Faults can be studied at different scales and through different methods. Hence, every method provides some insight into our understanding of fault geometry and properties despite the limitations inherent to these methods. Among fault characteristics, fault geometric attributes such as fault plane roughness, displacement, length, and width can be studied using both outcrops and 3D reflection seismic data. Fault geometric attributes from different scales of study are then used within fault scaling laws to increase our knowledge of fault growth mechanism and mechanics. In addition, fault rock properties from fault core and damage zone allow us to study the effect of faults on the fluid flow behavior of rocks as well as their mechanical integrity. In order to have a better knowledge of fault characteristics and mechanical behavior, it is important to integrate fault geometric studies with fault rock properties. This needs an interdisciplinary approach, combining geomechanics, earthquake and exploration seismology with structural geology, hence, allowing us to study both active and non-active faults from different perspectives.
How to cite: Torabi, A.: Fault characteristics from outcrop and reflection seismic studies, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-11403, https://doi.org/10.5194/egusphere-egu25-11403, 2025.
Reconstructing the spatiotemporal evolution of fault systems in rift basins is essential for characterizing reservoirs used in geothermal exploration and CO2 or hydrogen storage projects. This study aims to elucidate the growth and reactivation of inverted normal faults in the West Netherlands Basin and their implications for subsurface renewable energy projects. With a complex tectonic history, the area experienced multiple rifting phases and basin inversion. Fault displacement-distance diagrams were produced by using an updated semi-automated workflow for nine major basin-scale faults, providing new insights into the lateral and vertical growth of faults in inverted rift basins.
This study demonstrates that the faults in the West Netherlands Basin developed their lateral lengths during the early stages of Triassic rifting. Subsequent Jurassic extensional phases caused reactivation, leading to a consequent increase of vertical displacement and creating accommodation space for the deposition of the study area’s main reservoir rock. Variations in the reactivation behaviour along the different fault segments were promoted by stress field rotations, which significantly influenced the distribution and extent of sediment deposition. This resulted in a complex reservoir architecture that is characterized by spatial heterogeneities in porosity and permeability.
We identified contractional features, such as pop-up structures and fault-propagation folds, formed by positive fault reactivation during Late Cretaceous basin inversion. The strength of inversion was influenced by the geometry and orientation of pre-existing faults and the thickness of the underlying sedimentary cover. Inversion-related structures further complicate the basin’s architecture, by compartmentalizing the reservoir rock and influencing sediment distribution patterns.
Our findings show an example of how fault dynamics can affect the geothermal reservoir quality and storage capacity of subsurface exploration targets. This study provides valuable insights for optimizing exploration strategies and storage site selection by integrating fault growth and reactivation analysis. This helps to further reduce geothermal exploration risks and enhances storage efficiency in rift basin settings.
How to cite: Weert, A., Camanni, G., Mercuri, M., Ogata, K., Vinci, F., and Tavani, S.: Fault growth and reactivation in the West Netherlands Basin: Implications for subsurface renewable energy projects, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-1144, https://doi.org/10.5194/egusphere-egu25-1144, 2025.
Structural highs formed during the extensional tectonics of the Mesozoic rifting phase, represent important petroleum systems in Italy and hold potential for geothermal energy exploitation and geofluid storage. These structural highs, commonly exposed in the Southern Alps, Northern Apennines, or buried beneath the Po Plain foredeep, were generally overprinted by later tectonic phases. Specifically, the compressional tectonics associated with the Alpine orogeny since the Eocene often mask the original structure of mesozoic structural highs. The Gaggiano structure, located in the southwestern Po Plain, offers a rare opportunity to study a Mesozoic high only partially affected by subsequent tectonics.
This study investigates the geological and structural evolution of the Gaggiano area using a 3D seismic volume covering an area of approximately 180 km2. Seven seismic horizons, spanning from the Lower/Middle Triassic to the Miocene, were interpreted with detailed mapping, using a spacing of 300 m and 250 m between adjacent inline (N-S) and crossline (E-W) sections, respectively. The interpreted fault network, combined with thickness and top maps of key horizons, allowed the reconstruction of the structural evolution of the area and its associated fault system.
Well data from the topographically most elevated portion of the Gaggiano high highlight a stratigraphic succession that includes continental to evaporitic rocks (Lower Triassic), overlain by organic-rich mudstone limestones and dolomitized intervals (Middle Triassic), followed by Jurassic and Cretaceous pelagic limestones. The succession is highly condensed on the structural high, with significant stratigraphic gaps, including the absence of Upper Triassic rocks, most of the Jurassic (~30 m preserved), and Lower Cretaceous deposits. From the Middle Eocene onward, siliciclastic sediments of the Po Plain foredeep were deposited.
During the Middle Triassic, the structural evolution of the Gaggiano high was controlled by a N-S trending, east-dipping domino-style extensional fault system, with associated E-W striking normal faults. During the Late Triassic to Early Jurassic, the N-S striking east-dipping faults dominated. The Gaggiano high formed at the footwall of an E-dipping fault which accommodates a maximum throw of ~700 m. By the Early Jurassic, pelagic carbonates were deposited unconformably over the structural high, progressively leveling the paleotopography during the deposition of the Scaglia Formation (Upper Cretaceous-Middle Eocene). During or shortly after the deposition of Scaglia Fm., the Gaggiano area was affected by extensional tectonics testified by NW-SE striking normal faults affecting the top of Scaglia Fm. and of older formations. The NW-SE striking faults locally reactivate the pre-existing fault system. Partial involvement in Miocene compressional tectonics is evident from gentle folds affecting the sedimentary succession near Mesozoic extensional faults, suggesting positive fault reactivation.
The findings provide key insights into the interplay of extensional and compressional tectonics in shaping the evolution of the Gaggiano area. This study contributes to a better understanding of Mesozoic reservoirs and their potential reuse in sustainable energy applications.
