dolostones, pertain to 10’s of m- to 100’s of m-throw extensional fault zones, and include two main domains named as inner and outer fault cores, respectively. The inner fault cores are made up of main slip surfaces (MSS), matrix-supported cataclasites, and fault breccia. Cement-supported cataclasites, if present in the limestone-hosted fault cores, localize around the MSS. The outer fault cores mainly include grain-supported cataclasites, subsidiary slip surfaces, and lithons of fragmented host rocks. In order to assess the fluid flow properties of the carbonate fault cores, the results of microstructural, petrophysical, and ultrasonic studies are first presented, and then discussed in terms of pore type, geometry, textural anisotropy, and poro-perm relationships. Overall, the documented pore distribution is mainly function of both deformation micro-mechanisms and diagenetic processes, which took place in the carbonate fault cores during faulting and fault rock exhumation from depth. In the limestone-hosted fault cores, the experimental results show that the cross-fault fluid flow properties are affected by the the irregular geometry of the cement fronts. These fronts, which depart from the MSS, are due to calcite precipitation in vadose environments from meteoric-derived fault fluids. At depths of about 1 km, these fluids merge with the local freshwater aquifers, and cement the whole fault cores. Overall, these fault cores include a stiff pore networks, and are thought to behave like a granular medium. There, it is proposed that the cross-fault permeability can be computed by applying the Kozeny-Carmen correlation. For any given value of effective porosity, the value of permeability is therefore proportional to the average value of the pore throat, which characterize the aperture of capillary tubes with a geometrical tortuosity of ca. 2.5. On the contrary, the dolostone-hosted fault cores include a soft pore network made up of elongated pores, and are thought to behave like an elastic cracked medium. Accordingly, it is proposed that the cross-fault permeability can be computed by following percolation theory by considering the values of dynamic elastic moduli measured during ultrasonic tests at Pc=30 MPa, and almost isotropic fracture networks. Results of this work could be helpful during appraisal and development operations of hydrocarbon reservoirs, for freshwater aquifer protection, and activities of CO₂ storage in depleted carbonate reservoirs.

How to cite: Agosta, F.: Permeability models for carbonate fault cores, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-3360, https://doi.org/10.5194/egusphere-egu2020-3360, 2020.

Quantification of the geometry, distribution, and dimension of fracture networks is key to fully understand the petrophysical properties of outcrop-to-reservoir scales rock volumes. On these regards, Discrete Fracture Network (DFN) modeling is a very useful tool to compute the values of fracture porosity and equivalent permeability of geo-cellular volumes populated with stochastic or deterministic fracture networks. Independently of their size and cell dimensions, the single geocelullar volumes are populated by inputting the following parameters for each fracture set: (i) length; (ii) aspect ratio; (iii) mechanical and hydraulic apertures; (iv) fracture intensity, and (v) attitude. A sensitivity analysis is always carried out in order to test the seeding procedure of the employed software, and to check the validity of the fracture aperture values employed as input data. The latter values, in fact, are the most critical to assess from outcrop and laboratory analyses. The present contribution focuses on the results of recent works performed on the fractured limestone rocks of the Apulian Platform, which are widely exposed along the Italian peninsula. Outcrops are first introduced in order to define the fracture stratigraphy and fault architecture of the Meso-Cenozoic limestone rocks. Then, the criteria behind the construction of DFN models are illustrated. Methods employed for the build of individual fracture units and single fault damage zone domains are illustrated. Finally, the computed values of fracture porosity and equivalent horizontal permeability obtained for multiple DFN models are presented. Discussion of the data focuses on the fluid accumulation and migration properties of the fractured limestone rocks by considering their amount of exhumation experienced during Plio-Quaternary times. Results of DFN modeling could be helpful to optimize the appraisal and development operations of hydrocarbon reservoirs, and minimize the pollution of freshwater aquifer. In fact, the Apulian carbonates host in the underground significant amounts of freshwater of the Mediterranean Region, and the largest oil and gas reserves of continental Europe. Furthermore, the results could shed new lights into the role exerted by faults and fractures on subsurface CO₂ storage in depleted carbonate reservoirs, a practice that envisioned to decrease the greenhouse gas concentration in the atmosphere in the next future.

How to cite: Agosta, F.: Fracture stratigraphy, fault architecture and DFN modeling of both diffuse and localized fracture networks, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-3370, https://doi.org/10.5194/egusphere-egu2020-3370, 2020.

Finding an adequate bridge between direct and continuum modeling approaches has been the fundamental issue of upscaling fluid flow in rock masses. Typically, numerical simulations of direct fluid flow (e.g. Stokes or Lattice-Boltzmann) in fractured or porous media serve as small-scale building blocks for larger-scale continuum flow simulations (e.g. Darcy). For fractured rock masses, the discrete-fracture-network (DFN) modeling approach is often used as an initial step to upscale flow properties by parameterizing the permeability of each fracture with its hydraulic aperture and solving steady-state flow equations within the fracture system. However, numerical simulations of Stokes flow in small fracture networks (FN) indicate that, depending on the orientation of the applied pressure gradient, fluid flow tends to localize at places where fractures intersect. This effect causes discrepancies between direct and equivalent continuum flow modeling approaches, which ought to be taken into account when modeling flow at the network scale.

In this study, we compare direct flow simulations of small fracture networks to their continuum representation obtained with several techniques in order to find an upscaling approach that takes these intersection effects into account. Direct flow simulations are conducted by solving the Stokes equations in 3D using our open-source finite-difference software LaMEM. Continuum flow simulations are realized with a newly developed parallel finite-element code, which solves fully anisotropic 3D Darcy flow with specific permeability tensors for each voxel. The direct flow simulations serve as benchmarks to optimize the continuum flow models by comparing resulting permeabilities. We tested two different schemes to generate the equivalent continuum representation: 

(1) Fully resolved isotropic permeability discretizations (fracture permeability is obtained from a refined cubic law) where voxel sizes are a fraction of the minimal hydraulic aperture of the FN or

(2) coarse anisotropic permeability discretizations (permeability tensors are rotated according to fracture orientation) with voxel sizes larger than the minimal hydraulic aperture of the FN.

We then assess different scenarios to incorporate the intersection effects by adding, averaging and/or multiplying the permeabilities of the intersecting fractures within intersection voxels. Preliminary results for scheme 1 suggest that a simple addition of both intersecting fracture permeabilities delivers the best fit to the results of the direct flow simulations, if the voxel size is about 68% of the minimal hydraulic aperture. Scheme 2 systematically underestimates the direct flow permeabilities by about 26%.

How to cite: Kottwitz, M. O., Popov, A. A., Abe, S., and Kaus, B. J. P.: Linking direct and continuum fluid flow models for fractured media: The intersection problem, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-4902, https://doi.org/10.5194/egusphere-egu2020-4902, 2020.

Hydraulic stimulation is the process of initiating fractures in a target reservoir for subsurface energy resource management with applications in unconventional oil/gas and enhanced geothermal systems. The fracture characteristics (i.e., number, size and orientation with respect to the wellbore) determines the modified permeability field of the host rock and thus, numerical simulations of flow in fractured media are essential for estimating the anticipated change in reservoir productivity. However, numerical modeling of fluid flow in highly fractured media is challenging due to the explosive computational cost imposed by the explicit discretization of fractures at multiple length scales. A common strategy for mitigating this extreme cost is to crudely simplify the geometry of fracture network, thereby neglecting the important contributions made by all elements of the complex fracture system.

The proposed “Hierarchical Finite Element Method” (Hi-FEM; Weiss, Geophysics, 2017) reduces the comparatively insignificant dimensions of planar- and curvilinear-like features by translating them into integrated hydraulic conductivities, thus enabling cost-effective simulations with requisite solutions at material discontinuities without defining ad-hoc, heuristic, or empirically-estimated boundary conditions between fractures and the surrounding formation. By representing geometrical and geostatistical features of a given fracture network through the Hi-FEM computational framework, geometrically- and geomechanically-dependent fluid flow properly can now be modeled economically both within fractures as well as the surrounding medium, with a natural “physics-informed” coupling between the two.

SNL is managed and operated by NTESS under DOE NNSA contract DE-NA0003525.

How to cite: Chang, K. W., Beskardes, G., and Weiss, C.: Modeling fluid flow through complex fracture network in geological media by using hierarchical hydraulic properties , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-5677, https://doi.org/10.5194/egusphere-egu2020-5677, 2020.

The topological and geometrical description of fault and fracture networks is an essential first step in any investigation of fractured or faulted media. The spatial arrangement, density, connectivity, and geometry of the discontinuities strongly impact the physical properties of the media such as resilience and permeability. Obtaining reliable metrics for characterizing fault and fracture networks is of interest for mining engineering, reservoir characterization, groundwater management, and studies on the regional fluid flow history. During large-scale studies, we mostly rely on two-dimensional lineaments obtained through structural mapping, outcrop analysis, or remote sensing. An efficient and widely applicable framework for discontinuity network characterization should therefore be based on the analysis of the frequently available two-dimensional data sets.

Here, we present an automated framework for efficient and robust characterization of the geometric and topologic parameters of discontinuity networks. The geometry of the lineaments is characterised based on orientation, length, and sinuosity. The underlying distribution of these parameters are determined, and representative probability density functions are reported. The connection between the geometric parameters is validated, e.g. correlation between orientation and length. The spatial arrangement is determined by classical line- and window-sampling, by assessing the fractal dimension, and via graph-based topology analysis.

In addition to the statistical analysis of lineament networks, we show how the graph data structure can be utilized for further characterization by linking it to raster data such as magnetic, gravimetric, or elevation. This procedure not only yields an additional means for lineament characterization but also allows users to assess dominant pathways based, for instance, on hydraulic gradients. We demonstrate the applicability of our algorithm on synthetic data sets and real-world case studies on mapped fault and fracture networks.

We finally show how our framework can also be utilized to design detailed numerical studies on the fluid flow properties of analysed networks by conditioning mesh refinement on the type and number of intersections. In addition, due to known scaling relationships our framework can help to determine appropriate parameters for the simulations. We provide examples of statistically parametrized fluid flow simulations in natural discontinuity networks and show the impact of conceptualizing the lineaments as conduits, barriers or conduit-barrier systems.

How to cite: Poulet, T., Kelka, U., Westerlund, S., and Peeters, L.: Computational framework for discontinuity network characterization , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-6577, https://doi.org/10.5194/egusphere-egu2020-6577, 2020.

