Faults and fractures in rocks : mechanics, occurrence, dating, stress history and fluid flow

Fractures and faults are common tectonic features within shallowly deformed rocks. Fracture networks play a fundamental role in fluid migration. Understanding the mechanical and chronological development of fracture networks is therefore key for tectonic studies as well as for resources exploration and waste repositories studies.

This session aims at bringing together scientists working in the field, in the lab, and on simulations to foster discussion towards improving our understanding of (1) the mechanics, occurrence, timing and stress history of fractures in upper crustal rocks, and (2) the role fracture networks play on subsurface fluid flow. We welcome contributions from all fields, including structural geology, mechanics, isotope geochemistry, and hydrogeology that aim at comprehending the development of fracture systems in time and space and their co-evolution with fluid flow in a variety of geological settings.

Co-organized by GMPV6
Convener: Olivier Lacombe | Co-conveners: Stefano Tavani, Juliette Lamarche, Fabrizio Agosta, Fabrizio Balsamo
vPICO presentations
| Mon, 26 Apr, 11:00–12:30 (CEST), 13:30–15:00 (CEST)

vPICO presentations: Mon, 26 Apr

Chairpersons: Juliette Lamarche, Fabrizio Agosta, Olivier Lacombe
Faults and fractures : mechanics, occurrence, modelling
Pascal Richard and Loic Bazalgette

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 2021, online, 19–30 Apr 2021, EGU21-14729, https://doi.org/10.5194/egusphere-egu21-14729, 2021.

Erik Toller and Otto Strack

Understanding and modelling hydraulic fractures and fracture networks have a fundamental role in mapping the mechanical behaviour of rocks. A problem arises in the discontinuous behaviour of the fractures and how to accurately and efficiently model this. We present a novel approach for modelling many cracks randomly using analytic elements placed under plane strain conditions in an elastic medium. The analytic elements allow us to model the assembly computationally efficiently and up to machine precision. The crack element is the first step in the development of a model suitable for investigating the effect of fissures on tunnels in rock. The model can be used to validate numerical models and more.The solution for a single hydraulic pressurized crack in an infinite domain in plane strain was initially developed by Griffith (1921). We demonstrate that it is possible, by using series expansions in terms of complex variables, based on the Muskhelisvili-Kolosov functions, to generalize this solution to the case of an assembly of non-intersecting pressurized cracks. The solution consists of infinite series for each element Strack & Toller (2020). The expressions for the displacements and stress tensor components approach the exact solution, as the number of terms in the series approaches infinity.We present the case where two cracks approach each other orthogonally to less than 1/2000th of the cracks length. We show the effect of increasing the number of terms in the expansion and how this influences the precision, demonstrating that the result approaches the exact solution. We also present a case with 10,000 cracks; the coefficients are determined using an iterative solver. By using analytic elements, we can both present the corresponding stress and deformations field for the global scale and for small scales in the close proximity of individual cracks.ReferencesGriffith, A. A. (1921). The phenomena of rupture and flow in solids. Philosophical Transactions of the Royal Society of London. Series A, Containing Papers of a Mathematical or Physical Character, 221(582-593):163–198.Strack, O. D. L. and Toller, E. A. L. (2020). An analytic element model for highly fractured elastic media, manuscript submitted for publication in International Journal for Numerical and Analytical Methods in Geomechanics.

How to cite: Toller, E. and Strack, O.: Modelling Thousands of Fractures Using Analytic Elements, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-235, https://doi.org/10.5194/egusphere-egu21-235, 2021.

Jessica McBeck, John Mark Aiken, Ben Cordonnier, Yehuda Ben-Zion, and Francois Renard

The geometric properties of fractures influence whether they propagate, arrest and coalesce with other fractures. Thus, quantifying the relationship between fracture network characteristics may help predict fracture network development, and hence precursors to catastrophic failure. To constrain the relationship and predictability of fracture characteristics, we deform eight rock cores under triaxial compression while acquiring in situ X-ray tomograms. The tomograms reveal precise measurements of the fracture network characteristics above 6.5 microns. We develop machine learning models to predict the value of each characteristic using the other characteristics, and excluding the macroscopic stress or strain imposed on the rock. The models predict fracture development more accurately in the experiments performed on granite and monzonite, than the experiments on marble. Fracture network development may be more predictable in these igneous rocks because their microstructure is more mechanically homogeneous than the marble, producing more systematic fracture development that is not strongly impeded by grain contacts and cleavage planes. The varying performance of the models suggest that fracture volume, length, and aperture are the most predictable of the characteristics, while fracture orientation is the least predictable. Orientation does not correlate with length, as suggested by the idea that the orientation evolves with increasing differential stress and thus fracture length. This difference between the observed and expected predictability of orientation highlights the significant influence of local stress perturbations on fracture growth within brittle material in laboratory-scale systems with many propagating and interacting fractures.

How to cite: McBeck, J., Aiken, J. M., Cordonnier, B., Ben-Zion, Y., and Renard, F.: Predicting fracture network development in crystalline rocks, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-2965, https://doi.org/10.5194/egusphere-egu21-2965, 2021.

Riccardo Inama, Yuri Panara, Niccolò Menegoni, Filippo Carboni, Giovanni Toscani, Carlo Alberto Brunori, and Cesare Perotti

In the last years, several studies investigated the Mt. Vettore Fault Zone (MVFZ), activated during the 2016 Central Italy seismic sequence. Research has focused mainly on aftershocks and mainshock locations, coseismic slip and surface cracks, while an accurate study of the fracture network in the MVFZ was never conducted.

In this study we present a fracture analysis performed using very high resolution (0.5 – 5 cm) Digital Outcrop Models (DOMs) that developed by Unmanned Aerial Vehicle (UAV)-based digital photogrammetry. The UAV gave the possibility to investigate outcrops with dimensions up to hundreds of metres high and wide, and acquire big and precise fracture data using 3D digital automatic and manual mapping techniques. To investigate the structural variability of the MVFZ fracture network, we realized several DOMs located in different positions, along and around the major fault. All the selected outcrops are formed by Calcare Massiccio Fm., which better records brittle deformation in the study area.

This analysis aimed to better understand the MVFZ fracture network, including mechanics, kinematics and local structural evolution. In particular, it allowed to determine: 1) the main sets of fractures; 2) the geometrical parameters of the fracture network (e.g. fracture density, persistence, roughness and aperture); 3) the relative timing of the main tectonic brittle events. The preliminary analysis of the DOMs suggests a variability of the fracture network parameters over the MVFZ, especially for what concerned fracture set orientation and density. 



How to cite: Inama, R., Panara, Y., Menegoni, N., Carboni, F., Toscani, G., Brunori, C. A., and Perotti, C.: 3D Digital Outcrop Model analysis of fracture networks: insights from the Mt. Vettore Fault Zone., EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-14876, https://doi.org/10.5194/egusphere-egu21-14876, 2021.

Attoumane Abi, Julien Walter, Ali Saeidi, and Romain Chesnaux

It is well known that fracture networks play an important role in fluid circulation in crystalline rock mass. Given that crystalline basements have a negligible primary porosity (porosity of the rock matrix) in comparison to their secondary porosity (porosity due to fractures), fracture characterization generally constitute the most important parameter for the determination of the hydraulic characteristics of the rock mass. Fracture characterization may involve fracture samples from different surveying sources such as outcrops, tunnels and boreholes. For a matter of building a conceptual model, for a study area, the geologist compartmentalizes the study area into several structural homogeneous sub-areas. Those homogeneous sub-areas are called structural domains and how fracture samples are grouped in the same structural domain is the question treated in this presentation.

From field investigations to grouping fracture samples into structural domains, geologists have used methods that are mainly based on the geologist experience and use major structural elements such as faults as domain boundaries. In the case of total absence or limited presence of major structural elements, grouping fracture samples into structural domains becomes complicated. Therefore, several statistical methods which use fracture characteristics have been developed to assist the geologist for that matter. Those methods can be classified into two approaches, which have been introduced by Miller (1983) and Mahtab and Yegulalp (1984). Miller’s approach consists of grouping fracture samples which are totally homogeneous with regard to the fracture characteristic(s) of interest, especially fracture orientation. On the other hand, Mahtab and Yegulalp’s approach consists of grouping fracture samples which share a similar fracture set. While, Miller’s approach got a lot attention, especially in the engineering fields, Mahtab and Yegulalp’s method has the advantage of allowing taking into consideration the blind zones of fracture samples as in practice a fracture sample can hardly be constituted by all the fracture sets of its belonging structural domain. However, Mahtab and Yegulalps’s method ignore fracture characteristics such as fracture spacing, aperture and persistence which are important for fluid circulation in the rock mass.

This presentation proposes a new method that improves Mahtab and Yegulalp’s method by including fracture characteristics such as aperture, persistence and fracture spacing in addition to the fracture orientation considered in the original method. The field investigations took place in the Greenville geological province of the Canadian shield, in Lanaudière region, in Quebec; where fractures were sampled from 30 outcrops and four boreholes. The new method adds a higher level of confidence with regard to the similarity of samples within a structural domain. As a result of the new method, each structural domain has a unique combination of fracture set(s) characteristics which characterize its fracture network. The structural domain compartmentalization impact on the hydrogeological behavior of water flow within the rock mass constitutes the topic of an ongoing research project.

How to cite: Abi, A., Walter, J., Saeidi, A., and Chesnaux, R.: A similarity test for the compartementalization of crystalline rocks into structural domains, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-9979, https://doi.org/10.5194/egusphere-egu21-9979, 2021.

Fatemeh Nazari Vanani and Oscar Fernandez

Adequately characterizing the properties of a fracture network is the first step in accurately modelling its behavior, be it mechanically or hydraulically. Characterizing fracture networks requires determining fracture frequency, orientation, connectivity, and fracture properties. This becomes particularly challenging in subsurface systems, where hard data on fracture networks comes mainly from boreholes, that are samples of very limited volume in relation to the fracture network. Because of this scale relationship between sample dimension and the dimension of natural fracture networks, boreholes capture a very partial picture of the fracture network. This is particularly relevant when attempting to estimate fracture frequency and network connectivity from borehole data. Corrections are normally used to account for sampling bias related to fracture size and orientation. Whereas these corrections are valid for the sample itself, the topology and heterogeneity of fracture networks means that measurements obtained in any given borehole are not necessarily representative of the broader fracture network.