How to cite: Mercuri, M., Bigi, S., Doglioni, C., Trippetta, F., and Carminati, E.: Anatomy and Structural Evolution of a Mesozoic Structural High: An Example from the Western Po Plain (Italy), EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-15680, https://doi.org/10.5194/egusphere-egu25-15680, 2025.
Faults accommodate shear motion in the upper crust through brittle deformation such as cataclastic flow. Plenty of field evidence suggests that the width of the embrittled volume, the lateral extent of the fault plane, and the grain size distribution within the fault core scale with shear displacement, indicating fairly solid scaling laws. However, at the onset of brittle failure, faults commonly exploit pre-existing anisotropic structures that facilitate slip localization and affect the early fault geometry as well as the fabric of the cataclastic products. The Bedretto Underground Laboratory represents a unique chance to study the structure of immature faults, whose pristine brittle structures are hardly preserved elsewhere during exhumation and exposure to weathering. We present the case study of the Waterfault (WF), a brittle shear zone hosted in the Bedretto tunnel within the Rotondo granite, which exploits pre-existing Alpine mylonites.
On the tunnel wall, the brittle products of the WF are confined within a 50 cm thick volume, comprised between two boundaries defined by the local mylonitic foliation. The WF is characterized by a large water outflow, suggesting that the fault behaves as a major permeable conduit inside the granite host. Such high permeability is commonly related to the occurrence of intense but localised brittle damage. However, careful sampling across the fault and detailed microstructural investigations revealed that the brittle damage introduced by shear displacement is much less volumetrically widespread than expected. Cataclasis is in fact confined to a thin (5 mm) fine-grained gouge shear band developed at the boundary of the fault zone. The damaged rock surrounding this layer presents intense fracturing, dominated by oriented fractures at high angle to the shear plane, but no sensitive displacement down to the grain-scale. Damage distribution and geometry is further controlled by the anisotropic mylonitic fabric and its mineralogy, showing grain boundary cracking and shard-like fragmentation of quartz and feldspar, as well as micro-boudinage-like cracking of mica-grains. More than 50% in weight of the material recovered both within and in proximity of the gouge layer is finer than 125 µm. These observations are evidence of dynamic rock shattering, pointing to a seismic origin of the embrittlement. The seismic damage preferentially fractured along the grain boundaries of the mylonite, producing a fine-grained material that eased the onset of localised brittle shear.
Our microstructural observations suggest that the WF might represent an optimal model for the early onset of faulting in anisotropic rocks driven by initial seismic damage, which unlocked the cohesive shear zone to favour slip localization. The “shallow” estimated depths of this event (< 5 km) might also open an interpretative window for the small-magnitude natural and induced seismicity (M<2) recorded in the Rotondo massif.
How to cite: Pozzi, G., Ceccato, A., Aretusini, S., Spagnuolo, E., and Cocco, M. and the FEAR Team: Shattering and structural inheritance at the onset of faulting in the Rotondo granite, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-18550, https://doi.org/10.5194/egusphere-egu25-18550, 2025.
Layer-bound faults in fine-grained sediments, observed in sedimentary basins worldwide, are frequently interpreted as the result of polygonal faulting. In clay formations, these faults are linked to clay tectonics—post-depositional processes that generate intraformational fault systems with no predominant orientation. As part of subsurface investigations for future wind farm development in the Princess Elisabeth Zone (PEZ), offshore Belgium, we analyse faulting observed within the Eocene-aged Kortrijk Clay Formation, a lateral equivalent of the London Clay Formation.
Using ultra-high-resolution seismic and acoustic reflection data with dense grid spacing (40-60 m), we identified two distinct fault groups that transition sharply between the southern and northern PEZ. The first group, dominant in the southern PEZ, comprises ENE-WSW oriented faults (N75°E-N85°E) with steep dips of 60°-70°, narrow spacing of 50-160 m, and fault lengths typically under 0.5 km. In contrast, the second group, prominent in the northern PEZ, features NNE-SSW faults (N355°E–N25°E) with shallower dips of 35°–45°, wider spacing of 100–500 m, larger displacements of up to 5 m, and fault lengths extending to 1.7 km. This group also includes a major fault with a displacement of up to 15 m, aligned parallel to the other faults. Unlike the southern faults, some of these northern faults propagate deeper, potentially reaching the top of the Cretaceous. Additionally, folding structures are also observed, with fold axes oriented both parallel and oblique to the fault directions.
The distinct partitioning of fault orientations and distributions between the two areas raises critical questions about the origin and controls of the faulting process. The presence of dominant fault orientations contradicts the diagenetic-related polygonal faulting model, which typically predicts faults lacking a preferred orientation. These dominant orientations, along with the occurrence of major faulting—particularly in the northern PEZ—point to the potential influence of regional tectonic stresses. However, the controls on the spatial partitioning of these fault groups remain unclear, highlighting the need for deeper seismic data that would allow further investigation into the deeper regional structure to better understand the faulting processes within this clay formation.
How to cite: Andikagumi, H., Mestdagh, T., Saritas, H., Missiaen, T., de Batist, M., Stuyts, B., and Pirlet, H.: Faulting variability in the Kortrijk Clay Formation, Princess Elisabeth Zone, offshore Belgium: Implications for clay tectonics, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-9308, https://doi.org/10.5194/egusphere-egu25-9308, 2025.
Studying fault zone properties is crucial in addressing key challenges in subsurface exploration, resource management, and seismic risk evaluation. As global interest in geothermal energy, hydrogen, and carbon storage intensifies, the mechanical and structural characterization of faults, as well as their impact on fluid migration, reservoir integrity, and fault sealing analysis, is becoming increasingly important.