The exploration of the underground is a complex, but common task. Structural characterisation of a given sub-surface volume is of interest, for many purposes including deep geological repositories for radioactive waste, exploitation of mineral resources and geothermal energy production. One major challenge in this regard relates to the identification of sub-seismic scale faults and fractures. Discrete fracture network modelling is one possible technique for this purpose. Ideally, it is supported by borehole data. Even so, the results of stochastic models require critical verification to determine resulting uncertainties and model robustness.

We present a case-study from the village of Schlattingen, located the northernmost Molasse Basin in Switzerland, that is devoted to such a verification aimed at improvement of the DFN modelling workflows. Two boreholes were drilled at this location, a vertical cored borehole reaching into the crystalline basement and a deviated borehole running sub-horizontally for 464 m in the Schinznach Formation (Upper Muschelkalk), a potential geothermal reservoir (Frieg et al. 2015). This borehole layout allows testing the workflow for discrete fracture network modelling from a single borehole and assessment of the the added value of a deviated borehole (and vice versa).

The modelling workflow used borehole data and outcrop descriptions from a range of locations as input data. The spatial distribution of features was simulated using a Poisson distribution. The aims of the study were to investigate the workflow’s ability to account for the different orientation biases in the two boreholes and develop understanding of spatial variability in fracture orientation and frequency.

It was found that reasonable consistency in orientation and overall frequency could be achieved using the borehole orientation distributions but that the spatial variability in fracture frequency and clustering of fractures were significant. It was also necessary to critically evaluate the borehole imagery from the deviated borehole.

Current efforts are focused on better constrain the spatial fracture distribution along the deviated borehole using correlation analysis (Marett et al. 2018, Gale at al. 2018) and assess its influence on the discrete fracture network model. In addition, it is anticipated to integrate observation from nearby outcrops into the modelling strategy.

References:

Frieg, B., Grob, H., Hertrich, M., Madritsch, H., Müller, H., Vietor, T., Vogt, T., and Weber, H.P. (2015). Novel Approach for the Extrapolation of the Muschelkalk Aquifer in Switzerland for the CO₂-free production of vegetables. Proceedings World Geothermal Congress, Melbourne, Australia

Gale, J. F. W., Ukar, E., & Laubach, S. E. (2018). Gaps in DFN models and how to fill them. 2nd International Discrete Fracture Network Engineering Conference, DFNE 2018.

Marrett, R., Gale, J. F. W., Gómez, L. A., & Laubach, S. E. (2018). Correlation analysis of fracture arrangement in space. Journal of Structural Geology, 108, 16–33. https://doi.org/10.1016/j.jsg.2017.06.012

How to cite: Schneeberger, R., Lanyon, B., Herbert, A., Habermüller, M., and Madritsch, H.: Predictive DFN modelling for the Upper Muschelkalk aquifer in the northernmost Swiss Molasse Basin based on vertical and horizontal borehole records , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-19664, https://doi.org/10.5194/egusphere-egu2020-19664, 2020.

Fracture networks exist at a wide range of scale in the earth crust and strongly influence the hydraulic behaviour of rocks, providing either pathways or barriers for fluid flow. Many oil, gas, geothermal and water supply reservoirs form in fractured rocks. The main challenge is the development of numerical models that describe adequately the fracture networks and the constitutive equations governing the physical processes in fractured reservoir. The hydraulic properties of fracture networks, derived from Discrete Fracture Network (DFN), models are commonly used to populate continuum equivalent models at reservoir scale, to reduce the computational cost and the numerical complexity. However, the efficiency of fracture networks to fluid flow is strongly tied to their connectivity and spatial distribution, that continuum models are not able to capture explicitly.In this work we used field data and synthetic models to introduce a new parameter to evaluate the efficiency of fracture networks to fluid flow, reflecting a range of variability in fracture network characteristics (e.g. P32, number of fractures, stress field). This alternative method allows to model fractured systems at reservoir scale, in a variety of geological settings, using exclusively a DFN approach.

How to cite: Proietti, G., Romano, V., Conti, A., Tartarello, M. C., and Bigi, S.: An alternative method to evaluate fracture network efficiency to fluid flow , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-4799, https://doi.org/10.5194/egusphere-egu2020-4799, 2020.

Fluid transport within the Earth’s crust is predominantly controlled by planar void space like fractures and crack networks. Characterizing the time dependent hydro-mechanical properties of these rock-structural elements therefore is of paramount importance for natural geosystem understanding and geotechnical applications alike.

In this contribution we outline the protocol and results of a long term flow-through experiment of more than 4 months conducted with one single-fractured, pure quartz, and centimeter-sized Fontainebleau sandstone sample displaying very low matrix permeability.

The cylindrical sample was axially split to generate one single and rough tensile fracture and the obtained sample halves were manually offset in axial direction by 200 µm resulting in geometric mismatch of the two fracture faces yielding asperity contacts and high contact stresses upon loading.

The experiment was conducted at constant temperature (333 K) and pore fluid pressure (1 MPa), three different confining pressure levels (2, 18, and 30 MPa), and with two different fluids (deionized water and 0.3 mM SiO₂ solution).

The sample was continuously flown through and the experimental procedure consisted of several successive stages during which confining pressure and fluid type were systematically varied in time intervals of several weeks each.

The experiment yielded results of continuous sample and fracture permeability measurements, the derivation of time dependent changes in hydraulic fracture aperture, a complete ICP-OES chemical analysis of Si concentrations in the effluent in one day time intervals, and a full before/after microstructural investigation of mechanical aperture, contact area ratio, as well as asperity and free fracture face morphology.

Overall, this experiment yields evidenced insights into the low-temperature dynamics of fracture permeability when, concurrently, chemical interactions between fluid and rock are taking place. Moreover, the investigations emphasize the role of pressure solution (creep) in this context as opposed to, e.g., free face dissolution or subcritical crack growth. Finally, conclusions are drawn on the rate-limiting sub-process of pressure solution with possible implications for fluid history matching in quartz-rich fractured rock masses.

How to cite: Milsch, H. and Cheng, C.: Fluid flow through rock fractures undergoing chemical interactions: A look at pressure solution from an experimental perspective, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-4712, https://doi.org/10.5194/egusphere-egu2020-4712, 2020.

Dissolution process is a complex phenomenon controlled by several factors such as the nature of chemical dissolution, lithology, porosity, stress orientation, environmental conditions, networks of fractures. In the karst field, however, compressional tectonic structures, as like stylolites, are never been taken into consideration for fluid flow. Stylolites are formed by a pressure solution processes that dissolves the soluble particles and leads to an enrichment in insolvable, non-carbonate particles (NCP) along their surfaces. Potentially they play an important role in fluid circulation during carbonate deformation.

Although they seem macroscopically planar, stylolites have an extremely variable shape from the meso- to microscale, with variable porosity and permeability. Because of this, they have a strong effect on regional fluid flow and the formation of reservoirs since they can act as barriers or conduits for flow.

In this research we investigated the distribution of voids and pores present both within and near the stylolites.This task is challenging because the pore sizes are small and therefore difficult to investigate. To determine which role the NCP and these structures have on fluid circulation, a comparison is herein presented between two different methods used to map the submicroscopic arrangement of pores and voids in and around stylolites. Because the investigated stylolites are relatively narrow, around 30-50 µm, we decided to use a classical micro Computed Tomography (µCT) technique supported by the ¹⁴C-PMMA impregnation method on two marble samples. These two comprelementary methods characterize the spatial distribution of connected voids in and around stylolites. µCT analysis provide adequate information on the 3D distribution of voids even if nanometer scale pores and small fractures are difficult to observe using µCT.

¹⁴C-PMMA method is however able to reveal connected porosities from mineral areas that consist of nanometer scale pores. Methylmethacrylate intrudes into nanometer scale pores, and autoradiography is used to visualize the porosities thanks to ¹⁴C beta emissions. Combining these two techniques, CT tomography and PMMA autoradiography we can visualize the 3D pore structures of the studied samples.

The results show that the micro CT technique supported by the PMMA autoradiography technique provides a useful tool to characterize the voids and pore structures of geomaterials.

The results allowed a more accurate description of the behavior of stylolites in fluid-rock interaction.

Key words: stylolites, permeability, microCT and ¹⁴C-PMMA impregnation

How to cite: Repetto, G., Magni, S., Sardini, P., Sattari, M. S., Sammaljärvi, J., and Mazurier, A.: Stylolites:when they became conduits for fluid pathway ?, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-5791, https://doi.org/10.5194/egusphere-egu2020-5791, 2020.

A multi-scalar, multi-methodological approach has been used to characterize the deformation mechanisms and fluid-rock interaction processes within the Belluno Thrust (BT), a regional-scale thrust cutting through Mesozoic carbonates of the eastern Southern Alps of Italy. We report the first results of a systematic analysis of the deformation mechanisms that steered strain localization within the BT fault zone during seismogenic faulting. The WSW-ENE-striking BT contributed to development of the south-verging thrust-and-fold belt of the Southern Alps during the Late Oligocene – present time interval.        We studied an outstanding exposure of the BT in the greater Feltre region, where the BT juxtaposes an Early Jurassic oolitic and micritic limestone (the Calcari Grigi Group) in the hanging wall against an Upper Jurassic-Early Cretaceous pelagic and cherty limestone (the Maiolica Fm.). The BT is defined by a 2 m-thick damage zone formed at the expense of both the hanging wall and footwall blocks. Atop the damage zone is a millimetric principal slip surface (PSS) that strikes WSW-ENE and dips 40° to the NNW. Kinematic analysis confirms the top-to-the SSE transport along the BT. Several structural facies have been identified by means of detailed structural mapping and sampled from the damage zone (from within both the hanging- and the footwall blocks) and the PSS. The outcrop structural characterization has revealed a number of physically juxtaposed, yet different, structural facies: i) cohesive, weakly foliated proto- to ultracataclasite; ii) uncohesive, clay-rich gouge; iii) foliated domains with SC-C’ structures. Relatively unstrained host rock lithons are wrapped by these variably strained domains. Petrographic and microstructural analyses show evidence of pervasive pressure solution, with abundant stylolites, slickolites and foliated domains indicating an overall ductile behaviour. Calcite veins are also common in all recognised structural facies showing mutual cross-cutting relationships with the pressure-solution seams. This structural characterization has provided the basis for detailed image analysis of selected cataclastic textures to calculate fractal parameters for the particle size distribution (Ds) and morphology (Dr) of the clasts aiming at better understanding the cataclastic flow active in the BT fault rocks. Results from a range of representative samples suggest corrosive wear to be the main cataclastic process (Ds 1,41 ÷ 2,00; Dr 1,51 ÷ 1,88). Cathodoluminescence imaging revealed multiple generations of cement and permitted discriminating the first-order chemical characteristics of parental fluids and constraining the relationships between calcite veining and cementation. Two syn-tectonic cements have been identified: i) a bright-orange cement, preferentially surrounding carbonate clasts with highly irregular margins, indicative of the involvement of carbonate-reactive fluids; ii) a dull, homogeneous brown/black cement coexisting with a siliceous matrix, mantled clasts and local sigmoidal structures. The latter is at times observed as thin injections and fluidized structures.        Our preliminary results suggest that overall deformation was accommodated by creep and low-T crystal-plastic deformation possibly during inter-seismic phases as indicated by the presence of pressure-solution seams and foliated fabrics. Transient spikes of coseismic rupturing possibly promoted by multiple batches of overpressured fluids were accompanied by significant cataclasis and brittle strain localization.