To determine how “wrong” single-borehole analyses can be, we have conducted experiments on synthetic datasets to quantify how representative borehole samples are of entire fracture networks. Results show that properties that have an impact on the anisotropy of the fracture network (orientation, number of fracture sets) can be accurately resolved even in low data-density scenarios. On the contrary, accurately determining fracture frequency (which also impacts connectivity) for the entire fracture network is strongly dependent on the ratio between fracture frequency and the sampled volume. Measurements of fracture frequency in individual boreholes indicate that it frequency easily be overestimated or underestimated by a factor of 2 relative to the real network’s fracture frequency. The application of sampling bias corrections has a limited impact on reducing this error.

Based on the results from our experiments, we present methods to assess how representative of a fracture network a single borehole is. Representativity can be translated into uncertainty in fracture frequency, a metric that can be used in fracture modelling.

How to cite: Nazari Vanani, F. and Fernandez, O.: Now you see me... : Impact of sample representativity in fracture network characterization., EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-16341, https://doi.org/10.5194/egusphere-egu21-16341, 2021.

Kyriaki Drymoni, John Browning, and Agust Gudmundsson

Dykes and inclined sheets are known occasionally to exploit faults as parts of their paths, but the conditions that allow this to happen are still not fully understood. Here we report field observations from a well-exposed dyke swarm of the Santorini volcano, Greece, that show dykes and inclined sheets deflected into faults and the results of analytical and numerical models to explain the conditions for deflection. The deflected dykes and sheets belong to a local swarm of 91 dyke/sheet segments that was emplaced in a highly heterogeneous and anisotropic host rock and partially cut by some regional faults and a series of historic caldera collapses, the caldera walls providing, excellent exposures of the structures. The numerical models focus on a normal-fault dipping 65° with a damage zone composed of parallel layers or zones of progressively more compliant rocks with increasing distance from the fault rupture plane. We model sheet-intrusions dipping from 0˚ to 90˚ and with overpressures of alternatively 1 MPa and 5 MPa, approaching the fault. We further tested the effects of changing (1) the sheet thickness, (2) the fault-zone thickness, (3) the fault-zone dip-dimension (height), and (4) the loading by, alternatively, regional extension and compression. We find that the stiffness of the fault core, where a compliant core characterises recently active fault zones, has pronounced effects on the orientation and magnitudes of the local stresses and, thereby, on the likelihood of dyke/sheet deflection into the fault zone. Similarly, the analytical models, focusing on the fault-zone tensile strength and energy conditions for dyke/sheet deflection, indicate that dykes/sheets are most likely to be deflected into and use steeply dipping recently active (zero tensile-strength) normal faults as parts of their paths.

How to cite: Drymoni, K., Browning, J., and Gudmundsson, A.: Modelling of interactions between dykes, inclined sheets and faults at Santorini volcano, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-6309, https://doi.org/10.5194/egusphere-egu21-6309, 2021.

Henry Gage, Carolyn Eyles, and Rebecca Lee

Little research has been conducted to clarify the mechanism, extent, or factors involved in the fracturing of rocks exposed along the Niagara Escarpment in Ontario. Of particular interest are the effects of fluctuating temperatures during region’s cold winters which may be a contributor to the formation and expansion of fractures within these rocks. The results of a preliminary field-based study of temperature changes in fractured sedimentary rocks exposed at several sites along the Escarpment in Hamilton, Ontario are reported here. The objectives of the study were to examine the characteristics of operant thermal processes and to determine the effectiveness of mechanisms such as freeze-thaw and thermal stress in contributing to fracture formation and development. Fractured dolostone units were identified at three field sites along the escarpment that varied in their aspect, vegetation, and proximity to water. At each site, temperature probes were affixed to the exposed rock surface and inserted into a nearby fracture. Temperature measurements were taken at one-minute intervals throughout the winter of 2020-21.  In-situ field measurements of thermal changes within fractured dolostones on the escarpment were supplemented with recordings of rock surface and interior temperatures taken from unfractured dolostone blocks placed in a ‘controlled’ outdoor environment throughout the winter.  Initial results from the escarpment probes in the early winter show frequent, rapid shifts from warm to sub-zero temperatures and indicate that changes in temperature recorded at the rock surface closely follow diurnal atmospheric oscillations in both magnitude and timing.  However, temperature changes recorded within fractures are less extreme and show a temporal lag. Temperature fluctuations recorded at the field site with the highest degree of exposure, a southeasterly aspect, and little vegetation cover, are significantly higher and show larger thermal responses within fractures. Temperature fluctuations recorded from unfractured blocks in the ‘controlled’ outdoor environment show similar diurnal trends to those recorded on the escarpment but with reduced differential between temperatures at the block surface and interior. Together, these data indicate that temperature fluctuations sufficient to generate freeze-thaw cycles are abundant during the early winter months, temperature variability within fractures does occur, and slope aspect and exposure plays an important role in the determining the magnitude of diurnal temperature fluctuations experienced by surface rocks on the escarpment. The role of thermal stress in fracture development, created by rapid and substantial thermal changes, has yet to be determined.  

How to cite: Gage, H., Eyles, C., and Lee, R.: Thermal controls on the development of fractures in dolostones of the Niagara Escarpment, Hamilton, Ontario, Canada, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-9140, https://doi.org/10.5194/egusphere-egu21-9140, 2021.

Serena Formenti, Alexander Peace, John Waldron, Carolyn Eyles, and Rebecca Lee

The Niagara Escarpment is a geological feature comprised of highly fractured Ordovician and Silurian shales and carbonates stretching through southern Ontario and parts of the north-eastern United States. Differential erosion of the shale and carbonate strata has generated a steep cliff face bisecting the city of Hamilton, Ontario. Fractures occur throughout the cliff face and result in the formation of loose blocks of rock that are subject to erosion through rockfalls. This presents structural stability issues and an associated geohazard, which is of particular concern due to the proximity of the escarpment to city infrastructure. Previous work has alluded towards the role of geologic fractures in controlling erosion and stability of the Niagara Escarpment, but the causal mechanisms and extent to which these processes operate remains unclear. As such, the aim of this study is to quantify and analyse fracture networks using a combined field and numerical modelling-based approach to understand the distribution and nature of fractures throughout the escarpment, their connectivity, fluid flow properties, and relationship to structural stability. The location, orientation, and aperture of fractures were systematically quantified and documented around Hamilton. Data were plotted and analysed using the software Orient to identify clusters representative of fracture sets and to calculate average fracture set orientations and the respective confidence intervals. Three primary sets of geological fractures were identified including: 1) a near-vertical bedding confined set oriented north-south, 2) a near-vertical bedding confined set oriented east-west and 3) sedimentary bedding planes which have facilitated fracture migration and controlled resultant fracture geometry. Discrete fracture network modelling of these fracture sets in MOVE highlights their high degree of connectivity and indicates that the distribution and nature of these discontinuities are predominant controls on the locations and sizes of rock fragments generated on the cliff face resulting in rockfalls. Moreover, fracture-controlled porosity is a significant contributor to fluid flow throughout the escarpment. We conclude that geologic fractures present a first-order control on the stability of the Niagara Escarpment near Hamilton.

How to cite: Formenti, S., Peace, A., Waldron, J., Eyles, C., and Lee, R.: The influence of fracture networks on stability and geohazards of the Niagara Escarpment in southern Ontario, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-9094, https://doi.org/10.5194/egusphere-egu21-9094, 2021.

Åse Hestnes, Deta Gasser, Thomas Scheiber, Joachim Jacobs, Anna K. Ksienzyk, Roelant van der Lelij, and Jasmin Schönenberger

Brittle fracture and fault networks control the location of topographic features such as valleys and ridges and active faulting can lead to topographic rejuvenation. In Western Norway, however, it is debated how much faulting has contributed to rejuvenating of the topography during the late Mesozoic and Cenozoic. Geometric and temporal constraints on the brittle evolution are therefore important to obtain a comprehensive picture of the post-Caledonian topographic evolution of this region. In this study, we combine remote sensing, structural field measurements, paleo-stress analysis and isotopic dating to study the brittle evolution of a larger region of Western Norway. The region spans from the Sognefjord in the south to the Møre margin in the north. Lineament studies reveal important lineament sets trending N-S, NE-SW, E-W and NW-SE. Field observations show that these lineament sets correspond to both dip-slip and strike-slip faults, some of them parallel to ductile precursor structures and some cutting the ductile fabric. Epidote, chlorite, quartz and zeolite are the dominant mineralizations on fracture and fault surfaces. There is no clear correlation between the type of mineralization and fracture orientation in the region. Paleostress analysis on fault-slip data (n = 173), including faults reactivating older structures, show a good fit with a general E-W extensional regime. However, a considerable amount of faults (n = 115) formed under a strike-slip regime, which has so far not been documented in the region. We combine these findings with K-Ar fault gouge dating from six faults where five fractions (6-10 µm, 2-6 µm, 0.4-2 µm, 0.1-0.4 µm, <0.1µm) from each sample were analysed. These faults represent two of the four fracture sets observed, trending N-S and NE-SW, respectively, and show either strike-slip or dip-slip kinematics. XRD-data from these gouges show that K-feldspar and smectite are the main sources of potassium. The ages show a spread from the Triassic to the Cretaceous, where older ages can be affected by K-feldspar inherited from the host rock. Our results point to an important phase of Mesozoic strike-slip faulting in the region, with steep faults controlling the location of several major valleys. Extensional dip-slip faults might have contributed to the rejuvenation of the footwall topography.

How to cite: Hestnes, Å., Gasser, D., Scheiber, T., Jacobs, J., Ksienzyk, A. K., van der Lelij, R., and Schönenberger, J.: Structural framework and the timing of landscape-forming faults - a study from Western Norway, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-10972, https://doi.org/10.5194/egusphere-egu21-10972, 2021.