The presented study focuses on the structural and mechanical properties of superposed fault zones in dolostones, an issue that has been poorly investigated. In particular, we addressed how the architecture of an earlier, large-scale normal fault (F1) influences the geometry and deformation mechanisms of younger, superposed strike-slip faults (F2). The F1 fault consists of four sub-parallel fault rock units, each several tens of meters thick: (i) a cataclastic core (Cu), bounded in the hanging wall by (ii) cemented micro-mosaic breccia (MB), and in the footwall by (iii) high-strained (HS) and (iv) low-strained (LS) fault rocks. To achieve this aim, we performed some geotechnical and morphometric analyses. Uniaxial compressive strength (UCS) tests revealed mean values of 83 MPa and 120 MPa for MB and LS, respectively, whereas Cu and HS exhibit lower UCS values of 58 MPa and 62 MPa, respectively. MB and Cu exhibit heterogeneous particle size distributions (PSD) and porosities of 4.02% and 4.96%, respectively, while HS and LS show more homogeneous PSDs with porosities of 2.87% and 2.19%, respectively.
The F2 faults developed a spectrum of structural facies such as cataclastic shear bands (CSBs) in the LS and HS, and as compaction bands (CBs) in the MB and Cu. In the LS, cataclasis is highly localized within widely spaced, thick, tabular, and cemented CSBs. In the HS, deformation occurs through anastomosing CSBs, accommodating diffuse cataclasis and dissolution-precipitation mechanisms. In the Cu, anastomosing CBs develop through pore collapse and dissolution-precipitation processes. In the MB, compaction is driven by pore collapse and grain crushing, forming well-localized, widely spaced CBs.
Overall, the microstructural properties (PSD and porosity) and mechanical strength of the F1 fault rocks are key factors influencing the deformation mechanisms (cataclasis vs. compaction) and the geometry (localized vs. anastomosing) of the F2 faults. These findings contribute to the understanding of fault permeability and fluid flow dynamics in multi-faulted dolomite reservoirs.
How to cite: Vitale, S., Diamanti, R., Vitale, E., Russo, G., and Camanni, G.: Deformation mechanisms and geometries of superposed fault zones in dolostones, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-18371, https://doi.org/10.5194/egusphere-egu25-18371, 2025.
Drilling through fractured carbonate formations often presents significant geomechanical challenges, including borehole collapse, cavings, tight spots, stuck pipe and lost circulation issues. These instabilities arise due to complex interactions between in-situ stresses, natural fractures, and thermal effects, requiring a robust geomechanical model for risk mitigation.
The LTG-01 well, drilled in the Dutch subsurface, encountered severe wellbore instability issues while drilling the 8.5-inch section through the Dinantian carbonates. The well exhibited tight hole conditions, stuck pipe events, severe mud losses, wellbore breathing, along with washouts identified from caliper logs. Borehole resistivity imaging further revealed borehole breakouts and drilling-induced tensile fractures (DIFs) within the carbonate interval. To investigate these geomechanical challenges, a 1D Mechanical Earth Model (MEM) was constructed using well log data available from the NLOG public database.
Elastic properties such as Young’s modulus and Poisson’s ratio, along with strength parameters including Unconfined Compressive Strength (UCS), tensile strength, friction angle, and cohesion, were computed using empirical correlations. Pore pressure was estimated using Eaton’s method for the clastic overburden and a gradient-based approach in the carbonate section. Vertical stress was computed via density log integration, while horizontal stresses were derived from the poroelastic horizontal strain equation, constrained by LOT and FIT data.
A key finding was that the drilling-induced fractures in the carbonate interval could be linked to thermal stress effects, caused by the temperature contrast between the borehole fluid and formation temperatures which were in order of ~160-190°C. The breakdown gradient computed from the MEM approached the equivalent circulating density (ECD) in zones where DIFs were observed, suggesting that thermal stress significantly reduced the rock’s tensile strength, leading to DIF formation. Additionally, borehole washouts observed in calipers, along with vuggy and brecciated intervals, highlighted the presence of mechanically weak zones.
Furthermore, borehole breakouts appear to correlate more strongly with plane of weakness (PoW) shear failure rather than intact rock failure. In addition to Mohr-Coulomb intact rock failure, alternative shear failure mechanisms were assessed—one using bedding planes as failure surfaces, and another considering natural conductive fractures as weakness planes. A notable correlation between PoW shear failure gradients and breakout intervals suggests that pre-existing weak planes significantly influenced wellbore instability.
Despite these insights, some uncertainties remain, particularly regarding fracture connectivity and fluid interaction effects, which merit further investigation. Nonetheless, this study provides critical geomechanical insights for future drilling in fractured carbonate formations, emphasizing the need for thermal stress considerations and plane of weakness analysis in wellbore stability assessments.
How to cite: Konwar, D., Mishra, C., and Chatterjee, C.: Wellbore Stability Challenges in Fractured Carbonates: Analyzing Stresses, Planes of Weaknesses, and Thermal Effects , EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-10835, https://doi.org/10.5194/egusphere-egu25-10835, 2025.
Crystalline rocks can be highly fractured at shallow depths due to tectonic stresses and topographic effects. This fracturing largely controls the rheological properties of the rock in a way that depends on the properties of the fracture networks, including the distribution of fracture sizes and orientations, the distribution of sealing minerals in the fracture network, and the normal and shear stiffnesses. Better knowledge of these properties at the site scale would improve both mechanical and hydrological models, essential for risk assessment or resource management. To this end, we combine models of mechanical properties based on Discrete Fracture Network (DFN) properties (Davy et al., 2018), stress measurements, and strain deduced from large-scale lithosphere models. The consistency of this rheological equation provides a basis for discussing the hypothesis used to infer the mechanical properties of the rock mass.
We apply this methodology to the Forsmark site in Sweden, which is being studied as the future location for a deep nuclear waste repository, supported by a comprehensive database of fractures. Fracture network models have been developed based on core and outcrop observations. Fracture density decreases with depth, showing a significant reduction down to 300-400 meters, followed by a more gradual decline. Fracture stiffnesses and matrix elasticity have been extensively measured in the laboratory. As a rule for upscaling, we assume that openness and stiffness are influenced by fracture size and normal stress. The complete 3D compliance matrix of the effective properties is calculated, facilitating the identification of the primary anisotropy planes of the rock mass.