How to cite: Diamanti, R., Zuccari, C., Bonini, S., Vignaroli, G., and Viola, G.: Textural evolution and fluid-rock interaction during upper crustal, seismic deformation: Insights from the carbonate-dominated fault rock suite of the Belluno Thrust, Italian Southern Alps, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-5851, https://doi.org/10.5194/egusphere-egu2020-5851, 2020.

Faults, joints and stylolites are ubiquitous features in fold-and-thrust belts, and have been used for decades to reconstruct the past fluid flow (or plumbing system) at the scale of folded reservoirs/basins. The textural and geochemical study of the minerals filling the fractures makes it possible to unravel the history of fluid flow in an orogen, when combined with a knowledge of the burial history and/or of the paleothermal gradient. In most cases, the latter derives from the former, itself often argued over, limiting the interpretations of past fluid temperatures. Yet, recent methodological developments applied to carbonates and calcite fillings provide new perspectives for a more accurate reconstruction of the temperature, pressure and timing of the fluids that were present in the strata at the time they deformed, at every stage of fold development. Indeed, the temperature at which fluids precipitated can be obtained by Δ⁴⁷CO2 clumped isotopes while the timing of calcite precipitation in veins and faults is given by U-Pb absolute dating. Also, the maximum burial depth of strata before contraction can be estimated using sedimentary stylolite paleopiezometry, hence in a way free of any consideration about the geothermal gradient.

These techniques were jointly applied at the scale of the Umbria-Marches arcuate belt (UMAR, Northen Apennines, Italy). Mesoscale faults and vein sets were measured and sampled in the Cretaceous-Eocene rocks. Focusing on those fractures that developed during Layer Parallel Shortening (LPS, i.e. oriented NE-SW to E-W) and during folding (i.e. oriented parallel to local fold axis), paleofluid sources, temperatures and timing were reconstructed using U-Pb absolute dating, Δ⁴⁷CO2 clumped isotopes as well as δ¹⁸O, δ¹³C, and ^87/86Sr signatures of calcite veins. Results show a regional divide in the fluid system, with most of the belt including the foreland recording a fluid system involving basinal brines resulting at various degree from fluid-rock interactions (FRI) between pristine marine fluids (δ¹⁸O_fluid= 0‰ SMOW) and surrounding limestones (δ¹⁸O_fluid= 10‰ SMOW). Precipitation temperatures (35°C to 75°C) appear consistent with the burial history unraveled by sedimentary stylolite roughness paleopiezometry (600 m to 1500m in the range) and estimated geothermal gradient (23°C/km, Caricchi et al., 2004). As the degree of FRI increases forelandward, we propose a lateral, strata-bound, squeegee-type migration of fluids during folding and thrusting. In the western hinterland however, the fluid system rather involves hydrothermal fluids with a higher degree of FRI, the corresponding precipitation temperatures (100°C to 130°C) of which are inconsistent with local maximum burial (1500m). As the Sr radiogenic signatures preclude any deep origin of the fluids, we propose that the fluid system prevailing in the hinterland during LPS reflects the eastward migration of formational fluids originating from the Tuscan basin, located west from the UMAR, where studied Cretaceous rocks were buried under more than 4 km of sediments during the Miocene.

Beyond being the first combination of paleofluid geochemistry and burial estimates through paleopiezometry, this fluid flow model illustrates how the large scale structures may control the fluid system at the scale of a mountain belt.

How to cite: Beaudoin, N., Labeur, A., Lacombe, O., Hoareau, G., Marchegiano, M., John, C., Koehn, D., Billi, A., Boyce, A., Pecheyran, C., and Callot, J.-P.: Long-term burial history and orogenic-scale fluid flow depicted from stable isotopes and stylolite paleopiezometry in the Umbria-Marches arcuate belt (Northern Apennines, Italy)., EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-19184, https://doi.org/10.5194/egusphere-egu2020-19184, 2020.

The Têt fault is a crustal scale major fault in the eastern Pyrenees that displays about 30 hot springs along its surface trace with temperatures between 29°C and 73°C. The regional process of fluid circulation at depth has previously been highlighted by thermal numerical modelling supported by hydrochemical analyses and tectonic study. Numerical modelling suggests the presence of a strong subsurface anomaly of temperature along-fault (locally > 90°C/km), governed by topography-driven meteoric fluid upflow through the fault damage zone (advection). On the basis of this modelling, we focused our thermochronological study on 30 samples collected close and between two hot spring clusters in both the hanging wall and the footwall of the Têt fault, where the most important thermal anomaly is recorded by models. We analysed apatite using (U-Th)/He (AHe) dating combined with REE analyses on the same dated grains.

Along the fault, AHe ages are in a range of 26 to 8 Ma in the footwall and 43 and 18 Ma in the hanging wall, and only few apatite grains have been impacted by hydrothermalism near the St-Thomas hot spring cluster. By contrast, particularly young AHe ages below 6 Ma, correlated to REE depletion, are found around the Thuès-les-bains hot spring cluster. These very young ages are therefore interpreted as thermal resetting due to an important hydrothermal activity. A thermal anomaly can be mapped and appears restricted to 1 km around this cluster of hot springs, i.e. more restricted than the size of the anomaly predicted by numerical models. These results reveal that AHe dating and REE analyses can be used to highlight neo- or paleo-hydrothermal anomaly recorded by rocks along faults.

This study brings new elements to discuss the onset of the hydrothermal circulations and consequences on AHe and REE mobilisation, and suggest a strong heterogeneity of the hydrothermal flow pattern into the fault damage zone. Moreover, this study suggests that crustal scale faults adjacent to reliefs can localise narrow high hydrothermal flow and important geothermal gradient.  Besides these results, this study provides new constraints for geothermal exploration around crustal faults, as well as a discussion on the use of thermochronometers into fault damage zones. 

How to cite: Milesi, G., Patrick, M., Münch, P., Soliva, R., Mayolle, S., Taillefer, A., Bruguier, O., Bellanger, M., Bonno, M., and Martin, C.: Thermochronology and REE analyses as new tools to track thermal anomaly and fluid flow along a crustal scale fault (Têt fault, French Pyrenees), EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-8850, https://doi.org/10.5194/egusphere-egu2020-8850, 2020.

Degassing of volatiles across the Earth crust towards the atmosphere is mainly controlled by long last diffusion. However, it can also be episodic. In fact, tectonic control of solid phase release of radiogenic helium (⁴He) due to, e.g. fracturing, may contribute to explain variance of the continental ⁴He degassing flux over multiple time and space scales. Rock rheology have a controlling influence on a wide range of crustal-scale processes including fluid flow, tectonic deformations and seismicity. Though faults comprise a small volume of the crust, they influence the mechanical and fluid flow properties of the crust, and are mechanisms for accommodating most of the elastic strain in the crust through a variety of slip-behaviours. Helium isotopes (³He,⁴He) are useful tracers for investigating many important geological processes because helium is a stable and conservative nuclide that does not take part in any chemical or biological process. Indeed, ⁴He released from rocks in the porefluid can be used to trace the deformation of rocks in a field of stress [Bauer 2017; Torgersen and O’Donnell 1991]. In fact, a volume of rock starts to be affected by micro-fractures from since  it is subjected to stress conditions exceeding about half its yield strength [Bauer, 2017]. Hence, the network of fractures evolves in a volume of rock progressively increase as a function of the evolution of deformation, improving the release of ⁴He that is trapped since its production. Consequently, ⁴He in natural fluids that outgas in a region of active tectonic can record the evolution of the field of stress and this volatile component could be used to trace changes in stress and deformation field. For the purpose of quantifying the amount of ⁴He present in the geological traps that feed the mud volcanoes of Regnano-Nirano mud volcanoes systems (Bonini et al., 2007), in the north Italy, in our study we have reconstructed the 3D geological model of the reservoirs, and proceeded to estimate the gas contained in them. Fluids emitted from these systems are thermogenic-CH₄ rich, which vertically migrates towards the surface. Helium is in traces and its isotopic signature (≈0.01-0.02Ra, Ra is the ³He/⁴He in air) shows that ⁴He is mainly produced in the crust by U-Th decay. We have found that the present ⁴He is greater than what should be available taking into account only the steady-state crustal production. Therefore, we compared the excess helium present in the reservoirs with the contribution coming from the seismic activity of the area, which is sufficient to explain this excess. Our study highlights that an intense fracturing of a volume of rock, due to the recent seismicity below the studied area, may explain the accumulation of helium in the reservoir higher than the steady-state condition. Therefore, the effective vertical rate of fluid transport in the Earth's continental crust can be characterized by episodic events controlled by fracturing.

Bonini (2007) - JGRes Solid Earth 

Torgersen & O’Donnell (1991) - GRL, vol.18

Bauer (2017) - SAND2017-9438

How to cite: Buttitta, D., Caracausi, A., Favara, R., Chiaraluce, L., Gasparo Morticelli, M., and Sulli, A.: Tectonic control on crustal degassing in continental region: the role of rock fracturation, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-15407, https://doi.org/10.5194/egusphere-egu2020-15407, 2020.

Karst geothermal systems fluid flow is dominated by structurally controlled porosity, which constrains the paths of aquifer recharge and the upwell of geothermal fluids. In fold-and-thrust belt settings associated with continental collision, geothermal fields occur within basins generally interested by low-enthalpy geothermal systems. Despite that, the deeper and warmer levels of multiply stratified aquifers within the detached sedimentary covers are vertically connected to shallower depths by high-angle faults, thus making of them interesting targets for exploration.