Séko Sanogo, Cyril Durand, Michel Dubois, and Ousmane Wane

The Bougouni region is located in the southern Mali, 170 km south-east of Bamako. It is part of the northern edge of the Leo-Man [UdMO1] shield (southern part of the West-African Craton). It is known for its swarm of pegmatites, most investigated as a lithium resource (spodumene pegmatites) and its gold potential. It is made up of metavolcanosedimentary rocks and plutonic complexes of the Birimian (Paleoproterozoic).

The objectives of this study are: 1) to make a review of the main regional structural features, 2) to define the structural control of the pegmatite emplacement. To reach these aims a structural analysis was carried out using the information collected during our field campaigns (2018 and 2019) and data from previous works.

Metavolcanosedimentary rocks had undergone a low grade metamorphism degree. Schistosities in these rocks (n=156 measurements) are distributed along 2 main directions which indicate two deformation phases. The first one, D1, oriented NNW-SSE to N-S. It is ductile, responsible for low grade regional metamorphism (Baratoux et al., 2011) and crustal thickening of the volcano-sedimentary rocks (Wane et al., 2018). It is linked to a compression event oriented E-W to ESE-WNW (Baratoux et al., 2011; Wane et al., 2018). The second one, D2, oriented NNE-SSW to SE-NW, is a ductile-brittle transpressive deformation. It affects metavolcano-sedimentary rocks. It would be contemporaneous with the location of most birimian granitoids (Baratoux et al., 2011; Wane et al., 2018) and it would be responsible for the gold mineralization (McFarlane et al., 2011). The third one, oriented E-W is marked by fracture cleavage and extensional cracks sometimes sigmoidal and generally filled by quartz. It has a brittle-ductile character. The faults have been observed in the field but weren’t measured. Feybesse et al. (2006) showed that they intersect all the lithologies without having any direct link with the different deformation phases.


Pegmatites are hosted in both metavolcanosedimentary and granitoids. Lithium bearing pegmatites are characterized by an assemblage containing spodumene (15-65%; main lithium-host mineral), albite (10-55%), microcline (1-10%), quartz (25-50%) and muscovite (2-10%). The accessory minerals are: apatite; garnet; columbo-tantalite, tourmaline, beryl and rutile. Lithium pegmatites are distributed in three directions (n=209): NNE-SSW (minor, ≃10 %?), ENE-WSW to E-W and ESE-WNW to SE-NW. Most of them are characterized by a steep dip (≃90°). The dyke-host unit contacts are generally sharp and brittle. These results suggest that the emplacement of the lithium bearing pegmatites of Bougouni even of all the Birimian pegmatites (Example of Issia pegmatite, côte d'ivoire Allou, 2005) could be related to the brittle deformation phase D3. This phase is also thought to be linked with the gold mineralization (McFarlane et al., 2011).


Ref: Allou, 2005, thèse à l’université de Chicoutimi ; Baratoux et al., 2011, Precamb. Res. 191, 18–45 ; Feybesse et al., 2006, Carte et Notice explicative de la Carte du Birimien du Mali ; McFarlane et al., 2011, Econ. Geol. 106, 727-750; Wane et al., 2018, Precamb. Res. 305, 444-478

Key words: pegmatite, mineralization, lithium, spodumene, granitoid, Leo-Man shiel, Mali, West African Craton

How to cite: Sanogo, S., Durand, C., Dubois, M., and Wane, O.: Structural constraints of the Birimian lithium pegmatites of Bougouni (Southern Mali, Leo-Man shield), EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-6752, https://doi.org/10.5194/egusphere-egu21-6752, 2021.

Stefano Patruno, Ian Abdallah, Vittorio Scisciani, Ernestos Sarris, and Fabio Colantonio

The core of the island of Cyprus hosts the inverted Troodos ophiolitic zone, whose flanks are overlain by autochthonous sedimentary rocks, mostly comprising Cenozoic-age carbonate shelf units. Some of these units are likely analogous to the Miocene part of the Levantine Basin “Zohr-like” carbonate buildup reservoirs, which are playing a dominant role in the present-day gas prospectivity and long-term potential for CO2 carbon capture and storage in the Eastern Mediterranean.

The study location hosts a steep sided carbonate hill (c. 90 m elevation and about 0.35 km2 total area. This hill corresponds to a lower Miocene shallow-water Terra Member carbonate buildup (Pakhna Formation), inclusive of a well-developed reefal biohermal fossil community at the summit. The buildup can be subdivided into four main depositional sub-units (informally called ‘beds’). Each of these approximately horizonal “beds” is about 5-20 m thick and hosts a number of near-vertical open fracture and minor fault sets, further enlarged by meteoric diagenesis. The lack of vegetation makes this a world-class example of shallow-water buildup available for geological analyses.

In this work, we have focused on the reservoir-scale physical properties and stratigraphic architecture of the reef outcrop, and in particular on the impact that the fracture and karst networks can be expected to play on the porosity and permeability properties of these rocks. We have utilized 133 drone photographs, subsequently “patched” together in a 3D Digital Terrain Model (DTM) using CMD-MVS; this software takes a series of pictures and creates a 3D point cloud from them thereby solving the problem of structure from motion (SFM). Several photographs have been additionally georeferenced and the visible fracture networks mapped in GIS. Furthermore, fieldwork analyses have been carried out and the following fracture properties measured at several representative locations utilizing linear scanline sampling and circular scanline methods: fracture orientation, aperture, spacing, length, intensity. Finally, representative samples have been collected from the field in order to measure their porosity and permeability properties.

Our analysis suggests the presence of a dominant fracture and fault set, striking approximately NE-SW to ESE-WNW. Additional relatively randomly-oriented, minor fracture sets are also present. Fracture intensity from the linear scanline method varies from 3 fractures per meter to the north-east to 6 fractures per meter to the south-west. The fracture aperture ranges from 0.01 to 1 meter. The studied shallow-water carbonate is characterized by high permeability and moderate porosity, with likely anisotropic flow properties along the main fracture sets. The presence of fractures enlarged by subaerial dissolutions is likely the key property controlling the reservoir parameters of these rocks, although further analyses are needed to find out whether such dissolution is associated with the present-day outcrop exposure to meteoric leaching, or was developed earlier on and can be reasonably expected in the subsurface.

How to cite: Patruno, S., Abdallah, I., Scisciani, V., Sarris, E., and Colantonio, F.: The anatomy of a fractured reef from Cyprus: a possible analogue for the Eastern Mediterranean carbonate reservoirs, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-6485, https://doi.org/10.5194/egusphere-egu21-6485, 2021.

Marco Mercuri, Eugenio Carminati, Luca Aldega, and Fabio Trippetta

Faults and fractures play a key role in the permeability of the upper crust. Since anticlines represent very common structural traps for fluids, geometrical (i.e., orientation, length distribution) and topological (i.e., cross-cutting and abutting relationships, intensity) characterization of their fracture network is crucial to assess the migration and accumulation of fluids for CO2 sequestration or hydrocarbon exploitation purposes. For this reason, many previous studies focused on anticlines worldwide, and in particular on the Zagros fold-and-thrust belt where they represent the outcropping analogs of oil fields in SW Iran.

The Zagros fold-and-thrust belt involve sediments of the pre-collisional Arabian plate passive margin, arranged in folds elongated in a NW-SE direction and tectonic transport toward SW. The belt is dissected by N-S dextral strike slip transfer faults reactivating former rift-related normal faults. Most of the studies on fracturing in the Zagros belt are based on fracture orientation data collected mainly in the field, or alternatively coming from satellite images, and deal with the origin of fracture sets (fold-related or not). Although two of the classical fold-related sets, oriented roughly parallel and perpendicular to fold axis (i.e., NW-SE and NE-SW striking respectively) can be generally recognized everywhere in the belt, other fracture orientation (e.g., N-S and E-W striking) are locally predominant and there is still no consensus on the nature of all fracture sets. For example, the role of the strike-slip reactivation of N-S and E-W striking inherited faults on fracture set distribution is still not clear.

In this study we leverage on high quality Bing Maps satellite images of the Zagros anticlines and on scanlines performed in the field to provide a multiscale investigation of geometry and topology of the fracture network affecting three anticlines, namely Sim, Kuh-e-Asmari, and Kuh-e-Sarbalesh. The three anticlines have similar dimensions and are variably affected by ~N-S striking dextral strike slip tectonic lineaments. In particular, Kuh-e-Asmari and Sim anticlines are located ~10km far from the Izeh and Sabz-Pushan faults respectively, whilst the Kuh-e-Sarbalesh anticline is characterized by an evident drag in map view against the Kazerun fault.

We manually interpreted the fracture network on satellite images at different scales (1:100 to 1:100.000), producing fracture maps with resolution ranging from 10m to 1km. Each fracture map was then analyzed using the NetworkGT plugin in QGIS. In particular, we were able to identify fracture sets, their spatial distribution and, were possible, the topology of the fracture network. In this framework, scanlines performed in the field represent punctual observations at furtherly higher resolution (~1 cm). Following the same procedure for the three anticlines enables us to test the role of N-S faults on fracture set distribution at various scales.

With such a multiscale approach we provide a “big picture” that can help to shed light on the nature and distribution of the various fracture sets in the anticlines of the Zagros belt. Moreover, fracture sets identified at different scales in this study can be used to better interpret previous and future fracture data collected in the field.

How to cite: Mercuri, M., Carminati, E., Aldega, L., and Trippetta, F.: Geometry and topology of fractures and faults affecting anticlines in the Zagros fold-and-thrust belt: a multiscale approach, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-7774, https://doi.org/10.5194/egusphere-egu21-7774, 2021.

Manuel Ignacio de Paz Álvarez, Sergio Llana-Fúnez, Stefano M. Bernasconi, Juan Luis Alonso, and Heather M. Stoll

The Esla Nappe is located in the foreland and thrust belt of the Variscan Orogen (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 (ENSZ) at a minimum depth of 4 km 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.