Our findings indicate that, under specific conditions, the closure of the crustal-scale rheological equation can be guaranteed, i.e. the combination of DFN-inferred mechanical properties and measured stresses gives reasonable deformations, compatible with most lithospheric models. Using additional glaciation models, we then infer paleostresses and paleostrains during the Last Glacial Maximum, 20,000-10,000 years ago.
References:
Davy, P., Darcel, C., Le Goc, R., Mas Ivars, D., 2018. Elastic Properties of Fractured Rock Masses With Frictional Properties and Power Law Fracture Size Distributions. JGR Solid Earth 123, 6521–6539. https://doi.org/10.1029/2017JB015329
How to cite: Vairé, E., Davy, P., Darcel, C., Steer, P., and Mas Ivars, D.: Evaluating a Crustal-Scale Rheological Equation Using Stress Measurements and Mechanical Property Estimations from Discrete Fracture Network Models: A Case Study of Fennoscandia, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-16531, https://doi.org/10.5194/egusphere-egu25-16531, 2025.
The Triassic Evaporites (TE), made of alternating dolostones and anhydrites, hosted the mainshocks and most of the aftershocks of the Mw 6.0 1997 Colfiorito and Mw 6.5 2016 Norcia earthquakes in central Italy. Their complex rheology features elasto-frictional behavior and ductile deformation. The latter is highlighted by the spatio-temporal evolution of the Norcia aftershock sequence showing widespread distributed seismicity within a large crustal volume, suggesting a rheological embrittlement of the whole evaporitic layer.
To understand the main factors causing rheology variations of TE, we performed triaxial compression experiments on TE borehole samples varying strain rate, confining pressure, lithology and fabric. We tested pure anhydrite, foliated anhydrite-dolostone and mixed-chaotic dolostone-rich specimens. Samples, subjected to confining pressures of 10 and 20 MPa, were loaded up to failure at strain rates of 10-4 and 10-5 s-1, and then reloaded after a holding time of 1000 s to evaluate fault reactivation characteristics. We also performed friction experiments on TE gouge at normal stress of 20 MPa and load-point velocity of 10 μm/s, to capture fault structure development starting from randomly distributed anhydrite-dolostone particles. Mechanical measurements were coupled by microstructural analyses to elucidate the deformation processes operating at different boundary conditions.
In triaxial experiments, all samples exhibited brittle shear failure with associated stress drop. Upon rock failure, we observed a spectrum of fault slip behaviors ranging from slow (< 50 μm/s) to fast (> 600 μm/s) fault slip. We observe that dynamic faulting occurred preferentially at 10 MPa, and at higher strain rate in dolostone-rich samples . Upon fault reactivation, we recorded fault slip instabilities, mainly for the same type of conditions: dolostone-rich samples and at confining pressure of 10 MPa. On the contrary, neither fabric nor textural heterogeneities appeared to influence rock failure properties or fault reactivation behavior. During friction experiments on gouge, we measured similar friction coefficients between anhydrite and dolostone (μ ∼ 0.65), detecting minor fault slip instabilities within dolostone-rich samples. Microstructural investigations revealed the enrichment of dolostone within the experimentally developed fault slip zones, characterized by grain size reduction and shear localization.
The analysis of mechanical data suggests that rheological embrittlement of TE is facilitated by low confining pressure, high strain rate and high dolostone content. The simultaneous occurrence of these conditions promotes dynamic faulting upon rock failure and fault slip instabilities during fault reactivation. Shear localization favors dolostone concentration along slip planes, implying that the shear strength of TE-hosted faults is primarily controlled by frictional properties of dolostone, which create favorable conditions for the development of slip instabilities. When upscaling laboratory results to the crustal scale, we can speculate that low effective pressure is given by pressurized fluids while mainshock-induced stress changes facilitated a strain rate increase. These processes together contribute to the embrittlement of the evaporitic layer, explaining the distributed seismicity observed after the Norcia mainshock.
How to cite: Guglielmi, G., Giorgetti, C., De Paola, N., Mauro, M., Collettini, C., and Trippetta, F.: Rheology and fault slip behavior of Triassic Evaporites: an experimental study, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-6237, https://doi.org/10.5194/egusphere-egu25-6237, 2025.
The Coulomb stress transfer analysis is based on the hypothesis that failure on a fault plane occurs when the Coulomb stress exceeds a certain threshold. Positive Coulomb stress changes are thought to bring faults closer to failure, whereas negative ones inhibit failure. The Colca Region in Central Andes, southern Peru, is prone to small- to moderate-sized (Mw ≤ 6.0) shallow (< 20 km) earthquakes associated with normal and strike-slip crustal faults within the overriding plate in the Nazca-South American subduction zone. Along with the activity of the Sabancaya volcano, which in recent years comprised mainly of ash and fumarole emissions, this region offers an opportunity to investigate the complex relationship between seismic and volcanic activity, their potential interplay, and triggering factors.
To explore these dynamics, we carried out a Coulomb stress transfer analysis examining the interactions between source faults of 28 significant recent earthquakes (1991-2022), as well as the impact of magmatic inflation (2013-2022) on seismic events. The results confirm a tectonic origin for most earthquakes, while the magmatic source appears to play a secondary role, primarily amplifying the effects of prior seismic activity. However, the Coulomb stress transfer does not seem to be the main factor impacting the seismicity of the Colca Region, as most of the analyzed source faults were not brought closer to failure due to a positive stress change. Preceding seismic activity induced positive Coulomb stress changes on source faults in 43% of the events, while negative stress changes potentially inhibited 25%. Magmatic inflation contributed to positive stress changes in 22% of cases but also induced negative changes in a similar proportion. Notably, no direct connection was identified regarding the significant increase in seismic activity in 2013, which appeared to be potentially correlated with the start of the fumarolic emissions (late 2012) by the Sabancaya volcano.