In the frame of the geothermal exploration steered by the Geneva Canton, this work aims at determining how fracture connectivity, orientation and permeability anisotropy has implications on fluid flow within high-angle faults. Recent software development (e.g., FracPaQ) allows to quantify such interconnection providing insights into spatial variation of multiscale fault-controlled porosity in order to have dynamic feedbacks between fluid flow, permeability rise/fall. We use the inner Jura fold-and-thrust belt and the other carbonate relieves surrounding Geneva as an outcrop analogue for the deeper carbonate reservoir, lying at depth beneath the siliciclastic Molasse deposits. Hereby, we present new structural and morphostructural lineament maps and scan box analyses from outcrops that provide a multiscale analysis on fracturing across the study area. The sampling sites are representative of fractured fold hinges constituted of Mesozoic carbonates crossed by high-angle faults.

The map analysis show that the late Oligocene-early Miocene growing carbonate anticlines are shaped by a series of fore- and back-thrusts resulting in salient-and-recess curvy thrusts accommodating different amount of shortening across high-angle tear-faults. With the support of high-resolution LIDAR images, we observe that at the large scale (e.g., five kilometers), as fault zone broadens across transfer zones, the background fracture network is more intense at the salient flanks. Major faults occur as segmented, thus not providing near-surface structure capable of giving any earthquake significantly larger than the already measured ones (e.g., M_L 5.3, Epagny earthquake 1996). Our preliminary results identify the W- and the NNW- striking systems strike-slip faults as the preferred patterns of fluid flow. Cross-cutting relationships vary with their position into the bended belt, thus making them suitable to be multiply reactivated during the Jura arc indentation. At the outcrop scale, the most mature fault zones associated with larger displacement are characterized by high fracture intensity and connectivity. Field evidences show that NNW- and W/NW- striking systems are vein-rich whereas N- and NE-striking systems are accompanied by open fracture sets although they may work with opposite fluid-flow vertical directivity. Mechanical and regional chronological development of the fracture network is also discussed as related to the regional fault evolution.

How to cite: Cardello, G. L. and Meyer, M.: Faults controlling geothermal fluid flow in a karst geothermal system (Western Alpine Molasse Basin, France and Switzerland), EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-15861, https://doi.org/10.5194/egusphere-egu2020-15861, 2020.

Coupled fluid characterization and absolute dating of fracture networks may provide insights into the understanding of critical stages of evolution of a growing orogen. As a result of the collision between the European and Apulian plates, the Alps have experienced several evolutionary stages comprising continental subduction, nappe stacking, thick- to thin-skin tectonics in relation with the frontal propagation of a fold and thrust belt, and extensional reactivation of the major Penninic Frontal Thrust (PFT). Current evolution of the orogen (Tricart et al., 2001, 2007 and Sue et al., 2007) shows an ongoing extensional seismic activity along PFT while borders of the orogenic system remain in compression. The transition from compression to extension along the PFT remains unconstrained.

This study aims to constrain the time of the PFT inversion and provide a characterization of the tectonic structures through time during the formation of upper Durance normal fault system. For this, we applied several novel dating techniques (in-situ U-Pb calcite and (U-Th)/He hematite dating techniques). In addition, we determined the geochemical signature of the fluids trapped (calcite crystallization) deformation by δ¹³C and δ¹⁸O stable isotope analysis of calcites to constrain the fluid reservoirs, and thus the size of the involved tectonic structures. Stable isotopes show that the fluids associated with the early extensive structures bear isotopic signatures close to those of their host rocks, indicating a fluid at equilibrium and thus a close system in agreement with the small (mm-cm) size of mostly ductile structures. In a second stage, connection of veins and fractures lead to major fault formation (metric to kilometric scale structures) show isotopic signatures in agreement with ascending metamorphic fluids, featuring an open system along the PFT.

U-Pb dating on calcite was successful on several samples despite high common lead concentrations. Two fault gouge samples associated with kilometric scale faults gave ages between 3.5 Ma and 2.5 Ma. These structures are a signature of the paleoseismic activity wich occured some 2.5-3.5 Ma ago when the wall domain of PFT was few km depth. Moreover, (U-Th)/He hematite dating was used on slickensides of the same fault system. Preliminary ages of 2.5, 1.5 and 15 Ma were obtained. The 15 Ma age is interpreted as a minimum age inversion of the PFT, while other ages overlap with the U-Pb calcite ages. This multidisciplinary inverstigation in the Western Alps helps to constrain the exhumation history of the paleo-seismogenic zone related to the inversion of the PFT.

How to cite: Bilau, A., Rolland, Y., Schwartz, S., Dumont, T., Brigaud, B., Gautheron, C., Pinna-Jamme, R., Deschamps, P., Godeau, N., Guihou, A., and Melleton, J.: U-Pb calcite dating  and isotopic fluid signatures of extensional fault gouges affecting the Penninic Frontal Thrust : implications for exhumation of the seismogenic zone., EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-4762, https://doi.org/10.5194/egusphere-egu2020-4762, 2020.

In the eastern Paris Basin, the Oxfordian (Upper Jurassic) and Bathonian to Bajocian (Middle Jurassic) carbonate platforms have been intensively cemented, despite rather low burial (< 1000 m). These limestone units are separated from each other by a 150 m thick succession of Callovian - Oxfordian clay-rich rocks. These claystones are currently under investigation by the French national radioactive waste management agency (Andra).

Most of the initial porosity in the Middle and Upper Jurassic limestones is now sealed by successive stages of calcite precipitation, which have been thoroughly characterized both petrographically and geochemically over the last fifteen years (Buschaert et al., 2004; Vincent et al., 2007; Brigaud et al., 2009; André et al., 2010; Carpentier et al., 2014). However, despite these research efforts, the timing and temperature of the fluids involved in the cementation of these carbonate rocks were still uncertain.    

Here, we present and discuss newly acquired ∆₄₇ temperatures and U-Pb ages of calcite cements filling the intergranular pore space, as well as vugs and microfractures.

The Middle Jurassic limestones were largely cemented during the Late Jurassic / Early Cretaceous period, as shown by our new LA-ICP-MS U-Pb ages that agree with the previous Isotope Dilution-TIMS U-Pb age of 147.8 ± 3.8 Ma from Pisapia et al. (2017). This event is believed to be associated to the Bay of Biscay rifting. Our data also reveal a second and more discrete crystallization event during the Late Eocene / Oligocene period, related to the European Cenozoic Rift System (ECRIS). In both cases, calcite was precipitated from fluids in thermal disequilibrium with the host rocks. 

By contrast, the Upper Jurassic limestones were largely affected by the successive deformation events that occurred during the Late Mesozoic / Cenozoic period. New LA-ICP-MS U-Pb ages acquired in ca. 200 µm-thick fractures reveal that calcite crystallized during three successive periods corresponding to the Pyrenean compression, the ECRIS extension and, finally, during the Alpine compression. These compression phases generated late stylolitization and subsequent dissolution/recrystallization in the Upper Jurassic limestones, while such tectonic features are rare in the Middle Jurassic.

Therefore, as opposed to the more conventional « burial-induced » model, our study highlights the role of stress propagation in the cementation of carbonate rocks hundreds of kilometers away from the rifting or collisional areas.

References:

Buschaert et al., 2004. Applied Geochemistry 19, 1201 – 1215. Vincent et al., 2007. Sedimentary Geology 197, 267 – 289. Brigaud et al., 2009. Sedimentary Geology 222, 161 – 180. André et al., 2010. Tectonophysics 490, 214 – 228. Carpentier et al., 2014. Marine and Petroleum Geology 53, 44 – 70. Pisapia et al., 2017. Journal of the Geological Society of London 175, 60 – 70.

How to cite: Blaise, T., Brigaud, B., and Carpentier, C.: Large-scale intraplate deformation caused the cementation of Jurassic carbonates in the eastern Paris Basin, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-7405, https://doi.org/10.5194/egusphere-egu2020-7405, 2020.

In order to elucidate the regional variation of stress field in the eastern part of Japan after the 2011 Tohoku earthquake of M=9.3, we tried to analyze focal mechanism data of earthquakes that occurred in 2011, presented by the Japan Meteorological Agency (JMA). Although earthquakes (aftershocks) occurred largely in the offshore area along the subduction zone of the Pacific plate under the North American and Eurasian plates, focal mechanism data presented by JMA are mainly those on land. For fault tectonic analysis, the suggested focal mechanism data are classified into appropriate populations on the basis of clusters and focal depths to reduce the bias and errors of stress tensors resulting from areal stress variation and varying vertical load. According to the results, the stress types of determined stress tensors consist of reverse, wrench and normal faulting ones. As for reverse faulting stresses in which the vertical load is the minimum principal stress axis, those of NW-SE compression prevail, which may be tightly related to northwestward movement of the Pacific plate. Those of E-W compression are determined in the continental crust deeper than about 9 km around Yamagata and in the lower part of subducting oceanic crust. In the Kanagawa and Chiba areas, determined stress tensors display NNW-SSE compression as well as NW-SE and E-W compressions. The NNW-SSE compression seems to be related to the movement of the Philippine Sea plate. Stress tensors of wrench faulting type are found in the continental crust far from the subduction zone of the Pacific plate, displaying NW-SE and E-W compressions in the shallower and deeper parts of crust, respectively. The E-W compression is presumably associated with the Himalayan tectonic domain. Determined stress tensors of normal faulting type show diverse extension directions: NW-SE extension in the coastal area, parallel to the Pacific compression, and E-W or NE-SW extension elsewhere. Especially, numerous focal mechanism data showing normal faulting stresses are present in the coastal area of Fukushima and Ibaraki, from which Poisson’s ratio of shallow crust was determined to be 0.25 to 0.27 using friction lines on Mohr’s circles and focal depths (or corresponding vertical loads). Additional horizontal stress related to the northwestward motion of the Pacific plate was estimated to be 46, 122 and 286 MPa in three groups of 0 to1.5, 1.5 to 4.5 and 3.5 to 11.5 kilometers in depth, respectively.

How to cite: Choi, P.: Fault tectonic analysis of aftershocks of the 2011 Tohoku, Japan, earthquake: interaction between three different tectonic domains and approximation of stress magnitude, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-4391, https://doi.org/10.5194/egusphere-egu2020-4391, 2020.

Basement terranes commonly contain complex fault networks developed during repeated episodes of brittle deformation. The Mid-Norwegian margin (from 62 to 63.8 °N) exposes a complexly fractured terrane formed mainly by Caledonian basement rocks. The margin recorded a prolonged brittle deformation history spanning the Devonian to Paleogene time interval. It is characterised by a pervasive NE-SW structural grain due to the ductile-brittle multiphase activity of the Møre-Trøndelag Fault Complex (MTFC).