Following emplacement, the ENSZ was breached by clastic dykes and sills injected within re-opened previous anisotropies such as bedding planes, thrust surfaces, joints and stylolites. Together, they constitute an interconnected network of quartz sand-rich lithosomes that reach structural heights in excess of 20 m above the ENSZ. The orientation of the dykes suggests that the injection process took place under low differential stress conditions in the hangingwall and near-lithostatic fluid pressure conditions in the footwall. The injected slurry consisted of overpressured pore fluid, quartz-sand grains derived from the footwall and entrained host-rock fragments. The temperature of the fluids estimated from the clumped isotope composition of calcite cements is 71–86 °C, with an average of 80 ± 4 °C. The calcite isotopic composition (δ13C = -0.15, δ18O = -9.53, both VPDB) is well within the typical values of the host Láncara Fm., which suggests that the fluids achieved equilibrium with the host prior to calcite precipitation. Using this calculated temperature and depth estimates for the base of the Esla Nappe, the geothermal gradient during deformation is estimated to be in the order of 16–24 °C/km, a relatively low value.

Flow conditions within the injections have been inferred from properties such as the particle drag coefficient, morphology, diameter and concentration, and the fluid density and viscosity, necessary for the calculation of the terminal fall velocity of the particle array. Thin injections formed of pure quartz, with a thickness <1 cm, are consistent with flow velocities of 0.01–0.35 m/s and a laminar flow (Reynolds number (Re) <800). Thicker pure quartz injections (<10 cm), on the other hand, required faster flow velocities (0.35 m/s) and transitional to turbulent flows (800 < Re < 8000). The thicker injections (≈1 m) that entrained larger host-derived fragments would require transitional to turbulent flows (1200 < Re < 1.2×104) at fast velocities (0.35 m/s).

The estimated geothermal gradient is consistent with the lower estimations for current foreland basins, and very similar to ocean trenches. The velocities and Reynolds numbers derived for the Esla Nappe are larger than usually estimated for deep seated injections without hydraulic connection with the surface, where the vertical pressure gradient driving them is limited. In those cases, laminar flow conditions are usually invoked, but our results suggest that turbulent flow is possible in the thicker injections. Nonetheless, the values are lower than those reported for shallow injections in connection with the surface.

How to cite: de Paz Álvarez, M. I., Llana-Fúnez, S., Bernasconi, S. M., Alonso, J. L., and Stoll, H. M.: Temperature and flow conditions of quartz-sand injections at the base of the Esla Nappe (Cantabrian Zone, NW Iberia), EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-8415, https://doi.org/10.5194/egusphere-egu21-8415, 2021.

Bülent Tokay and Erdin Bozkurt

Transfer faults are generally identified as transversely oriented discrete faults linking normal fault segments in extensional tectonic settings.  The presence of the transfer faults in fault networks provides displacement transfer between the normal faults. The role and tectonic significance of transfer faults in overall extensional deformation of the upper crust is however not known very well. Micropolar theory extended by J-2 plasticity facilitates evaluation of a deforming medium in which cataclastic flow takes place with respect to each component of deformation. In this study, a series of experiments based on the Micropolar theory are performed, using fault-slip patterns, to better understand interplay among dip angle of normal and transfer faults connecting to each other, angle of linkage, and extensional direction. Synthetic linkage cases are created systematically considering various orientation of both faults sharing common stretching direction.
Our findings reveal that in orthogonal and oblique linkage cases, 3D strain field is mostly observed; a few cases exhibit plane strain. All cases are subjected to simple shearing. In cases of orthogonal linkage, extensional direction is predominantly oblique to the strike of the normal faults. Many of these cases have no block rotation (microrotation) independent from macrorotation. No particular relationship between changing dip amount of faults and direction of extension is observed. In cases of oblique linkage, (sub)orthogonal direction of extension appear in nearly half of experiments, especially those including normal faults dipping less than 60˚. The frequency of non-zero microrotation is seen apparently more than that in orthogonal linkage cases.
The study represents that structural togetherness of the transfer and normal faults essentially can accommodate complete micropolar strain in a region. This further suggests that not only the normal faults but the transfer faults should also be considered as major primary structural elements in extending domains.

How to cite: Tokay, B. and Bozkurt, E.: Strain Analysis of Transfer Faults in Extending Regions: Inferences from Micropolar Theory, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-14589, https://doi.org/10.5194/egusphere-egu21-14589, 2021.

Lunch break
Chairpersons: Stefano Tavani, Fabrizio Balsamo, Olivier Lacombe
Faults and fractures : dating, stress history and fluid flow
Paul D. Bons, Tamara de Riese, Enrique Gomez-Rivas, Isaac Naaman, and Till Sachau

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 2021, online, 19–30 Apr 2021, EGU21-14503, https://doi.org/10.5194/egusphere-egu21-14503, 2021.

Daniel Koehn, Gary Mullen, Adrian Boyce, Kelka Ulrich, and Renaud Toussaint

Fluid mixing is one of the important ore-forming processes for hydrothermal mineralization. A common hypothesis envisages hot metal-bearing fluids entering rocks that contain brines derived from paleo-seawater. Upwelling of pressurized hydrothermal fluids may occur along permeable faults or fracture systems along fluid pressure gradients. When the upwelling- and pore fluids mix, the conditions for ore-precipitation are met for instance by varying pH, temperature, or redox-conditions. This would be reflected best by changes in the saturation states of the respective mineral phases.

However, how does this process work in detail? How are the two fluids moving, where are they are meeting, how are they mixing and what minerals are precipitating? In order to study such a system, we link a transport model in the software ELLE with the IPHREEQC library of the USGS. We investigate the case of two fluids, a pore fluid representing paleo-seawater and a hot metal-rich fluid that enters the system at high fluid overpressure. As an initial condition, the pore fluid is equilibrated with different mineral phases reflecting different lithologies with varying permeabilities. Temperature as well as multi-components in the system are advecting along pressure gradients as a function of the local Darcy velocity that is calculated as the influx of a compressible fluid initially entering the system and subsequently leading to a flux through the system. In addition, the different species are diffusing as a function of the timescales set by the pressure diffusion into the system. IPHREEQC then calculates the fluid properties and mineral saturation states for every node in the system.

We show that a reactive wave develops between the two fluids, pore matrix, and infiltrating fluid. The incoming fluid pushes the pore fluid out of the system and the mixing process is mainly governed by diffusion. Depending on the time scales involved, minerals will preferably precipitate between the fluids in relatively “quiet” domains where the reactive zones prevail for an extended time. An example are fault walls where the reactive zone is stable for a longer time, whereas it is moving relatively fast along the faults. We discuss the implications of our observations with respect to low temperature ore deposits, present a first model of reactive domain development in a two-fluid system, and compare the results obtained by utilizing different thermodynamic databases.

How to cite: Koehn, D., Mullen, G., Boyce, A., Ulrich, K., and Toussaint, R.: Fluid mixing and mineralization along faults – the role of stable and traveling reactive zones, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-16293, https://doi.org/10.5194/egusphere-egu21-16293, 2021.

Irène Aubert, Juliette Lamarche, and Philippe Leonide

Understanding the impact of fault zones on reservoir trap properties is a major challenge for a variety of geological ressources applications. Fault zones in cohesive rocks are complex structures, composed of 3 components: rock matrix, damage zone fractures and fault core rock. Despite the diversity of existing methods to estimate fault zone permeability/drain properties, up to date none of them integrate simultaneously the 3 components of fracture, fault core and matrix permeability, neither their evolution with time. We present a ternary plot that characterizes the fault zones permeability as well as their drainage properties. The ternary plot aims at (i) characterizing the fault zone permeability between the three vertices of matrix, fractures and fault core permeability ; and at (ii) defining the drain properties among 4 possible hydraulic system: (I) good horizontal and vertical, fault-perpendicular and -parallel; (II) moderate parallel fluid pathway; (III) good parallel fault-core and (IV) good parallel fractures. The ternary plot method is valid for 3 and 2 components fault zones. The application to the Castellas Fault case study show the simplicity and efficiency of the plot for studying underground and/or fossil, simple or polyphase faults in reservoirs with complete or limited permeability data.

How to cite: Aubert, I., Lamarche, J., and Leonide, P.: Partitioned Permeability Diagram: an innovative way to estimate Fault Zones hydraulics., EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-12238, https://doi.org/10.5194/egusphere-egu21-12238, 2021.

Nicolas Beaudoin, Aurélie Labeur, Olivier Lacombe, Daniel Koehn, Andrea Billi, Guilhem Hoareau, Adrian Boyce, Cédric John, Marta Marchegiano, Nick Roberts, Ian Millar, Fanny Claverie, Christophe Pecheyran, and Jean-Paul Callot

Faults, joints and stylolites are ubiquitous features in fold-and-thrust belts commonly used to reconstruct the past fluid flow (or plumbing system) at the scale of folded reservoir/basins. Through the textural and geochemical study of the minerals that fills the fractures, it is possible to understand the history of fluid flow in an orogen, requiring a good knowledge of the burial history and/or of the past thermal gradient. In most of the case, the latter derives from the former, itself often argued over, limiting the interpretations of past fluid temperatures. We present the results of a multi-proxy study that combines novel development in both structural analysis of a fracture-stylolite network and isotopic characterization of calcite vein cements/fault coating. Together with new paleopiezometric and radiometric constraints on burial evolution and deformation timing, these results provide a first-order picture of the regional fluid systems and pathways that were present during the main stages of contraction in the Tuscan Nappe and Umbria-Marche Apennine Ridge (Northern Apennines). We reconstruct four steps of deformation at the scale of the belt: burial-related stylolitization, Apenninic-related layer-parallel shortening with a contraction trending NE-SW, local extension related to folding and late stage fold tightening under a contraction still striking NE-SW. We combine the paleopiezometric inversion of the roughness of sedimentary stylolites - that provides a temperature-free constraint on the range of burial depth of strata prior to layer-parallel shortening -, with burial models and U-Pb absolute dating of fault coatings in order to determine the timing of development of mesostructures. In the western part of the ridge, layer-parallel shortening started in Langhian time (~15 Ma), then folding started at Tortonian time (~8 Ma), late stage fold tightening started by the early Pliocene (~5 Ma) and likely lasted until recent/modern extension occurred (~3 Ma onward). The textural and geochemical (δ18O, δ13C, ∆47CO2 and 87Sr/86Sr) study of calcite vein cements and fault coatings reveals that most of the fluids involved in the belt during deformation are basinal brines evolved from various degree of fluid rock interactions between pristine marine fluids (δ18Ofluids = 0‰ SMOW) and surrounding limestones (δ18Ofluids = 10‰ SMOW). The 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 geothermal gradient (23°C/km). However, the western edge of the ridge recorded isotopically depleted past fluids of which corresponding precipitation temperature (100°C to 130°C) are inconsistent with local burial history (1500m). We interpret then pulses of eastward migration of hydrothermal fluids (>140°C), driven by the tectonic contraction and by the difference in structural style of the subsurface between the eastern Tuscan Nappe and the Umbria-Marche Apennine Ridge. Allowed by an unprecedented combination of paleopiezometry and isotopic geochemistry, this fluid flow model illustrates how the larger scale structures control the fluid system at the scale of the range.