While the coseismic static Coulomb stress change does not fully account for the complexity of seismic and volcanic activity and their interplay in the Colca Region, it provides valuable insights into active geological processes and highlights open questions warranting further investigation.
This research was funded by the National Science Centre (Poland), Grant Number 2020/39/B/ST10/00042.
How to cite: Woszczycka, M., Gaidzik, K., Anccasi, R., Mendecki, M., and Benavente, C.: The Coulomb stress transfer and possible interactions between seismic and volcanic activity in the Colca Region (Central Andes), EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-368, https://doi.org/10.5194/egusphere-egu25-368, 2025.
Paleostress reconstruction in fractured rocks is generally conducted through fault-slip inversion. Estimating spatio-temporal variation of paleostress directions from deformed rocks with prolonged deformation history is extremely challenging due to heterogeneity, non-coaxiality of deformation, ascertaining relative timing of formation of different fracture sets, genetic association with the larger structure, and probable reactivation among other factors. Therefore, fault-slip-based kinematic studies from roof thrusts of duplexes are generally less common. In this context, we attempt to deduce the fault kinematics from the leading-edge exposure of the Ramgarh thrust (RT) sheet in Darjeeling-Himalaya, which hosts a thrust-related antiform. The RT also acts as the roof thrust of the Lesser Himalayan duplex (Bhattacharyya et al., 2015). We studied the slickenline data from near the footwall contact of the RT zone (part of the overturned forelimb of the antiform) up to ~3.4km into the RT sheet (backlimb). As part of an ongoing study (Ammu and Bhattacharyya, in revision), we have deciphered a first-order relative timing among the fracture sets along with fold-fracture relative timing using various factors, for example, spatial variation of shear fracture-bedding angles, offset, relative abundance at different locations, fold test, correlation between the dihedral angles of conjugate faults and depth. We used the PBT method (Huang and Charlesworth, 1989; Sperner et al., 1993) to invert the fault-slips and reconstruct the stress regimes. This multi-proxy workflow can be used to systematically reconstruct the fault kinematics from structurally complex settings.
The major fault, RT, is oriented ~72°, 304°, along a cross-section with a bearing of ~130-310° and has a top-to-the-south vergence. The shear fractures (n=208) record normal (~61%), inverse (~39%), sinistral (~54%), dextral (~46%), oblique- (~74%), dip- (~18%) and strike- (~8%) slip movement. Although the RT sheet records a heterogeneous fault-slip population, proximal to the RT zone, the shear fractures show dominantly inverse (~58%) and sinistral (~67%) sense, i.e., a similar sense of slip as that of the major fault. We divided the heterogeneous fault-slip data into fourteen homogeneous subsets, which were categorized into pre-, syn-, and post-folding stages. The RT sheet records eight post-folding fault-slip subsets with ~NNE-SSW compression, ~NW-SE, and ~NE-SW extension, and strike-slip regimes with ~NE-SW, ~E-W, and ~NW-SE compression. The post-folding ~NNE-SSW compressional regime conforms to the present-day orientation of the regional tectonic stress field. Most stress regimes exhibit an anticlockwise rotation of stress fields proximal to the fault as compared to the interior of the RT sheet. The rotation of stress fields is observed across the pre-, syn-, and post-folding stages. Thus, the RT caused stress perturbations and influenced fracture kinematics within the RT sheet across space and time.
How to cite: Jayalakshmi Krishnankutty, A. and Bhattacharyya, K.: Decoding fault kinematics from the roof thrust of the Lesser Himalayan duplex: Insights from paleostress reconstruction, Ramgarh thrust, Darjeeling-Himalaya, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-791, https://doi.org/10.5194/egusphere-egu25-791, 2025.
Fault rocks are influenced by physical conditions such as frictional properties, temperature, effective normal stress, and differential stress. Their formation is examined with respect to energy distribution in fault zones, fault slip velocity, and etc. In this study, the fault-zone materials were retrieved from Hole-A of MiDAS project and were examined at 491.3 m in which the extremely fractured quartz was found in the vicinity of upper boundary of the active Milun Fault zone. The quartz was analyzed using optical microscopy, X-ray powder diffraction (XRD), and synchrotron XRD and Laue diffraction to understand their microstructures and potential deformation mechanisms. Microstructural observations showed angular, extremely fractured quartz grains with intragranular fracturing and no significant shear strain. XRD analyses showed a notable rightward peak shift in the 491.3 m quartz compared to quartz from other depths (389.1 m and 505.45 m), suggesting compressive stress-induced strain. Synchrotron-based XRD confirmed the absence of amorphous phases, indicating the quartz experienced rapid brittle deformation rather than prolonged shear. Laue diffraction demonstrated significant lattice distortion and high residual stress within the quartz, further supporting a mechanical origin. On ther other hand, triaxial compression tests on synthetic quartz were conducted to simulate deformation under semi-static deformation conditions. These tests revealed that strain localization are inconsistent with observations from the extremely fractured quartz. Based on these findings, thermal fracturing and comminution due to shear deformation were excluded as primary mechanisms. Instead, the results suggest that the pulverized quartz formed under extreme high strain rates likely associated with seismic rupture dynamics. This study provides a comprehensive microstructural characterization of pulverization, advancing our understanding of fault zone processes during earthquakes.
How to cite: Wu, W.-J., Lien, P., Huang, T.-H., Chiang, C.-Y., Kuo, L.-W., and Ma, K.-F.: Extremely fractured quartz within Milun fault zone: implications for pulverization, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-5986, https://doi.org/10.5194/egusphere-egu25-5986, 2025.