In order to develop a time-constrained tectonic model of the area, we applied a multidisciplinary approach combining remote sensing, field work, paleostress inversion, microstructural analysis, mineralogical characterization, clumped isotope thermometry on carbonates and K-Ar dating of fault rocks from key representative faults. We present herein the preliminary structural-geochronological data of a still ongoing study of two regions along the Mid-Norwegian margin, the Hitra-Frøya and Kråkenes-Runde areas. These key areas represent the intersection regions between the Mid-Norwegian- and the other sectors of the margin.

The brittle structural record of the entire Mid-Norwegian margin was analysed by remote sensing of lineaments using high resolution LiDAR data followed by ground-truthing of the obtained results during field work. Three main sets of lineaments were identified: i) (E)NE-(W)SW-trending lineaments, parallel to the coastline and to the MTFC; ii) N(NW)-S(SE)-trending lineaments; iii) WNW-ESE-trending lineaments. The main sets of faults and fractures were further characterised by their fault rock association and coating. All generations of faults contain thin coatings of chlorite, variably thick epidote and quartz mineralisations and calcite veins and coatings, locally associated with acicular zeolite. Samples of calcite and related gouges were collected from different sets of faults. Carbonate clumped isotope thermometry constrains the range of temperature of calcite growth between 140 and 30 °C, indicating that calcite precipitated at different thermal conditions during a multiphase structural evolution. K-Ar data collected so far from synkinematic illite separated from fault gouges yield Jurassic-Paleogene ages.

The structural network of the margin is interpreted as reflecting a sequence of different deformation episodes. In order to resolve the orientation of the stress field for each recorded event, we applied paleostress inversion with the Win-Tensor software [1]. The preliminary results suggest that at least three tectonic stages affected the margin. A NE-SW strike-slip dominated transpression possibly reflects the late stages of the Caledonian orogenic cycle. A pure and oblique extensional (E)NE-(W)SW stage is associated with the Jurassic North Sea rifting, followed by a NW-SE Paleogene extensional reactivation observable throughout the margin.

To conclude, a new multidisciplinary database for the reconstruction of the brittle deformation history of the Mid-Norwegian margin is presented. The proposed approach aims to define the temporal and structural characterisation of each single tectonic episode. Such an approach is also pivotal toward the correlation with the deformation history of the corresponding offshore domains, as well as the comparison in time with other segments of the Norwegian margin.

[1] Delvaux, D. and Sperner, B. (2003). Stress tensor inversion from fault kinematic indicators and focal mechanism data: the TENSOR program. Geological Society, London, Special Publications, 212: 75-100

How to cite: Tartaglia, G., Viola, G., Ceccato, A., Bernasconi, S., van der Lelij, R., and Scheiber, T.: Multiphase brittle tectonic evolution of the Mid-Norwegian margin, central Norway, reconstructed by remote sensing, paleostress inversion and K-Ar fault rock dating, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-5438, https://doi.org/10.5194/egusphere-egu2020-5438, 2020.

Quartz veins are produced from the crystallization of the last silica enriched hydrothermal phase from granitic magma circulating along the pre-existing fracture of rock. In many instances, these hydrothermal fluid act as a carrier for the ore minerals. The intrusion of quartz veins along fractures depends upon the tectonic stress conditions in the area. Fluid pressure (P_f) of these ascending liquids should be higher than the normal compressive stress (σ_n) to dilate the fractures. We are studying the quartz vein intrusion in the Cu‒Pb‒Zn mineralization belt of Ambaji, South Delhi terrane, Aravalli- Delhi mobile belt, NW India. The host rocks include mica schist, amphibolite, calc schist, talc tremolite schist, and four phases of granite intrusion (G₀‒G₃). The age of G₀, G₁, G₂ and G₃ granite are 960, 860, 800, and 750 Ma respectively. The rocks underwent three phases of folding (F₁‒F₃) and show greenschist to amphibolite facies metamorphism. The quartz vein intrusion is related to syn to post F₃ folding and G₃ granite magmatism. This final phase hydrothermal fluid extremely altered host rock and formed biotite-tourmaline-quartz and tremolite-actinolite-talc-chlorite greisen along the contact. The greisen host chalcopyrite-pyrite-galena-sphalerite mineralization suggesting the ore minerals were transported by the quartz vein. Vein orientation, stress condition, fluid pressure fluctuation, and fluid temperature can decide the fracture dilation and mineralization processes. Therefore, this work concentrates on the geometrical distribution of the vein orientation data. From this we deduced (i) girdle distribution pattern of vein data  (ii) σ₁ = 120º/75º, σ₂ = 052º/07º, σ₃ = 323º/07º indicate maximum extension was NW-SE and σ₁σ₂ plane strikes was N52ºE, (iii) θ₂ =12º, θ₃ = 40º  and (iv) R'(driving pressure ratio) = 0.95, ϕ (tectonic stress ratio) = 0.90 indicates high value for R' leading to dilation of wide range of fractures. Further, the high ϕ value suggests uniaxial extension. Microscopic petrography of fluid inclusions shows three generations of inclusion like primary inclusion, secondary inclusion, and pseudosecondary inclusion. Most of the inclusion has aqueous and vapour phase and some inclusions show solid halite phase. We observed different types of trail bound of inclusion like intragranular inclusion, intergranular inclusion and transgranular inclusion, which suggest deformation and recrystallization in the rock. We are studying microthermometry analysis of fluid inclusion present in the quartz vein and trying to estimate the fluid pressure. With the help of fluid pressure, the 3D Mohr circle will be constructed and paleostress will be quantified. That will help in understanding the stress condition and mineralization in the rock.

Keywords: Veins, Fractures, Paleostress, 3D Mohr Circle, Mineralisation, Fluid Inclusion, Microthermometry

How to cite: Sharma, N. K. and Biswal, T. K.: Estimation of paleostress from pore fluid pressure of the quartz veins and its significance in the Cu-Pb-Zn mineralization (Ambaji, Aravalli-Delhi mobile belt, NW India) , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-7788, https://doi.org/10.5194/egusphere-egu2020-7788, 2020.

  Friction coefficients along faults control the brittle strength of the earth's upper crust, although it is difficult to estimate them especially of ancient geological faults. This study proposes to estimate the friction coefficient of faults with stress condition which activated them by the following procedure. Stress tensor inversion using fault-slip data can calculate principal stress axes and a stress ratio, which allows us to draw a normalized Mohr’s circle. Assuming that faulting occurs when the ratio of shear stress to normal stress on the fault (the slip tendency) exceeds the friction coefficient, a linear boundary of distribution of points corresponding to the observed population of faults should be found on the Mohr diagram. The slope of the boundary (friction envelope) provides the friction coefficient. Since this method has a difficulty in the graphical recognition of the linear boundary, this study automated it by considering the fluctuations of fluid pressure and differential stress. The fluctuations yield a density distribution of points representing faults on the Mohr diagram according to the friction coefficient. Then we can find the optimal value of friction coefficient so as to explain the density distribution.

  The method was applied to some examples of natural outcrop-scale faults. The first example is from the Pleistocene Kazusa Group, central Japan, which filled a forearc basin of the Sagami Trough. Stress inversion analysis showed WNW-ENE trending tensional stress with a low stress ratio. The friction coefficient was calculated to be about 0.7, which is typical value for sandstone.

  Another example is from an underplated tectonic mélange in the Cretaceous to Paleogene Shimanto accretionary complex in southwest Japan along the Nankai Trough. The stress condition was determined to be an axial compression perpendicular to the foliation of shale matrix. The friction coefficient ranges from 0.1 to 0.3, which is extremely low indicating a weak plate boundary under the accretionary wedge.

How to cite: Sato, K.: Determination of stress condition and friction coefficient from orientation distribution of outcrop-scale faults, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-13864, https://doi.org/10.5194/egusphere-egu2020-13864, 2020.

The Miocene plate configuration along southwest Japan arc has been controversial as follows. According to Hall (1996), the Pacific Plate subducted dextrally against the Eurasia Plate before 15 Ma, whereas Seno and Maruyama (1984) suggested the Philippines Sea Plate subducted sinistrally. The shear sense of oblique subduction is possibly recorded as the deformation of forearc basin fill. This study performed a series of paleostress analyses by using outcrop-scale structures in the Miocene Tanabe Group to examine whether strike-slip fault stress conditions were detected or not and which sense of shear is expected along trench-parallel faults.

The study area extends roughly 10 km along the coastal area of the Shirahama Formation, the upper part of the Tanabe Group, Kii Peninsula, southwest Japan. The measured structures consist of outcrop-scale faults and veins. Fault displacements range from about several mm to 1 m. The thicknesses of veins are about several mm. 
In total, 245 faults and 245 veins were observed. They were analyzed by the stress inversion methods (Sato, 2006; Yamaji and Sato, 2011), which can detect multiple stress conditions from a dataset.

As the result, normal fault stresses were dominant in the whole Tanabe Group. The horizontal extension directions was spatially variable. It trends roughly N-S in southern area and E-W in northern area. This spatial variation is consistent with the report from the present-day forearc basin offshore the Kii Peninsula (Lin et al., 2010). In several areas, strike-slip fault stresses were detected. In these area, some map-scale faults subparallel to the ENE-WSW trending present-day trench were reported (Tanabe Research Group, 1984). Detected strike-slip fault stresses can induce dextral shear deformations on these map-scale faults, which is consistent with the dextral oblique subduction model of the Pacific Plate.

Hall, R., 1996. Reconstructing cenozoic SE Asia. Geological Society of London Special Publication, 106, 153-184.
Lin, W., M. L. Doan, J. C. Moore, L. McNeill, T. B. Byrne, T. Ito, D. Saffer, M. Conin, M. Kinoshita, Y. Sanada and others, 2010. Present-day principal horizontal stress orientations in the Kumano forearc basin of the southwest Japan subduction zone determined from IODP NanTroSEIZE drilling Site C0009. Geophysical Research Letters, 37.
Sato, K., 2006. Incorporation of incomplete fault-slip data into stress tensor inversion. Tectonophysics, 421, 319-330.
Seno, T., S. Maruyama, 1984. Paleogeographic reconstruction and origin of the Philippine Sea. Tectonophysics, 102, 53-84.
Tanabe Research Group, 1984. Stratigraphy and geological structure of the Tanabe Group in the Kii Peninsula, Southwest Japan. Earth Science (Chikyu Kagaku), 38, 249-263.
Yamaji, A., Sato, K., 2011. Clustering of fracture orientations using a mixed Bingham distribution and its application to paleostress analysis from dike or vein orientations. Journal of Structural Geology, 33, 1148-1157.