How to cite: Beaudoin, N., Labeur, A., Lacombe, O., Koehn, D., Billi, A., Hoareau, G., Boyce, A., John, C., Marchegiano, M., Roberts, N., Millar, I., Claverie, F., Pecheyran, C., and Callot, J.-P.: Long-term orogenic-scale paleofluid system across the Tuscan Nappe – Umbria-Marche Apennine Ridge (northern Apennines, Italy) as revealed by mesostructural and isotopic analyses of stylolite-vein networks, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-561, https://doi.org/10.5194/egusphere-egu21-561, 2021.

Luigi Berio, Silvia Mittempergher, Fabrizio Storti, Fabrizio Balsamo, Stefano M. Bernasconi, Anna Cipriani, Federico Lugli, and Andrea Bistacchi

Fluid systems in fold-thrust belts typically evolve from hydrologically closed to open, as a consequence of the incremental development of brittle deformation. The spatial distribution of fold-related fractures depends, among other factors, on the kinematics of folding and on the presence of inherited pre-folding structures. An improved understanding of the impact of the incremental evolution of deformation patterns on fluid migration and accumulation is crucial for industrial purposes. Here, we discuss the evolution of the fluid-rock system of the Parmelan anticline, in the Dauphinois units of the northern Subalpine Chains (Bornes Massif). We combined a detailed structural analysis in the Lower Cretaceous units (e.g. Urgonian Limestones) with the study of syn-tectonic calcite cements, by coupling stable and clumped isotope analysis with trace and major element geochemistry, radiogenic Sr isotopic data, and fluid inclusion microthermometry. The older calcite cements associated with the pre-folding structural assemblages precipitated from an 18O-enriched fluid at temperatures between 90 and 115 °C. This first fluid type was thermally equilibrated with the host rock under maximum burial conditions in the Alpine Foreland and its isotopic composition has been interpreted to reflect a high degree of fluid-rock interaction in a closed system. Trace and major elements and Sr isotopes support a mixed meteoric-marine origin of this fluid, possibly trapped during subaerial platform exposure in the forebulge and then mixed with Eocene seawater. Closed system and rock-buffered conditions persisted during incipient folding whereas, during late folding, longitudinal (i.e. axial parallel) deformation structures allowed fluid circulation in an open system. Open system conditions initially occurred only in crest-limb transitional domains characterized by an higher deformation intensity. By contrast, during post-folding transpression,  the formation of a persistent vein set oblique to fold axis allowed external fluids to migrate in the anticline crest. Younger calcite cements precipitated from moderately warm (55-66 °C) 18O-depleted meteoric fluids during the late- to post-folding stages. Our compositional and Sr isotopic data exclude any contribution from basement-derived ascending fluids and rule out a possible downward circulation of these meteoric fluids at basement depths. Our results indicate that, in regional anticlines of shallow crustal sectors in foreland fold-thrust belts, a significant amount of secondary porosity can be produced in the pre-folding stages when the hydromechanical stratigraphy likely preserves closed conditions and regional stratigraphic seals can prevent upward fluid migration during the entire tectonic evolution.

How to cite: Berio, L., Mittempergher, S., Storti, F., Balsamo, F., Bernasconi, S. M., Cipriani, A., Lugli, F., and Bistacchi, A.: Pre- to syn-folding closed paleofluid system conditions and their opening in late- to post-folding structural evolution: the case of Urgonian platform carbonates folded in the Parmelan anticline, Bornes Massif, external Western Alps, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-10204, https://doi.org/10.5194/egusphere-egu21-10204, 2021.

Sergio Llana-Funez, Manuel Ignacio de Paz-Álvarez, Marco Antonio Lopez-Sanchez, Stefano M. Bernasconi, Juan Luis Alonso, Edgar Berrezueta, Ana Méndez, and Heather Stoll

The isotopic carbon and oxygen isotope composition of carbonates (δ13C and δ18O), determined by temperature and the relative abundances of stable isotopes of both elements in water at the time the carbonate is precipitated, can be modified subsequently during geological processes that involve the recrystallization of carbonate. Temperature changes mostly affect δ18O, while additional sources of carbon have a greater impact on δ13C. Amongst the various processes that may alter the original isotopic signature of carbonate rocks are deformation processes, which can lead the dissolution and reprecipitation of carbonates during deformation, or the involvement of fluids of various origin during younger tectonic events.

Here, we present the results of isotope analysis in fault rocks from two distinct faults in the Cantabrian Zone (CZ) in northern Spain. It represents the foreland fold and thrust belt of the Variscan orogen in Iberia and is characterized by numerous and large thrust sheets that were emplaced during the Carboniferous. Subsequent rifting episodes in the Mesozoic and more recently Alpine North-South convergence produced the overprinting of some of the earlier Variscan structures. In all cases, brittle processes produced often similar-looking rocks as the fracturing occurred under upper crustal conditions, relatively close to the surface. Fluids involved during deformation on both cycles are likely to differ, so to evaluate alternative tools to distinguish the different cycles of fracturing in carbonates, a stable isotope analysis on carbon and oxygen was undertaken in two well-known structures in the region: the Somiedo nappe and the Ventaniella fault.

The Somiedo nappe is one of the largest thrust sheets in the Cantabrian Zone, with an estimated offset close to 20 km. The base of the thrust sheet is characterized by well-developed cataclasites and ultracataclasites that formed on Cambrian fine-grained dolostones. It has relatively minor vein activity associated, although the dolostones have been partially recrystallized. The Ventaniella fault is a dextral strike-slip structure cutting obliquely the Cantabrian Mountains. It runs for tens of kilometres inland and has an estimated offset of approximately 5 km. The fault zone in the studied area is characterized by the fracturing and dextral offset of Carboniferous micritic limestones and, more importantly, a relatively strong vein activity that formed a distributed network of calcite veins.

Cataclasite matrix and fragments, and associated veins were sampled for isotope analysis in the two fault zones. In both cases, the matrix has a signature which is intermediate between the undeformed rock and that of the veins. The fragments have a signature which is indistinguishable from the matrix, suggesting the reworking of the fault rock. The veins have a distinct pattern in both faults, but different from each other. Those related to the Ventaniella fault are mostly hydrothermal, with limited range in δ18O and δ13C, while the veins from the base of the Somiedo nappe have a larger range of δ13C, but limited δ18O variation.

How to cite: Llana-Funez, S., de Paz-Álvarez, M. I., Lopez-Sanchez, M. A., Bernasconi, S. M., Alonso, J. L., Berrezueta, E., Méndez, A., and Stoll, H.: Carbon-oxygen isotope analysis of fault rocks in carbonates from different orogenic cycles in the Cantabrian Zone (N Spain): similarities and differences, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-7335, https://doi.org/10.5194/egusphere-egu21-7335, 2021.

Thomas Blaise, Benjamin Brigaud, Cédric Carpentier, and Xavier Mangenot

In the eastern Paris Basin, the Oxfordian (Upper Jurassic) and Bathonian to Bajocian (Middle Jurassic) carbonate platforms have been intensively cemented, despite a relatively low burial history (< 1000 m). These limestones units are separated by a 150 m thick succession of Callovian-Oxfordian tight clay-rich rocks that are currently investigated by the French national radioactive waste management agency (Andra).

Most of the initial porosity in the Middle and Upper Jurassic limestones is now cemented by successive stages of calcite, which were 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 such research efforts, the timing and temperature of the fluids involved in the cementation of these carbonate rocks are still debated.    

Here, we complement these efforts by coupling ∆47 temperatures and U-Pb ages on calcite cement filling tectonic microfractures, as well as the intergranular pore space and vugs.

Our findings indicates that the Middle Jurassic limestones were largely cemented during the Late Jurassic / Early Cretaceous period, with new LA-ICP-MS U-Pb ages (Brigaud et al., 2020) in agreement with previously published Isotope Dilution-TIMS U-Pb age of 147.8 ± 3.8 Ma (Pisapia et al., 2017). This event is believed to be associated to the Bay of Biscay rifting. A second and more discrete crystallization event occurred during the Late Eocene / Oligocene period, related to the European Cenozoic Rift System (ECRIS).

The Upper Jurassic limestones were by contrast affected by a broader range of successive deformation events spanning the Late Mesozoic / Cenozoic period. New LA-ICP-MS U-Pb ages acquired in ca. 200 µm-thick fractures show that calcite crystallized during three successive periods corresponding respectively to the Pyrenean compression, the ECRIS extension and the Alpine compression.

Our study highlights tectonic stress propagation across hundreds of kilometers, from the rifting or collisional areas toward the cementation area of carbonate rocks. Thanks to the direct radiometric dating and clumped isotope thermometry of calcite cements in microfractures, a refined paragenetic sequence is proposed with emphasis on the genetic link between large-scale deformation and calcite precipitation.