The spatio-temporal evolution of seismicity is of paramount importance as it provides insights into the brittle deformation of the Earth’s crust. During a seismic sequence, the spatial distribution of aftershocks typically reveals the structural architecture of the fault zone activated during the mainshock. However, within the seismogenic crust, seismicity is not necessarily exclusive of the mainshock rupture plane. In the case of the Mw 5.9 and Mw 6.5 2016-2017 Visso-Norcia (Central Italy) seismic sequence, widespread distributed seismicity (Mw < 4.5) has been recorded down-dip in the hangingwall of the ruptured fault in the depth range of 4-9 km (down-dip hanging-wall seismicity, DHwS). This is the portion of the seismogenic crust where seismic reflection profiles identify the presence of large volumes of Triassic Evaporites, TE, a geological formation composed of anhydrites and dolostones. Field and laboratory observations show that, away from the major brittle faults, TE deformation consists of a background ductile deformation interspersed with brittle processes in the form of distributed failure and folding of the anhydrites associated with boudinage hydro-fracturing and faulting of dolostones.
In this work we used the DHwS to highlight the seismological evidence of distributed deformation within a layer of the seismogenic crust affected by a background ductile deformation.
Through the construction of a series of seismological cross sections oriented perpendicularly to the strike of the mainshock rupture plane, we identified two main types of DHwS:
- Diffuse, non-localized seismicity characterized by low variability of daily seismicity rate, and
- Localized seismicity featured by both swarm-like and Omori-like event decay.
In particular, localized seismicity is mostly located south of the mainshock, and illuminates kilometers-long structures with different orientations.
We interpret the occurrence of DHwS as the result of the embrittlement of the evaporitic layer induced by an increase of strain rate following the Norcia mainshock. This is supported by recent laboratory experiments showing that an increase in strain rate promotes brittle failure and faulting in TE samples, rather than ductile deformation. Furthermore, Coulomb stress changes simulated after the Norcia mainshock suggest an increase in strain rate within rock volumes where DHwS is recorded.
Our results suggest that the aftershock distribution during a seismic sequence can be strongly controlled by the rheology of the lithologies contained within the seismogenic crust.
How to cite: Rossi, F., Guglielmi, G., Trippetta, F., and Collettini, C.: Rheological control on distributed aftershock activity: insights from the Mw 6.5 Norcia seismic sequence, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-6171, https://doi.org/10.5194/egusphere-egu25-6171, 2025.
After the devastating Mw 7.3 Nanbu-Kobe earthquake of 1995, a 750 m deep borehole was drilled by the Geological Survey of Japan to intersect the Nojima fault at depth. The Hirabayashi borehole intersected the fault core at 625 m in 1996, less than one year after the earthquake. Such a short span provides an exceptional opportunity to assess the co-seismic damage generated by this earthquake.
As part of the AlterAction project (https://anr-alteraction.osug.fr/), which assesses the interplay between the alteration and deformation of faults embedded in crystalline rocks under hydrothermal conditions, several core samples were collected at regular intervals along the Hirabyashi borehole. X-ray CT scans with a voxel size of ~50 μm were systematically performed on the samples.
This allows to quantify the fracture pattern across the fault zone. The scans show a network of open fractures, whose density increases when approaching the principal slip zone at 625 m, suggesting that these fractures are related to fault activity. The damage appears symmetric extending ~70m above and below the fault (corresponding to an effective thickness of ~15m given the low angle between the borehole and the fault plane). The relative volume occupied by the open fractures strongly decreased in the vicinity of the principal slip zone. This is related to the sealing of the fracture network, which is assumed to have occurred during the short interval between the 1995 earthquake and the time of drilling.
Several samples exhibited a dense and diffuse fracture pattern, but with very moderate deformation. Such a damage pattern is reminiscent to the “pulverized rocks” found at the surface near active faults. This would provide the first direct evidence of pulverization at depth in the Nojima Fault and confirms that a high strain rate was achieved during the 1995 Nanbu-Kobe earthquake.
How to cite: Doan, M.-L., Iaquinta, R., and Nagy, C.: Co-seismic damage of the Nojima fault one year after the 1995 Nanbu-Kobe earthquake, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-7316, https://doi.org/10.5194/egusphere-egu25-7316, 2025.
The Borborema Province (NE Brazil) is a key region to investigate fluid-assisted brittle deformation related to the reactivation of continental-scale shear zones during South Atlantic rifting. The Cruzeiro do Nordeste shear zone (CNSZ), defines the northern boundary of the Jatobá Basin and displays brittle structures such as faults, veins, breccias and pseudotachylytes, which contain evidence of paleoseismic cycles. These structures formed during the brittle reactivation of pre-existing ductile and brittle-ductile fabrics. They are filled by multiple stages of mineralization, first epidote, followed by two phases of calcite. Calcitic carbonate veins appear as cement in tectonic breccias, or as late veins associated with quartz. Fault breccias are characterized by angular clasts with crackle textures, while hydrothermal-type breccias are distinguished by matrix-supported subangular to subrounded clasts, indicative of multiple fluid injection events. To determine the timing of these deformation events, five carbonate samples were dated using U-Pb geochronology. The samples include spatic calcite in hydrothermal breccias, late sub-horizontal veins, and spatic calcite that crosscuts epidote-filled veins. The textures of these carbonates range from coarse tabular crystals with mechanical twins to fine-grained, granular shapes. Cathodoluminescence imaging reveals two distinct calcite mineralization phases reflecting episodic fluid flow during deformation. The samples yielded ages from 140 to 114 Ma, spanning the Lower Cretaceous (Berriasian) to Early Cretaceous (Aptian), a timeframe associated with South Atlantic rifting and the development of the intraplate Brazilian sedimentary basins. The structural and chronological data suggest that reactivation of brittle-ductile structures played a crucial role in channelling carbonate-rich fluids during successive seismic cycles. The variations in orientations and ages of brittle structures indicates that deformation occurred episodically, driven by regional stress variations during rifting. These findings highlight the importance of brittle-ductile deformation in accommodating tectonic stresses and facilitating fluid circulation throughout rift evolution. This study enhances our understanding of the tectonic and fluid-flow processes associated with shear zone reactivation during South Atlantic opening. By integrating structural analysis with U-Pb dating of carbonate minerals, we provide a framework for reconstructing paleoseismic cycles and their role in shaping the geological record of intraplate deformation. These insights contribute to broader discussions on rifting dynamics and the evolution of continental margins.