How to cite: Abe, N. and Sato, K.: Strike-slip fault stress detected by paleostress analysis in paleoforearc basin: an example of the Miocene Tanabe Group, Southwest Japan, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-21186, https://doi.org/10.5194/egusphere-egu2020-21186, 2020.

The in-situ stress state in the upper crust is an important issue for diverse economic purposes and scientific questions as well. Several methods have been established in the last decades to estimate the present-day orientation of the maximum compressive horizontal stress (S_Hmax) in the crust. It has been assumed, that the S_Hmax orientation on a regional scale is governed by the same forces that drive plate motion too. The S_Hmax orientation data, compiled by the World Stress Map (WSM) project, confirmed that for many regions in the world. Due to the increasing amount of data, it is now possible to identify several areas in the world, where stress orientation deviates from the expected orientation due to plate boundary forces (first order stress sources), or the plate wide pattern. In some of this regions a gradual rotation of the S_Hmax orientation is observed.

Several second and third order stress sources have been identified which may explain stress rotation in the upper crust. For example, lateral heterogeneities in the crust, such as density, petrophysical or petrothermal properties and discontinuities, like faults are identified. Apparently, there are just a few studies, that deal with the potential extend of stress rotation as a function of second and third order stress sources. For that reason, generic geomechanical numerical models have been developed, consisting of up to five different units oriented at an angle of 60 degrees to the direction of contraction. These units have variable elastic material properties, such as Young’s modulus, Poisson ratio and density. In addition, an identical model geometry allows the units to be separated by contact surfaces that allow them so slide along the faults, depending on a selected coefficient of friction.

The model results indicate, that a density contrast or the variation of the Poisson’s ratio alone sparsely rotates the horizontal stress orientation. Conversely, a contrast of the Young’s modulus allows significant stress rotations. Not only areas in the vicinity of the material transition are affected by the stress rotation, but the entire blocks. Low friction discontinuities do not change the stress pattern when viewed over a wide area in homogeneous models. This also applies to models with alternating stiff and soft blocks - the stress orientation is determined solely by the boundary conditions, not the material transitions. This indicates that material contrasts are capable of producing significant stress rotation for larger areas in the crust. Active faults that separates such material contrasts have the opposite effect, they compensate for stress rotations.

How to cite: Reiter, K.: Impact of elastic material properties and discontinuities on the stress orientation, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-14540, https://doi.org/10.5194/egusphere-egu2020-14540, 2020.

The deformation history along the E-W trending Kachchh rift basin at the western continental margin of the Indian plate located in the state of Gujarat, India, has been controlled by activation of NW-SE, NE-SW and E-W trending, 0.25–50 km long oblique-slip and dip-slip faults.

The study is an attempt to establish the kinematic framework along sub-parallel, NW-SE striking group of intra-uplift, striated, high-angle reverse faults, consisting of, Vigodi Fault (VF) and its bifurcation – West Vigodi Fault (WVF), Gugriana Fault (GUF) and its bifurcation – Khirasra fault (KHIF) from the western part of the Kachchh basin in the northern part of Gujarat state in western India. They meet the E-W trending master faults – the Kachchh Mainland Fault (KMF) to the north and the Katrol Hill Fault (KHF) to the south at an acute angle.

Fault-slip data consisting of fault plane and slickenside attitudes along with other kinematic indicators were recorded along the faults at 69 structural stations. A total of 1258 fault-slip data were used to carry out paleostress analysis using Win-Tensor (v.5.8.8) and T-Tecto Studio X5 by executing the Right Dihedral Method.

The NW-SE trending fault system exposes highly porous and permeable deformed sandstones belonging to the Jhuran and Bhuj Formation. The pure compaction bands, cataclastic deformation band clusters, slipped deformation bands and deformation band faults are documented. These tabular structures are densely populated in the fault damage zones of VF, WVF, GUF and KHIF. The field observations related to fluid flow conduits are discussed. We also present the field characteristics and petrographic evidences of chemical bleaching caused by fluid-rock interaction found in the Bhuj and the Jhuran sandstones. The change in the coloration pattern of deformation bands in comparison with the host rock color, presence of iron concretions, iron rinds and liesegang rings are important records of the diagenetic control over the fluid flow. The study is an attempt to the link the tectonic activity and simultaneous chemical reactions that affect the fluid flow transport.

We attribute the deformation history in the western continental margin of the Indian plate has been dominantly controlled by intraplate compressional stresses induced by anticlockwise rotation and collision of the Indian plate with the Eurasian plate at ~55 Ma. This correlates well with the Kachchh basin where rifting aborted during the Late Cretaceous, accommodated syn-rifting extensional component in the intra-uplift VF, GUF and KHIF. It has then undergone inversion phase due to onset of compressive stresses during the Post-Deccan Trap time up to the present. The NW-SE trending intra-uplift faults reactivated multiple times and generated deformation bands having high porosity contrast with the host Bhuj sandstone.

How to cite: Shaikh, M., Maurya, D., Soumyajit, M., Vanik, N., Kumar, A., and Chamyal, L.: Tectonic evolution of the seismically active western continental margin of the Indian plate: Implications for kinematic history and fluid flow, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-18815, https://doi.org/10.5194/egusphere-egu2020-18815, 2020.

Carbonate rocks are among the most important targets for hydrocarbon exploration, and are considered of particular interest also for gas storage and carbon dioxide sequestration. The development of complex fracture networks in carbonates have a significant influence in fluid circulation, enhancing porosity and permeability and, therefore, modifying their storage capacity. The middle-Triassic Lastoni di Formin platform (Italian Dolomites) was studied by combining field measurements and photogrammetric techniques. The reconstruction of the Digital Model of the buildup allowed the analysis at the outcrop scale with a resolution of 5-10 cm, and gave the opportunity to focus on the behavior of sub-seismic (<10 m) structural elements. Even though their influence on the reservoir quality has been documented, heterogeneities of this order of dimensions are considered as part of the matrix properties in reservoir modeling: outcrop analogues represents a very good source of data that can help to fill this resolution gap. Many generations of fractures and faults can be distinguished at seismic and sub-seismic scale in the present-day fracture pattern of Lastoni di Formin, that is the result of different successive deformational events. In particular, the outcrop records the presence of two different tectonic phases: an E-W extension (Jurassic), that generate N-S trending joints and normal faults, and the Alpine compression (Neogene), that forms conjugate strike slip faults and flower structures. Moreover, an early fracturing gravitational event can be observed: is represented by opening-mode fractures and extensional faults sub-orthogonal to the direction of progradation of the buildup. The presence of platform-derived materials (oncoids) in the fracture fills allows to time-constrain the genesis of these fractures shortly after the deposition. Bed-perpendicular diffuse fractures, which are often strata-bound or terminate on bed-parallel stilolythes, were also detected. Both the margin-parallel early fractures and the Jurassic structures underwent strike-slip reactivations during the Alpine orogeny, which indicates a N-S to NNW-SSE shortening. Evidence of these movements can be inferred from riedel structures, en-chelon arrays, splays and fault jogs that can be observed at different scale. Reactivation of early structures can indicate that they influenced the distribution of subsequent faults and fractures affecting the platform.

How to cite: Inama, R., Menegoni, N., and Perotti, C.: Tectonic evolution of shallow water carbonates: the Lastoni di Formin platform (Dolomites - Italy), EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-19230, https://doi.org/10.5194/egusphere-egu2020-19230, 2020.

 Hazara Basin is a NE-SW trending fold and thrust belt, emerged as a consequence of ongoing collision between the Indian and Eurasian plates. Hazara Basin is bounded by Panjal Thrust (PT) in the North and Main Boundary Thrust (MBT) is located in the South. The present work deals with the paleostresses and outcrop fracture pattern (orientations, opening, fracture density) in different rock units exposed in Ghumanwan area located in the vicinity of Abbottabad, in Hazara Basin. PT and MBT juxtapose various lithological units along the Hazara Kashmir Syntaxes (HKS). The imbricate fault system between these two faults indicates a sinistral relative movement. We adopted circle inventory method in the field and collected data (fracture length, width, orientations and dip azimuth) from diverse rock units at 11 visited outcrop stations of the Ghumanwan Dome. These rock units include Upper Cretaceous (Kawagarh Formation) and Paleogene carbonates (Lockhart Formation and Margalla Hill Limestone). We observed highly dense, non-systematic fracture pattern in which mostly fractures are oriented in N-W direction normal to the bedding. Moreover, MOVE^TM 2018 (Midland Valley) Stress Analysis module (Stereonet Plot) was used for paleostresses analysis. The results show that the Slip Tendency (ratio of shear stress to normal stress) magnitude of σ2 lies closer to the σ3 (on Stereonet) and suggests compressional stresses in which NW-SE oriented fractures developed. The N-S compressive stresses which have mainly affected the concerned area are presumably linked to be late Eocene-Oligocene tectonic event.

How to cite: Ahsan, N., Armaghan Faisal Miraj, M., Tariq, H., and Qayyum, A.: Outcrop Fracture Pattern and Paleostress Analysis of the Ghumanwan Dome, Hazara Basin, NW Himalayas, Pakistan, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-19461, https://doi.org/10.5194/egusphere-egu2020-19461, 2020.

The southern Glade Fault Zone is a crustal-scale, subvertical dextral strike-slip fault zone on the eastern margin of Fiordland, New Zealand. For a distance of c. 40 km between Lake Te Anau and the Hollyford Valley, the fault cuts plutonic host rocks and has an estimated total dextral separation of c. 6-8 km. We report previously unidentified mylonites, cataclasites, pseudotachylites and fault gouge subparallel to pervasive sets of planar cooling joints in the Hut Creek-Mistake Creek area plutonic suites. The outcropping assemblage of joints and fault rocks record thermal, seismic and rheological conditions in the southern Glade Fault. Here we integrate methods to characterise the fault rocks and fracture damage zone of the southern Glade Fault from Glade Pass to Mt Aragorn. We use (i) EDS (Energy Dispersive x-ray Spectroscopy), XRD (X-Ray Diffraction) and EBSD (Electron Backscatter Diffraction) analysis to describe the mineralogy, kinematics and microstructures of fault rocks and, (ii) drone orthophotography and traditional structural measurements to detail geometrical relationships between structural features. Field mapping of glacially polished outcrops identifies the zone of brittle fault-related damage (i.e. damage zone + fault rock sequence) is up to one order of magnitude narrower than documented along other strike-slip faults with similar displacements, suggesting that the Glade Fault Zone represents an “end-member” of extreme localization of brittle deformation and fault displacement. This is interpreted to result from linkage of pre-existing cooling joints (and mylonitic shear zones), which allowed the younger brittle fault zone to establish its length and planarity relatively efficiently compared to the case of fault nucleation and growth in more isotropic host rocks.