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. Brigaud et al., 2020. Geology 48, 851 – 856.

How to cite: Blaise, T., Brigaud, B., Carpentier, C., and Mangenot, X.: Relationships between intraplate deformations and the cementation of Jurassic carbonates in the eastern Paris Basin revealed by calcite U-Pb geochronology and ∆47 thermometry, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-7400, https://doi.org/10.5194/egusphere-egu21-7400, 2021.

Antonin Bilau, Yann Rolland, Stéphane Schwartz, Nicolas Godeau, Abel Guihou, Pierre Deschamps, Cécile Gautheron, Rosella Pinna-Jamme, Benjamin Brigaud, Xavier Mangenot, Aurélie Noret, Jérémie Melleton, and Thierry Dumont

In the last decade, important improvements in dating methods have been made and make it possible to go into the details of fault gouge formation and evolution. Common minerals like calcite and hematite can now bring detailed information on timing of fault development and fluid-rock interaction. We applied those novel techniques to a tectonically well constrains alpine context, though still lacking key chronological constrains. The targeted fault zone is the Penninic Frontal Thrust (PFT) of SW Alps, which is a major tectonic boundary that juxtaposed the metamorphic internal Alps over the unmetamorphosed external Alps, primarily as a thrust during the Oligocene (Simon-Labric et al., 2009). The PFT was later reactivated as an extensional detachment in the Mio-Pliocene, though the age of this reactivation remained unconstrained. Sue and Tricart (2003) showed that ongoing extensional seismic activity along the PFT, corresponding to the High-Durance Fault System (HDFS), is characterized at the surface, by an extensional fault network. In this context, the HDFS corresponds to extensional reactivation of the PFT as a consequence of Pelvoux external crystalline massif exhumation.

In this study, we coupled field tectonic, in-situ calcite U-Pb and hematite (U-Th-Sm)/He dating to stable and clumped isotope analysis to infer the HDFS activation age and to investigate the related fluid circulations. Isotopic signature (δ13C and δ18O) of compressional veins, en-echelon extensional veins and cataclasite fault gouge have been determined (Bilau et al., 2020).

This study allows pinpointing the evolution of deformation and fluid-rock interaction in the PFT footwall during its progressive extensional exhumation. The older U-Pb ages obtained on the cement of the gouge fault range between 5 to 3.5 Ma and taking into consideration uplift rate, comparison to currently seismicity depth and calcite brittle/ductile transition temperature, calcite crystallization may have occurred between 5 to 2 km. The hematite crystallization appears at shallower levels in the latest stages of the fault displacement at 3-1 km depth. A transition in the nature of fluids, controlling the redox state, can be highlighted here. This transition occurs between the calcite and hematite forming events at 2-3 km depth, which is probably related to a significant influx of meteoric fluids into the drainage of the fault system.

How to cite: Bilau, A., Rolland, Y., Schwartz, S., Godeau, N., Guihou, A., Deschamps, P., Gautheron, C., Pinna-Jamme, R., Brigaud, B., Mangenot, X., Noret, A., Melleton, J., and Dumont, T.: Timing of fault gouge formation and fluid-rock interaction during tectonic inversion of the Penninic Frontal Thrust (SW Alps), EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-7161, https://doi.org/10.5194/egusphere-egu21-7161, 2021.

Vincent Monchal, Kerstin Drost, and David Chew

This project aims to refine direct dating of carbonates by the U-Pb system, using a new LA-ICP-MS imaging technique that incorporates complementary element and textural analysis information. The direct dating of carbonates in deep time has been considered desirable for decades (e.g. Jahn and Cuvellier, 1994) given their ubiquity in the Earth system, and carbonates are a key phase for dating geological processes such as brittle-ductile deformation in carbonate successions. This new method facilitates detailed (on the scale of tens of microns) mapping of U-Pb isotope and element distributions (cf Drost et al., 2018), and is here applied to carbonate vein dating to constrain local and regional histories of deformation or fluid activity.

In this presentation we focus on a sample from the Carboniferous North Dublin Basin, Ireland. The basin has been affected by deformation that led to tight chevron folds and kinematically-linked dextral en-echelon vein sets. Additionally bedding-parallel veins and  slickenfibres are common. The deformation has been conventionally assumed to be of Variscan age, and some Variscan U-Pb ages are recorded in this study. However many calcites analysed yield late Eocene ages, a deformation phase that is hitherto undetected on the Irish mainland. Our data indicate repeated fault slip over a peroid of at least c. 4 my during late Eocene times and, thus, demonstrate the ability of the LA-ICP-MS imaging approach to not only unravel complex polyphase deformation histories in carbonates but also to resolve processes on fine temporal and spatial scales.



DROST, K., CHEW, D., PETRUS, J. A., SCHOLZE, F., WOODHEAD, J. D., SCHNEIDER, J. W. & HARPER, D. A. T. 2018. An Image Mapping Approach to U-Pb LA-ICP-MS Carbonate Dating and Applications to Direct Dating of Carbonate Sedimentation. Geochemistry, Geophysics, Geosystems, 19, 4631-4648.

JAHN, B.-M. & CUVELLIER, H. 1994. Pb-Pb and U-Pb geochronology of carbonate rocks: an assessment. Chemical Geology, 115, 125-151.

How to cite: Monchal, V., Drost, K., and Chew, D.: Shallow deformation kinematic history, a new insight from carbonates U-Pb direct dating by LA-ICP-MS imaging technique., EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-2294, https://doi.org/10.5194/egusphere-egu21-2294, 2021.

Luca Pisani, Marco Antonellini, Francisco Hilario Bezerra, Augusto Auler, and Jo De Waele

Fractured and karstified carbonate rocks are important targets for the hydrocarbon exploration industry as they usually represent very productive reservoirs. Most of the karst features in carbonate reservoirs are the result of rising fluid flow in deeply buried settings (i.e., hypogenic karst), whose origin and solutional efficiency are not connected to surface processes. Hypogenic conduits are often intercepted by drilling during hydrocarbon exploration, and their occurrence is sometimes associated with high-permeability horizons characterized by intense silicification. Silicification is a common diagenetic process in sedimentary basins, in which Si-rich fluids modify textures, mineralogy, and petrophysical properties of the host rock.  

We present the preliminary results of a multidisciplinary study performed in a cave developed within a mixed carbonate-siliciclastic succession of the Salitre Formation, in Northeastern Brazil (Calixto Cave). This cave offers the opportunity to study an accessible and extensive (more than 1 km long) conduit system associated with silicification. We performed a detailed stratigraphic and structural characterization of the sedimentary sequence in the cave, identifying different SiO2 facies and textural associations. Furthermore, we described cave geometry and pattern by topographic and morphometric observations using terrestrial laser scanner 3D models. Petrographic observations at the optical microscope were complemented with porosity-permeability analyses on rock plugs, XRD, XRF, and SEM-EDX analyses to highlight composition and petrophysical properties of the different lithostratigraphic units in the cave.

We found that silicification and mechanical stratigraphy determined the formation of high-permeability and seal units, whose distribution was fundamental for controlling paleo-flow pathways, karstification, and the spatial-morphological organization of the resultant conduit system. Cave morphologies, evidence of silica dissolution, crystalline quartz deposits and their associated paragenesis suggest that the speleogenetic phase contributing to the main karst formation happened in deeply buried hypogenic conditions, involving rising alkaline fluids probably of hydrothermal origin.

How to cite: Pisani, L., Antonellini, M., Bezerra, F. H., Auler, A., and De Waele, J.: Silicification and mechanical stratigraphy control on flow pathways and hypogenic karst development in carbonate rocks, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-457, https://doi.org/10.5194/egusphere-egu21-457, 2021.

Aurélie Labeur, Nicolas E. Beaudoin, Olivier Lacombe, Guilhem Hoareau, Lorenzo Petraccini, Laurent Emmanuel, Mathieu Daëron, and Jean-Paul Callot

The timing and duration of fold-related deformation - including layer-parallel shortening (LPS) – related to fold growth, are difficult to estimate because of a lack of data in most natural cases. We propose an original combination of direct and indirect dating methods to reconstruct the burial-deformation history of the Mesozoic carbonates that crop out in the Cingoli Anticline in the Umbria-Marche Apennine Ridge.

The Cingoli anticline displays an arcuate shape in map view, trending N140 in its northern part and N160 in its southern part). We first study the fracture-stylolite network to characterize the successive stages of deformation. Several sets of mesostructures were discriminated according to their orientation and relative chronology:

  • (i) N-S trending vertical joints (after unfolding), likely related to foreland flexure/forebulge development;
  • (ii) N045 trending vertical, bedding and fold-axis perpendicular joints/veins, associated with early folding stylolites with N045-oriented peaks and reverse faulting associated with a N045 σ1 (after unfolding), reflecting LPS;
  • (iii) bedding-perpendicular and fold axis -parallel joints/veins, e., trending N140 in the northern part and N160 in the southern part of the anticline, reflecting outer-arc extension associated to strata curvature at fold hinge, the variation about 20° in orientation between the northern and southern parts of the fold being consistent with the arcuate shape of the anticline;
  • (iv) tectonic stylolites with horizontal peaks striking N045, along with conjugate vertical strike-slip faults, associated with a horizontal N045 contraction affecting the strata after the fold was locked, corresponding to the late stage of fold tightening (LSFT).

These results suggest that the Cingoli anticline developed under a continuous N045 contraction and that its arcuate shape is likely primary and was achieved in relation to the reactivation of an N-S normal fault inherited from the Tethyan rifting, without any vertical-axis rotation of the fold axis.