How to cite: Miranda, T., Barbosa, D., Correia Filho, O., Silva, A., Viegas, L. G., Neves, S., Carvalho, B., Reis, M. L., Roberts, N., and Neumann, V.: TRACKING PALEOSEISMIC CYCLES TROUGH U-Pb CARBONATE DATING: EVIDENCE FROM THE BRITTLE DEFORMATION OF THE CRUZEIRO DO NORDESTE SHEAR ZONE, NE BRAZIL, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-10920, https://doi.org/10.5194/egusphere-egu25-10920, 2025.
Stylolite, a discontinuity between blocks of rock with complex mutual columnar interdigitation, is a pressure solution dehydration structure and is useful for estimating the paleostress. Stylolites are found in lithofacies such as limestones and evaporites, although only those in limestones have been used to estimate the paleostress. Stylolite formation could be a major contributor to the creation of sedimentary space after evaporite deposition in general. Therefore, the state of stylolite formation in evaporite is necessary to understand basin evolution. This study analyzes stylolites in evaporites collected by IODP Expedition 402 at Hole U1617B, located in the Tyrrhenian sea about 110 km southwest of the Italian peninsula, with the aim of estimating paleostress value at the time of stylolite formation.
Stylolites were photographed on the vertical section of cores perpendicular to the stylolites. Their traces were analyzed using the discrete Fourier transform method to estimate crossover-length L which separates two scaling regimes with different roughness exponents for small and large scales. Most stylolites show L as ~2 mm. Corresponding overburden stress σzz ≈ 9 MPa assuming gypsum physical properties of Young's modulus E = 50 GPa, solid-fluid interfacial energy γ = 47 mJ/m2, and Poisson's ratio ν = 0.25. The corresponding depth z ≈ 330 m by assuming σzz = ρgz, with rock density ρ = 2.7 g/cm3 and gravitational acceleration g = 10 m/s2. The water depth of the hole was 2822.33 m and analyzed stylolites were located at ~328 m below the sea floor. An estimated overburden stress value is not unnatural compared with the sampling depth, suggesting that stylolites in evaporite would also be useful for stress estimation. The gypsum-anhydrite transition is thought to occur at a burial depth of 500~1000 m. Therefore, stylolite formation in Hole U1617B would have occurred before the transition.
How to cite: Abe, N. and the IODP Exp. 402 Scientists: Paleostress value estimation using stylolite in evaporite of IODP core sample from Expedition 402 at the Tyrrhenian Sea, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-14539, https://doi.org/10.5194/egusphere-egu25-14539, 2025.
Tectonic faults can slip with diverse behaviors: from aseismic creep to seismic slip. Such diverse slip behaviors of a fault are mainly controlled by the frictional stability of the rough fault interface, where a complex set of real contacts are established by numerous discrete asperities. However, understanding how these asperities precisely control the slip stability still remains elusive.
Here we develop a novel analog fault model to overcome the difficulty of imaging an exhaustive spatiotemporal variability of a natural fault interface at depth. Specifically, numerous identical rigid spherical PMMA (poly-methyl-methacrylate) beads, which are used to model the discrete frictional asperities, are embedded with height variations and random spatial distribution in a soft viscoelastic silicone block to establish numerous micro-contacts with a thick transparent rigid PMMA plate on the top. During the entire shear process of such a heterogeneous fault interface, not only the subtle motion of each local asperity can be directly measured by the high-resolution optical monitoring system, but also the seismic waves emitted from slip transients that occurred at local asperities can be captured by the acoustic monitoring system.
The synchronization of the local rapid slips at all asperities is responsible for the unstable system-size stick-slip of the macroscopic fault that generates large amplitude energetic acoustic event. It is interesting to observe that complex seismic activities initiate also early during the interseismic phases and are interpreted as the seismic signature of destabilizing transients that originate from spatiotemporal interactions of limited local asperities. We locate acoustic events at the asperity scale and correspond them with these slow transients. We further quantify the partitioning of the resolved slip taking place on the asperities as dynamic events to interpret the nature of the complex seismicity. Our results provide insights into a better understanding of the physical processes leading to the occurrence of foreshocks and complex seismic sequences.
How to cite: Shu, W., Lengliné, O., and Schmittubhl, J.: Complex seismic sequences originated from the collective behavior of asperities: an experimental approach, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-16530, https://doi.org/10.5194/egusphere-egu25-16530, 2025.
The ratio of compressional to shear wave velocity (Vp/Vs) provides critical insights into rock composition, fluid saturation, porosity, and fault characteristics. However, resolving fine-scale Vp/Vs structures is challenging with conventional tomographic imaging techniques due to uneven ray path coverage and regularization constraints in inversion. Alternatively, high-resolution Vp/Vs imaging can be achieved through in-situ Vp/Vs ratio estimation by analyzing differential P-wave and S-wave arrival times (Δtp,Δts) from closely located earthquake clusters, effectively circumventing the limitations of traditional tomography. Conventional in-situ Vp/Vs estimation method mitigates origin time difference errors by subtracting the mean Δtp and Δts value for each event pair. However, this approach clusters data points near the coordinate origin, increasing estimation uncertainty. In this study, we present an improved in-situ Vp/Vs ratio estimation technique that corrects the origin time difference errors for each event pair within a cluster using spatially dense seismic arrays. This correction ensures that P-wave and S-wave time differences precisely represent the actual travel time differences, thereby enhancing the reliability of Vp/Vs estimation. Synthetic tests confirm the robustness of the method and its ability to reduce uncertainty. Applying the improved method to the 2021 Ms 6.4 Yangbi Yunnan aftershock sequence, we identify significant spatial variation in Vp/Vs ratio along the fault zone, with an increase in Vp/Vs at greater depth and a strong correlation with fault structures and their geometries (figure 1). We further investigate how mineral composition, stress state, and fluid content influence Vp/Vs in the Yangbi region. Our method, suitable for dense array observations, demonstrates strong potential for application in other seismically active regions.