How to cite: Ofman, M. and Smith, S.: On the straight and narrow: extreme localization of brittle fault damage and displacement along the Glade Fault Zone, Eastern Fiordland, New Zealand, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-4878, https://doi.org/10.5194/egusphere-egu2020-4878, 2020.

Microfaults formed in continental carbonates reveal poorly known mechanisms of shear localization induced by early diagenesis during compaction. These faults are characterized by sinuous shape, bed-controlled, pervasive distribution, no calcite precipitation, and mainly disaggregation processes. Two main sets were described: (1) The first set is composed by normal-sense, high-angle microfaults affecting the top of carbonate beds showing undulating pedogenic bed surface. They show porosity increase and are sometimes organized in polygonal patterns. Their occurrence seems related to overconsolidation of pedogenic surface and density inversion – phreatic loading – fluid expulsion processes in the surficial carbonate bed. (2) The second set is composed by low-angle compactive microfaults with large slickenlines and incipient shear-offset. Their organization within two conjugate systems (normal-sense set and strike-slip set) almost contemporaneous is consistent with a NS extension following the slope induced by the basin subsidence to the south. Their occurrence seems related to vertical loading below few meters depth and occurred by shear-enhanced compaction and incipient pressure-solution process. The presence of such structures gives news information concerning dilatant or compactive shear processes and rheological properties of micritic carbonates during early diagenesis.

How to cite: Ballas, G., Girard, F., Caniven, Y., Soliva, R., Celerier, B., Mayolle, S., Hemelsdael, R., Loggia, D., Gay, A., Lopez, M., and Seranne, M.: Microfaulting by early diagenesis of micritic continental carbonates - dilatant and compactive shear localization (Montpellier area, France), EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-8497, https://doi.org/10.5194/egusphere-egu2020-8497, 2020.

Most of the present-day extensional systems formed in areas that already experienced an older phase of tectonic activity. Therefore, understanding how a pre-existing structural setting may affect the development of an extensional basin is a crucial interplay to decipher. Depending on the kinematics of these phases, the resulting inherited faults can be extensional, contractional, or transcurrent. Consequently, a new extensional basin forms atop or across pre-existing faults that can dip at a low- (e.g., inherited thrust faults) or high-angle (e.g., inherited extensional faults). Furthermore, the inherited structures can have a non-optimal attitude with respect to the new extensional stress field, thereby determining different instances for reactivation. In this study, we analyzed the impact of dip and strike of inherited faults on the development of an extensional basin using wet clay (kaolin) analogue modeling. We reproduced sixteen different setups by varying the dip (30°, 45°, 60°) and the strike (15°, 30°, 45°, 60°, 75°) of the pre-existing faults that we introduced in the experiments before applying extension. The results show that the orientation of pre-existing faults has a direct effect onto the shape of the new extensional basins. When the pre-existing faults are reused to accommodate the new extensional phase, the formed basins are asymmetric and the rate of growth of the new faults is lower.

How to cite: Bonini, L., Basili, R., Bertone, N., Fracassi, U., Maesano, F. E., and Valensise, G.: The effect of pre-existing faults on the development of extensional basins: insights from wet clay experiments, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-8516, https://doi.org/10.5194/egusphere-egu2020-8516, 2020.

Fault damage zones (DZ) are fractured volumes of rock that surround the fault core(s), and their structure can have an important role on the control of fault mechanics and of the hydraulic properties of the fault zone, with impact on groundwater flow, ore-deposits, hydrocarbon reservoirs, nuclear waste disposal and contaminant transport in the subsurface. It is generally accepted that DZ width is controlled by fault displacement, and that it increases with increasing offset. However, published data on DZ width in faulted carbonates show a scattering over two orders of magnitude, suggesting that this parameter is controlled also by other factors. Here we present the results of a study performed on two units of the platform carbonates of the Malta and Gozo Islands. These two units, that are cross-cut by normal faults, are characterize by different petrographical, petrophysical and mechanical properties and have completely different Damage Zone width along faults characterized by the same tectonic history and with comparable displacement. More competent and rigid grain-dominated carbonates show DZ thickness of several hundreds of meters, while fracturing in the less competent and more elastic micrite-dominated rocks is developed only very close to the fault core, with a DZ width of few tens of meters. In order to explain this counterintuitive facies-controlled behavior, we performed petrophysical (porosity, density, permeability) and geo-mechanical (Uniaxial, Brazilian, Triaxial tests) analyses to characterize the mechanical stratigraphy and develop a numerical modelling study. Results highlight the heterogeneous stress distribution in a multilayer with variable elastic parameters subjected to horizontal extension. The more elastic unit can more easily expand laterally with respect to the less elastic one with the consequence that σ₃ decrease faster in the last one and this can yield before the more compliant one even if it is stronger. Also the width of the yielding zone is increased in the stiffer layers, leading to a wider DZ.

How to cite: Martinelli, M., Bistaccchi, A., and Castellanza, R.: Stratigraphic control on damage zone width in faulted platform carbonates: an example from the Gozo Island, Malta, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-9747, https://doi.org/10.5194/egusphere-egu2020-9747, 2020.

Fault damage zones present a renewal of interest to better understand stress perturbations around faults, earthquake’s ground motions and fluid flow in the upper crust. Although numerous studies provide significant amounts of data from a broad variety of rocks, the processes controlling fault damage development are not clearly understood and scaling properties in carbonate rocks remain poorly studied. D-T (displacement - DZ thickness) data compilations show strong scattering and are acquired using different methods and at different places along the faults (including tip, wall, link, or inner and outer damages), therefore rendering difficult a proper definition of the scaling relationship.

First, we analyse fault/fracture systems at the outcrop and map scale and define displacement - thickness (D-T) scaling of fault damage zones using scanlines, in carbonate rocks in France and Spain. We determine fault displacement and damage zone thickness perpendicular to fault planes and far from fault tips for 12 selected faults in four study sites. The data show a logarithmic decrease of fracture frequency from the fault cores. This decrease is characterized by local frequency peaks corresponding to variably-linked secondary fault segments and abandoned tips within the fault damage zone. D-T data comprised between 0 and 100 m of net fault displacement show a nearly linear scaling with very little scattering. Including two additional data for D > 100 m, the best fit corresponds better to a power law. The linear scaling is explained by well-known processes of fault growth such as stress perturbations around faults and fault segment linkage. The non-linear trend shown by the largest faults suggests that at this scale the faults become restricted at their lower tips by the base of the brittle crust.

Secondly, we analyse fault damage growth using analogue modelling of normal faults in a sand box. The model is composed by a 5 cm thick layer of dry sand deposited above a 2 cm thick ductile “kinetic sand” (sand and silicone) layer. The experiment is analysed in cross-section using image correlation allowing to calculate the velocity field and strain tensor over the fault zones including their damage pattern. Fault damage thickness obtained using the strain field appears to grow linearly with respect to shear displacement when the fault is contained into the dry sand layer. When the fault lower tip reaches the kinetic sand, fault damage begins to growth non-linearly with shear displacement, revealing that the brittle layer thickness is the main parameter governing the non-linear scaling.

How to cite: Mayolle, S., Soliva, R., Dominguez, S., Caniven, Y., Wibberley, C., and Ballas, G.: Fault Damage Scaling: Insights from field data in carbonate rocks and analogue modelling, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-8709, https://doi.org/10.5194/egusphere-egu2020-8709, 2020.

Polygonal faults are ubiquitous features, commonly observed in seismic images of fine-grained sedimentary successions along many passive margins. They are characterized by covering large parts of the basin with a typical polygonal pattern. In the last decade, different mechanical models for the generation of polygonal faults have been proposed; however, as they are commonly formed at depth and not directly observable at the surface, their formation remains a matter of debate. As part of the GEOTREF Program (ADEME – Investissement d’avenir) we found polygonal fault structures exposed close to the surface in marine soft sediments at 5 m water depth at the western coast of Guadeloupe. The structures are associated with fault-bound thermal springs and clearly visible at the sea bottom due to preferential precipitation of sulfur minerals and concentration of diatoms. In a multidisciplinary study involving a team of hydrogeologists, marine micro-biologists, and structural geologists, we study the genesis of polygonal faults in this setting. We analyzed the sediments in which the polygonal faults formed structurally and geochemically. First results suggest that SiO2 precipitated from hydrothermal fluids increases the cohesion of the most permeable soft sediments. Dewatering of the underlying layers causes the formation of polygonal faults at a depth of <1 m. These polygonal faults then act as channels for hot fluids, resulting in accumulation of sulfur favoring the establishment of diatoms at the surface. This study offers the unique opportunity to study the formation of polygonal faults in situ. We compare the observed geometries of polygonal faults with GEOTREF cruise 2-D seismic data offshore Guadeloupe, and 3-D seismic data of polygonal faults in New Zealand and Australia with the goal to understand the variability of polygonal fault geometries, as well as their comparability across different scales of observation. On a more local scale, this study provides insights how fracture dynamics guides fluid flow, which in turn interacts with the marine biosphere.

How to cite: von Hagke, C., Leu, K., Luijendijk, E., Philippon, M., Back, S., Lebrun, J.-F., Cordonnier, S., Gros, O., Gonzalez-Rizzo, S., and Gay, A.: Linking the mechanics of polygonal faults with hydrothermal activity and marine biosphere – the Guadeloupe geothermal system, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-14084, https://doi.org/10.5194/egusphere-egu2020-14084, 2020.

Faulting in seismically active regions commonly involves the deformation of unconsolidated to poorly lithified sediments. The seldom occurrence of seismic slip within these deposits appears to be counterintuitive if compared to classic crustal strength profiles that predict a velocity-strengthening behaviour for the first few km of depth. Therefore, the investigation of geological evidence for coseismic faulting within unconsolidated deposits is a key step towards a broader understanding of mechanisms of strain accommodation at shallow to near-surface depth.