We further reconstructed burial curves considering sedimentary formation thicknesses, corrected from physical and chemical compaction. We also quantified the vertical stress experienced by sedimentary stylolites from a roughness inversion technique, allowing derivation of the maximum depth experienced by the strata prior to contraction (using bedding-parallel sedimentary stylolites) and during exhumation (using horizontal sedimentary stylolites related to a post-folding compaction). Oxygen and carbon isotope ratios measured in tectonic vein cements point towards a locally-sourced fluid system with limited vertical migration at the scale of the carbonate core, enabling the use of the absolute temperatures obtained from CO2 clumped isotope (D47) to reconstruct the depth during layer-parallel shortening and folding. The comparison of reconstructed depth at which each deformation phase occurred with the burial curve provides absolute timing for the development and exhumation of the Cingoli Anticline. Together with U-Pb ages of calcite vein cements and fault coatings from the nearby San Vicino Anticline, located west of the Cingoli Anticline, our data suggest that contraction started by ~8 Ma (LPS) and ended by ~3 Ma (LSFT), and that the growth of the Cingoli anticline lasted from ~5.5 to 4 Ma.  

How to cite: Labeur, A., Beaudoin, N. E., Lacombe, O., Hoareau, G., Petraccini, L., Emmanuel, L., Daëron, M., and Callot, J.-P.: Burial-deformation history of an arcuate fold unraveled by fracture analysis, stylolite paleopiezometry and vein cement geochemistry: a case study in the Cingoli Anticline (Umbria-Marches, northern Apennines), EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-12742, https://doi.org/10.5194/egusphere-egu21-12742, 2021.

Loïc Bouat, Pierre Strzerzynski, Régis Mourgues, and Yannick Branquet

Basement/cover interfaces are important transfer zones for hydrothermal fluids responsible for ore deposition, such as U and Pb-Zn deposits. Unconformities are peculiarly mixing zone where basement-derived fluids encounter sedimentary- and/or meteoric-derived fluids; leading to precipitation of these ores. Fluids are channelized by permeability contrast, i.e. impermeable barriers, until being trapped in porous units, i.e. intrinsic porosity and/or secondary porosity (dissolution and karstification process). In this configuration fracturing channelize the fluid flow by breaking impermeable barriers allowing external fluids to enter and react with the rocks (precipitation and/or dissolution). In this way, structural studies are crucial to highlight the fracture network and the potential of geological units to be good reservoirs.

In France, many occurrences of sediment-hosted deposits are known in Mesozoic basins (i.e. Aquitaine and Paris Basin) especially above the Variscan basement (Morvan district, SW Massif Central district, Poitou High district). The Vendée coast deposit (South Armorican Massif, France) is known for two Pb-Zn(-Ag) occurrences located in Liassic sediments overlying the Variscan basement. Previous works show that, during the Upper Jurassic extensional event (NNE-SSW horizontal stretching), the ore deposition results from the mixing of two different fluids: (1) low temperature brines following a horizontal path from evaporite to basin borders within Liassic sediments along the unconformity, (2) a high temperature and low salinity fluid rising up through the basement from several kilometres depth by a probable vertical pathway.

However, the permeability architecture leading to such mixing remains poorly constrained. The Vendée ore deposits present favourable outcrop conditions to study the structural control of the fluid plumbing system along the basement/cover unconformity. Structural studies assisted by drone imagery coupled with the characterization of the alteration-mineralization pattern show that:

(1) Horizontal path for basin brines is controlled by the impermeable barrier of the Toarcien layer overlying Liassic hosting karsts.

(2) Vertical path of basement-derived fluids is enhanced by new faults and inherited fractures, respectively generated and reopened by the Jurassic extension.

(3) Relative abundance of faults and veins in the Liassic sedimentary cover and the basement is consistent with a mechanical decoupling in a context of fluid overpressure.

How to cite: Bouat, L., Strzerzynski, P., Mourgues, R., and Branquet, Y.: Horizontal and vertical fluid flows as a key control of ore deposition at the basement/cover unconformity: insight from drone imagery of the Vendée coast, France, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-15409, https://doi.org/10.5194/egusphere-egu21-15409, 2021.

Tridib Kumar Mondal and Sreyashi Bhowmick

Archean greenstone belts gained prominence for its gold mineralization. Gold bearing vein infillings within fracture systems are significant for its economic utility. Fracture formation is often associated with reactivation of the pre-existing host rock fabric under a compatible stress field. Upper crustal fluids are mostly channelized through these fracture systems under variable fluid pressure conditions generating a widespread network of veins. A wide range of vein infilled crosscutting fractures of variable thicknesses, are investigated from the gold-bearing massive metabasalts (supracrustals) of the Chitradurga Schist Belt (CSB) adjacent to the Chitradurga Shear Zone (CSZ), Western Dharwar Craton, southern India. Anisotropy of magnetic susceptibility (AMS) studies are adopted for determining the internal anisotropy of the apparently massive metabasalt hosts. The study involves tensile strength determination of the metabasalts, deciphering the paleostress condition using fault-slip analysis and propensity of fracture/fault reactivation under the prevailing stress field. Parameters like stress ratio (ϕ) and driving pressure ratio (R´) are evaluated for understanding the conditions of fluid induced fracture opening/reactivation. Change in the opening angle (µ) of fractures with fluid pressure (Pf) variation, ϕ and R´ variations with the range of fracture orientations are also ventured upon.

            We conclude ~NW-SE oriented (mean 337°/69° NE) magnetic fabric in the metabasalts are a product of regional D1/D2 deformation on an account of NE–SW shortening. This was followed by the D3 deformation with NW–SE to E–W shortening that led to the sinistral movement along CSZ. Thus, prominent fracture orientations representing riedel shear components were formed as a consequence of this sinistral shearing. Under compatible fluid pressure conditions, all such cohesionless pre-existing pathways were reactivated. Schematic models help to understand the mechanism of vein emplacement under episodic fluid pressure fluctuations from high to low Pf at shallow crustal depth (~2.4 km). With respect to the prevailing stress field, fracture orientations coinciding with the host rock fabric show higher values of slip/dilatation tendencies justifying maximum vein thickness along this orientation. Multiple methods were integrated to develop a better understanding of the fracture networking system, channelizing fluids and assisting gold mineralization in the greenstone belt.


How to cite: Mondal, T. K. and Bhowmick, S.: Role of pre-existing fabric in abetting fracture formation, fluid flow and vein emplacement in the metavolcanics: a domain for shallow crustal gold mineralization in the Archean greenstone belt, India, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-1943, https://doi.org/10.5194/egusphere-egu21-1943, 2021.

Adriana Georgina Flórez-Rodríguez, Joaquín García-Sansegundo, and Agustín Martín-Izard

The Picos de Europa Region constitutes one of the outermost areas of the Cantabrian Zone, the foreland and thrust belt of the Variscan orogen in NW Iberia. It constitutes a thrust imbricate formed of Carboniferous limestones that was emplaced towards the S-SW during the latest Pennsylvanian. During the Permian and throughout the Mesozoic, the area was subjected to extension, as attested by the scarce remnants of contemporary sedimentary successions. During the N-S Cenozoic Alpine convergence between Iberia and Eurasia, the Picos the Europa Massif was deformed under shallow crustal conditions through the reactivation of previous structures.

Zn-Pb ores, in the form of sphalerite and galena, are abundant in the central and eastern sections of the Picos de Europa Massif, where they formed as Mississippi Valley-type deposits. Although a direct dating of the minerals has not been performed to date, indirect attempts have been made based on field observations and paleomagnetic studies that have resulted in a broad span of age estimations comprised between Permian and Cenozoic times. Our ongoing research includes the study of Pb isotopes within galena samples in several localities in the Picos de Europa. The measured Pb isotopic ratios (206Pb/204Pb = 18.604–18.771, and 207Pb/204Pb = 15.686–15.707) are comparable to those of other Mississippi-Valley-type and Sedex-type ore deposits situated further east in the Basque-Cantabrian Basin. This basin was formed throughout the Mesozoic as an extensional basin, and the associated ores have been dated through ore-typology (syn-sedimentary Sedex-type deposits), metallogenic data, and other geological criteria. The similarity of the isotopic ratios in these deposits and our samples from the Picos de Europa Massif suggests a similar ore formation age, around the Lower Cretaceous, based on the interpretation of a comparable Pb crustal source.

The ores from Picos de Europa are largely associated with kilometre-scale faults that have acted simultaneously as fluid conduits and zones of preferential mineralisation. Many of the studied localities display significant deformation of the ore deposits, suggesting subsequent fault reactivation events following precipitation. Thus, the age of the deposits is useful for determining the relative timing of fault reactivation. There are two main mineralised fault systems: faults trending W-E with a variable dip, and a subvertical NW-SE-trending set. Faults from the first system were originally developed as Variscan thrusts and in some cases reactivated as normal and/or, subsequently, reverse faults during the Alpine orogenic cycle (e.g. the Cabuérniga Fault System). In contrast, the age and kinematics of the second fault system are more debated. Zn-Pb deposits from the Ándara and Liordes mining districts constitute illustrative examples of ore precipitation and subsequent brittle deformation along the San Carlos N118E-trending subvertical fault and the Liordes N117E-trending high-angle fault. While the San Carlos Fault accommodated an oblique but mainly dextral strike-slip displacement during ore deformation, the Liordes Fault acted as a dextral oblique fault with a larger reverse component, likely as a result of its slightly different dip angle. The last activity on these structures post-dates the Lower Cretaceous, suggesting a clear linkage with the Alpine orogeny.

How to cite: Flórez-Rodríguez, A. G., García-Sansegundo, J., and Martín-Izard, A.: Zn-Pb deposits as a clue for recognising reactivated structures in the Picos de Europa area (NW Spain), EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-13558, https://doi.org/10.5194/egusphere-egu21-13558, 2021.

Espen Torgersen, Roy Gabrielsen, Johan Petter Nystuen, Roelant van der Lelij, Morgan Ganerød, Jasmin Schönenberger, Sofie Brask, and Alvar Braathen

It is well known that faults, once formed, become permanent weaknesses in the crust, localizing subsequent brittle strain increments. The case of repeated brittle reactivations localized along pre-existing plastic shear zones is less recognized, although this situation is frequently observed in many geologically old terranes.

We have studied the prolonged deformation history of the Himdalen–Ørje Deformation Zone (HØDZ) in SE Norway by combining K–Ar and 40Ar–39Ar geochronology with structural analysis. The HØDZ consists of a large variation of deformation products from mylonites and cataclasites to pseudotachylites and fault gouge. Several generations of mylonites make up the ductile part of HØDZ, called the Ørje shear zone, a km-think SW-dipping shear zone within the late Mesoproterozoic Sveconorwegian orogen. 40Ar–39Ar dating of white mica from one of these mylonites give a plateau age of c. 908 Ma, interpreted to constrain the timing of late-Sveconorwegian extensionial reactivation of the Ørje shear zone.

This mylonitic fabric is extensively reworked in a brittle fashion along the SW-dipping Himdalen fault, a 10–25 m thick fault zone of cataclasite, breccia, fault gouge and, in places, abundant pseduotachylite veins. 40Ar–39Ar dating of pseduotachylite material gives several small plateaus between c. 375 and 300 Ma, whereas K-feldspar clasts from the cataclasitically deformed host rock carry a Caledonian signal (plateau at c. 435 Ma). K–Ar dating of three fault gouges constrain the timing of gouge development at c. 270 and 200 Ma. Two of the fault gouges also contain protolithic K-bearing mineral phases that overlap in age with the c. 375 Ma pseudotachylite 40Ar–39Ar plateau age, consistent with field observations of the former reworking the latter.

In sum, the HØDZ records multiple Paleozoic and Mesozoic brittle reactivations of the early Neoproterozoic (and older) mylonitic Ørje shear zone. Most of the brittle deformation is interpreted to have accumulated during development of the Permian Oslo rift and its subsequent latest Triassic evolution. The suggested late Devonian (c. 375 Ma) initiation of brittle deformation does not have a clear tectonic association, but we speculate that it relates to strike-slip displacements caused by the Variscan orogen, as also suggested for the sub-parallel Tornquist zone to the south.

How to cite: Torgersen, E., Gabrielsen, R., Nystuen, J. P., van der Lelij, R., Ganerød, M., Schönenberger, J., Brask, S., and Braathen, A.: Geochronological evidence for repeated brittle reactivations of a pre-existing plastic shear zone: The Himdalen–Ørje deformation zone, Southern Norway, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-10545, https://doi.org/10.5194/egusphere-egu21-10545, 2021.

Deblina Ray and Kathakali Bhattacharyya

We analyze veins from the deepest exposure of the regionally folded Pelling-Munsiari thrust (PT), the roof thrust of the Lesser Himalayan duplex, in the Sikkim Himalaya. The PT is exposed as a discontinuous, ~970 m thick quartz-mica mylonite zone near Mangan (27°29′ N, 88°31′ E), and records progressive deformation path where shallow crustal deformation features overprint deeper crustal deformation structures. The mean mylonitic foliation is north easterly oriented in the studied location (mean ~31°, 042°).  Based on the angular relationship with respect to the mylonitic foliation, we recognize three different fracture- and associated vein-sets at the outcrop scale. These are low-angle set (<30° with respect to the mylonitic foliation), moderate-angle (30°-60°) and high-angle set (>60°).The high-angle fracture set overprints the mylonitic foliation and is the youngest set. These are also the most dominant fracture set (~58 %), followed by the moderate-angle (~32%) and low-angle (~10%) sets. Interestingly, the low-angle vein set (mean orientation ~ 29°, 054°) is the most  dominant set (~61%), followed by the moderate-angle set (~26%; mean orientation  ~ 19°,  055°),  and the high-angle set (~13% ; mean ~23°, 340°).Field analysis indicates that ~95% of low-angle, ~71% of moderate-angle and ~ 40% of high-angle fracture-sets form veins. Some of the low- and moderate-angle veins are locally folded along with the mylonitic foliation. The co-efficient of variation (Cv) of spacing of both the fracture and vein sets are less than 1, indicating that these follow anti-clustered distribution. The poles to the veins indicate two distinct patterns. The low- and moderate-angle veins define girdle distribution, implying pore fluid pressure (Pf) exceeded intermediate principal stress axis (σ2), whereas the high-angle set shows a clustered distribution indicating σ2 exceeded Pf. A preliminary study reveals presence of blocky texture in the low- and moderate-angle veins with quartz growing at high angles with respect to the vein walls. The average thickness of the low-angle, moderate-angle, and high-angle veins, measured along appropriate scan-lines are ~ 0.92 cm, ~1.03 cm and ~0.64 cm respectively. As the low- and moderate-angle vein-sets are the most dominant sets and both show girdle distribution, we estimated a driving pressure ratio (R' ~0.35-0.6) and stress ratio (ɸ~0.251) for these veins.  The estimated paleostresses from these veins are σ1 (28°, 058°), σ2 (2°, 327°), σ3 (62°, 233°).

How to cite: Ray, D. and Bhattacharyya, K.: Estimation of driving pressure ratio and paleostresses from veins in the Pelling-Munsiari shear zone, Sikkim Himalayan Fold Thrust Belt, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-3945, https://doi.org/10.5194/egusphere-egu21-3945, 2021.

Hamza Skikra, Khalid Amrouch, Youssef Ahechach, Muhammad Ouabid, Abderrahmane Soulaimani, Otmane Raji, and Jean-Louis Bodinier

The Moroccan High Atlas mountain range is an aborted Mesozoic rift basin that was moderately shortened during the Late Cretaceous‒Cenozoic inversion. The range is currently featured in its central part by the presence of conspicuous S-shaped open gentle synclines where Middle Jurassic strata crop out, with sub-horizontal bottom, separated by 15-to-80-km narrow faulted anticline ridges with two distinct directions: ENE and NE. The tight anticline ridges are cored by Triassic continental red-beds intruded by the CAMP basalts and subsequently by Upper Jurrasic‒Lower Cretaceous alkaline magmatism. Regional cleavage with very low-grade anchi- to epi-zonal metamorphism are depicted along several structures of the High Atlas, particularly the NE-trending anticlines. The sedimentary layers thickness, on the other hand, gets thinner towards the faulted anticlines with the development of intraformational truncations. The structural history of the High Atlas syncline-topped anticlinal ridges remains a controversial matter. Any attempt to reconstruct the evolutionary process of such folded structures must take into consideration the following circumstances:

  • After a Triassic rifting episode followed by the establishment of Liassic carbonate platform, the High Atlas basin underwent a wide spread exhumation event at the time interval between the Middle Jurassic and Lower Cretaceous leading to the deposition of continental detrital series and sedimentary hiatus;
  • The upward motion was accompanied with the emplacement of alkaline magmas in the Central High Atlas;
  • A complex halokinetic history characterizes the Central High Atlas salt province during both pre-orogenic and orogenic stages;
  • During the Late Cretaceous‒Cenozoic, the High Atlas experienced a moderate crustal shortening which was focused essentially within the range’s borders;

In order to bring new insights to the structural history of the High Atlas folded structures, a structural investigation was carried out in Tirrhist and Anemzi ridges. In each station, fractures measurements were taken, and oriented samples were collected for micro-structural analysis. First paleo-stress inversion in some stations reveals the presence of pre-folding bedding-parallel maximal horizontal stress oriented NE to NNE. For a deep analysis of pre syn and post-folding stresses history, we use a calcite stress inversion technique, namely Etchecopar’s method, to unravel the paleo-stresses orientations and to quantify the differential stresses during the different episodes of deformation. The present work is a preliminary attempt to quantify tectonic stresses in the hinterland of an arguably weakly deformed orogenic belt.

How to cite: Skikra, H., Amrouch, K., Ahechach, Y., Ouabid, M., Soulaimani, A., Raji, O., and Bodinier, J.-L.: Structural evolution of the Moroccan Central High Atlas Syncline-Topped Anticlinal Ridges: Insights from micro-structural analysis of Tirrhist and Anemzi ridges, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-7361, https://doi.org/10.5194/egusphere-egu21-7361, 2021.

Florian Duschl, Tobias Stephan, Saskia Köhler, Daniel Köhn, Harald Stollhofen, and Michael Drews

Detailed knowledge on the temporal and spatial distribution of faults and fractures not only reveals the geodynamic and tectonic evolution of the lithosphere. It is also of increasing importance with regard to economic, social, and environmental challenges such as nuclear waste disposal, gas storage, geothermal energy, natural hazards, and mineral resource exploration. In this context reliable data on both timing and kinematics of deformation and their regional impact on faulting and fracture formation provide crucial information to evaluate exploration, storage, and production risks, which in turn stresses the need for comprehensive data on paleostress fields and their influence on deformation, fault reactivation, fluid activity, and hydrothermal mineralization.

In this study we present a first comprehensive approach to compile and visualize information on the crustal paleostress field of Central Europe with a focus on northern Bavaria and adjacent areas. The compilation includes published structural data from kinematic paleostress analyses (e.g. fault-slip analysis, tectonic stylolites) and geo- and thermochronological ages of fracture mineralization and fault activity, respectively. The present compilation comprises structural records from more than 40 studies and age information from more than 100 geo-thermochronological studies. All structural data are categorized according to its tectonic stress regime and quality-ranked for reliability and comparability. The consequent linkage of structural data with thermochronological data wherever possible allows to correlate local paleostress fields and deformation patterns with regional to global tectonic events. As one result, the “Paleostress Chart for Northern Bavaria and adjacent Areas” visualizes the temporal and spatial evolution of several regions in Central Europe together with known tectonic phases, sedimentary unconformities and the plate kinematic framework since the Carboniferous.

This compilation may therefore help to better understand the timing and the spatio-temporal evolution of crustal stress patterns for tectonic events across Central Europe in the context of plate tectonics. 

We aim to supplement and improve existing paleostress models on both, regional, and temporal scale by compiling published and original data. In the long term the database is intended as a continuing compilation where data from all across Central Europe are supposed to be included and refined subsequently.

How to cite: Duschl, F., Stephan, T., Köhler, S., Köhn, D., Stollhofen, H., and Drews, M.: Compiling and correlating paleostress fields across Central Europe - A paleostress chart for northern Bavaria and adjacent areas, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-10098, https://doi.org/10.5194/egusphere-egu21-10098, 2021.