Here we document the occurrence of minor faults within an unconsolidated colluvial fan at the hanging wall of the Vado di Corno Fault Zone (VCFZ) in the Central Apennines, Italy. The VCFZ is part of the active Campo Imperatore Fault System and accommodated 1-2 km of displacement since Early-Pleistocene. The deposits lie in direct contact with the master fault surface, are Late-Pleistocene to Holocene in age, and consist of angular carbonatic clasts, up to tens of centimetres in size, derived from the dismantling of the VCFZ footwall.

Studied faults are organised in two main sets: (i) subvertical, N-S trending dip-slip faults, parallel to the fan long axis, and (ii) WNW-ESE striking faults, synthetic and antithetic to the VCFZ master fault surface (N195/55°). Both fault sets are striated and commonly have positive relief with respect to the host deposits. Some of these faults show a fault core up to 5-6 cm thick, bounded by discrete and well-developed polished surfaces. Locally, particularly in fine-grained gravel levels, the occurrence of extreme strain localisation (i.e. millimetric ultracataclastic layers with truncated clasts) along mirror-like fault surfaces is observed. Grain size analysis of undeformed and faulted gravels shows an increase of the power-law exponent (fractal dimension) from values of D = 1.65-2.2 in the undeformed host rocks up to D = 2.9 in the cataclastic slip zones. Microstructural analysis suggests cataclasis is the main deformation mechanism leading to grain size reduction along faults, whereas intergranular pressure solution becomes widespread moving away from the slip zone where fluid circulation was present.

Collectively, our observations provide new insights into the mechanics of faulting and strain accommodation in the shallowest part of the crust (< 1 km) and new evidence to understand the propagation of seismic ruptures within shallow unconsolidated deposits.

How to cite: Demurtas, M., Balsamo, F., and Pizzati, M.: Signature of coseismic slip in unconsolidated Quaternary gravels, Campo Imperatore, Italy, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-7192, https://doi.org/10.5194/egusphere-egu2020-7192, 2020.

Hydrological anomalies induced by the earthquakes are valuable research data to understand the hydrogeology structure. At the same time, a complete hydrogeological data is the key to the study of earthquake hydrology. In this research, we collected the anomalous hydrological data after the M_w 6.4 2016 Meinong Earthquake in Taiwan. The main purpose is to know the mechanism of hydrological changes triggered by earthquake and understand the local hydrogeological characteristics in the southern Taiwan.

From the distribution of the groundwater level change in the same location but different depths of aquifer, as well as the location of the rupture and liquefaction, it could be found that the co-seismic groundwater level change is large in Chianan Plain in the northwest of the epicenter and accompanied with a lot of ruptures and liquefactions located along the Hsinhua Fault. However, the observations in several wells around the Hsinhua Fault show a different water level change pattern compared with the other wells in Chianan Plain. Actually, these wells show that the co-seismic groundwater level decreases in the deep aquifer and increase in the shallow aquifer. It is shown that the Meinong Earthquake may enhance the connectivity between different aquifers near the fault zone and produce an increased vertical pressure gradient. The anomalous hydrological phenomenon also reflected in the river flow. Based on the river flow data we collected from five stations in the Zengwun River watershed, the river flow at two stations in the upstream dose not change after earthquake. There is a little increase at the midstream station. However, a large river flow increase is observed at the downstream station. After excluding the influence of rainfall, we think that the large amount of anomalous flow is caused by the rise of the co-seismic groundwater level between the middle and downstream sections, and a large amount of liquefaction in this area can prove this hypothesis.

The hypothesis of connectivity changes between different aquifers can be verified by analyzing the tidal response of different aquifers. Many studies have used the tide analysis to obtain the aquifer permeability and compressibility, and compared the changes in the analysis results before and after the earthquake. We think that if different aquifers are vertically connected after earthquake, the tidal analysis results should show a consistent permeability. Tidal analysis is executing now and the results will be provided at conference.

How to cite: Lin, Y.-Y., Wang, S.-J., and Lai, W.-C.: The research of Meinong earthquake induced hydrological anomalies and increased vertical permeability in southwestern Taiwan, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-7993, https://doi.org/10.5194/egusphere-egu2020-7993, 2020.

The Esla Nappe is located in the external foreland and thrust belt of the Variscan Orogen in the NW Iberian Massif (Cantabrian Zone, NW Iberia). It is formed by a near-complete Palaeozoic sedimentary succession. With a displacement of around 19 km, the nappe was emplaced along a thin (<2–3 m) basal shear zone (ENBSZ) located at an estimated minimum depth of 4 km. Emplacement took place during the Moscovian (ca. 312 Ma). Fault-rock assemblages record a variety of alternating deformation mechanisms and processes, including cataclastic flow,  pressure solution and hydrofracturing and vein precipitation. All these processes are considered evidence of an aseismic stable behaviour of the ENBSZ, where deformation was influenced by secular variations in the fluid pore pressure.

Following emplacement, the ENBSZ was breached by clastic dykes and sills which were intruded following re-opened previous anisotropies, including bedding planes, thrust surfaces, joints and stylolites. Together, they constitute an interconnected network of quartz sand-rich lithosomes reaching structural heights occasionally exceeding 20 m above the ENBSZ. The orientation of the dykes suggests that the injection process took place under low differential stress conditions in the hangingwall, and near-lithostatic fluid pore overpressure conditions in the footwall. The injected slurry consisted of overpressured pore fluid, quartz-sand grains derived from the footwall and entrained host-derived fragments. Depending on fracture aperture and slurry composition, a variety of fluid velocities can be inferred in the order of 15–30 cm/s. Thin pure injections of quartz grains (ca. <1 cm) were characterised by a laminar flow (Re<2100), whereas the thickest quartz and host-derived mixed injections (~1 m) displayed a fully turbulent flow (Re~2 x 10⁴).

The causes for the fluids to reach near-lithostatic fluid overpressures within the uppermost footwall remain unknown. It is not possible to rule out a seismic trigger, but the absence of extreme shear localization structures typical of seismic slip suggests that the injection process was driven by fluid progressive accumulation, possibly related with clay dehydration reactions, tectonic loading, pore compaction or fluid migration from underlying formations. Actual breaching and injection may have been allowed by a decrease in bedding-parallel compressive stresses in the Esla Nappe associated with the subsequent evolution of the thrust-wedge.

How to cite: de Paz Álvarez, M. I., Llana Fúnez, S., and Alonso, J. L.: Post-emplacement fluid-driven intrusion fracturing and quartz-sand injections at the basal shear zone of the Esla Nappe (Cantabrian Zone, NW Iberia), EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-17943, https://doi.org/10.5194/egusphere-egu2020-17943, 2020.

Based on the analysis of hydrocarbon source, reservoir forming period, composition and classification of transportation system, and the reasons of failure well in Chexi Depression of Bohai Bay Basin，Two types of hydrocarbon accumulation models in gentle slope belt of Chexi area are established and the main controlling factors of hydrocarbon accumulation are defined. There are three sets of source rocks（Es1、middle and lover submember of Es3、Es4）in Chexi area, the different strata of source rocks have great differences in the Pr/Ph and the content of gammacerane. It has been found that the crude oil of Es3 has a good geochemical correspondence with the middle and lower of Es3 source rocks, and has the characteristics of near source accumulation. The hydrocarbon accumulation in the study area exists in the sedimentary period of the Dongying formation and the sedimentary period of the Guantao formation to the present two stages, which is dominated by late filling. There are two stages of oil and gas filling in the inner and middle belts, and only late stage hydrocarbon filling in the outer slope belt. The hydrocarbon transportation system is mainly composed of faults and sand bodies. The effective source rocks in the middle and lover submember of Es3 are connected with the upper reservoir of Es3 in a small area, which can be directly migrated to the upper sandstone reservoir of Es3 to form lithologic oil and gas reservoir. However, most of the oil and gas in the upper Es3 reservoir need to be vertically migrated by means of oil source fault, and then through the contact of sand bodies such as main channel and fan body, the main oil and gas reservoir will gradually move up with the distance from the source rock. The area with direct contact source reservoir configuration relationship is a "sand body lateral migration" reservoir formation mode, and the main controlling factors of reservoir formation are sand body connectivity and reservoir porosity and permeability. The source reservoir configuration area with fault connection type is a "fault sand combination T-type migration" reservoir forming mode, and the main controlling factors of reservoir forming are migration convergence facies (structural ridge and cross-section ridge).The area of passive reservoir contact is "fault sand combination step migration" reservoir forming mode, and the main controlling factors of reservoir forming are migration convergence facies (structural ridge) and lateral sealing of faults in preservation conditions.

Key words: Chexi Depression; Source of hydrocarbon; Accumulation period; Fault sand transport combination; Reservoir forming mode

How to cite: Zhao, S. and Liu, H.: Study on the hydrocarbon accumulation models of the upper Es3 in the gentle slope belt of Chexi Depression, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-1784, https://doi.org/10.5194/egusphere-egu2020-1784, 2020.

Bleached rocks are commonly formed from CO₂-saturated water leakage to the surface. It could provide opportunities to understand the mechanisms and controlling factors associated with injected and sequestrated CO₂ leakages as well as fluid flow in the subsurface. In this study, we investigated the various bleaching patterns and their related structures in order to understand the characteristics of fluid-flow migration around the major structures. Also, we examined the effects of lithology and structural characteristics on fluid flow in detail along and around the major structures. For this purpose, we analyzed bleaching characteristics of multiple layered sedimentary rocks around two major faults (Moab and Salt Wash Faults) and fold axes (Green river anticline system) based on field observations and quantitative measurements (scanline survey and permeability) in the exhumed reservoir-cap rock systems in SE Utah, USA. The results showed that strongly bleached layers of sedimentary rock have a higher density of deformation bands compared to unbleached layers. This is consistent with the general property of deformation bands that frequently develop in layers with higher porosity and permeability. Although almost all fault zones act as conduits for fluid flow, some fault zones filled with clay-rich gouges could impede fluid flow. In addition, the internal sealing characteristics of the layer boundaries such as bedding planes could be important factors as they can act either as a pathway or a barrier for lateral fluid flow depending on the existence of filling materials such as calcite or kerogen. Our research may be useful for assessing CO₂ leakage in oil reservoirs or CO₂ sequestration sites located in a reservoir-cap rock system of sedimentary basins.

How to cite: Ko, K.: Characteristics of fluid-flow migration(bleached rock) around major structures in a reservoir-cap rock system, SE Utah, USA, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-6519, https://doi.org/10.5194/egusphere-egu2020-6519, 2020.