Figure1. Spatial Variations and uncertainties of in-situ Vp/Vs in the 2021 Ms 6.4 Yangbi Yunnan aftershock sequence. The red star is Yangbi earthquake. (a)In-situ Vp/Vs ratio of the Yanbi aftershock sequence. The black line indicates the fault trace, where F1 represents the Weixi-Qiaohou-Weishan Fault. (b)Uncertainties of the Yanbi aftershock sequence. The blue line represents the fault surface, and the gray triangles denote seismic stations. (c)Vp/Vs variation with depth.
How to cite: Zhang, Q., Yu, C., Meng, H., and Zeng, X.: Improved in-situ Vp/Vs Estimation Using Dense Seismic Array with Application to the 2021 Ms 6.4 Yangbi Yunnan Aftershock Sequence, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-17081, https://doi.org/10.5194/egusphere-egu25-17081, 2025.
Studying megathrust shear zones is crucial for understanding the mechanics of subduction zone earthquakes, as these zones are recognized as weak interplate faults that localize deformation under low shear stresses. The low effective friction coefficient of megathrust faults, often due to fluid overpressures, facilitates deformation and is evidenced by the occurrence of mineral-filled fracture sets. These fracture networks, which form in response to cyclic stress states and fluid pressures, provide valuable insights into the palaeostress orientations and the characteristics of fault zone fluids over time.
The Sestola Vidiciatico Unit (SVU) in the Northern Apennines is a tectonic unit interpreted as the plate boundary shear zone between the Ligurian prism and the underthrusting Adria microplate during early-to-middle Miocene, active at temperatures up to 170°C. The SVU is 200 - 400 m thick, composed of kilometer-sized tectonically juxtaposed slices of marls, shales, sandstones and mud-rich deposits. The hanging wall, formed by slope sediments along with Ligurian Units incorporated at the toe of the prism, was overthrusted along a basal décollement onto the younger foredeep turbidites of Adria microplate. Here we present the results of a mesoscale structural analysis of a well exposed sector of the basal contact, where the SVU overthrusts foredeep turbidites along a thrust ramp dipping to the south. Early stages of deformation involved soft-sediment deformation, with polygonal normal faults accommodating boudinage and flattening, producing a pervasive flattening foliation and secondary shear surfaces in the hanging wall. As lithification progressed, shear localization occurred, transitioning from distributed shearing to focused slip on a few dominant thrusts lined by thin calcite shear veins, including the basal contact with the turbidites. Along the footwall ramp, irregular and unfavorably oriented shear surfaces were gradually abandoned as slip localized along a sharp, smooth, and planar slip surface, incorporating slices of the hanging wall to the footwall. The deformation within the footwall includes an oblique cleavage in the fine-grained horizons, minor bed parallel shear planes exploiting pelitic horizons, and a conjugated set of NNE-SSW left-lateral and N-S right-lateral subvertical transtensional faults. The latter either crosscut or are crosscut by the basal thrust of the SVU, and are mineralized by at least two carbonate phases, including an early-stage light gray carbonate rich in organic matter. These results highlight a marked contrast in deformation style and stress state between the soft hanging wall and (relatively) strong footwall. Subvertical dilatant shear fractures served as fluid conduits for vertical fluid flow, which is instead very limited in the hanging wall. This study highlights the potential of combining structural and geochemical analyses in megathrust shear zones in providing insights into the interplay between stress state and fluid circulation in both fossil and modern shallow subduction zones.
How to cite: Rocca, M., Mittempergher, S., Remitti, F., Molli, G., and Tesei, T.: Shear localization and deformation patterns of a regional scale overthrust: an example from the Sestola Vidiciatico Unit (Northern Apennines) , EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-17416, https://doi.org/10.5194/egusphere-egu25-17416, 2025.
In 2011, the Tohoku subduction zone endured a Mw 9.0 large earthquake and a large tsunami with co-seismic displacement exceeding 40 meters at the trench. Such an unexpected tsunami earthquake is expected to have created intense damage across the plate boundary fault zone (PBFZ).
We propose to characterize the hydraulic damage pattern across the PBFZ of the Tohoku earthquake. To achieve this, we will use the rich dataset collected during IODP Expedition 405, during which the PBFZ was drilled directly several times between September and December 2024. In particular, we focus on drilling and logging data to estimate the flow entering the borehole. By carefully modelling the mud pressure, we can evaluate the hydraulic inflows and outflows to the borehole as drilling advances. The flow profile provides valuable insights into permeable and/or overpressurized intervals.
Using the Logging-While-Drilling data collected in Hole C0019H, we obtain a high-resolution and continuous fluid flow profile along the borehole. The results show that above the plate boundary (~815 mbsf), we observe the incoming fluid flow, with strong flow pulses at 775 mbsf and 805 mbsf. These two pulses are associated with variations in mud temperature and some clear fracture zones identified by the electrical images. This suggests the hydraulic structure of the PBFZ has two components highlighted by the two types of flow: (1) A background damage, increasing progressively in the hanging wall when approaching the PBFZ, with a potential for fluid flow across the fault zone. (2) A fracture-supported flow, related to the major subfaults which sustain a more longitudinal flow.
How to cite: Pei, P. and Doan, M.-L.: Hydraulic structure of the Tohoku plate boundary fault zone: insights and evidence from direct drilling (IODP expedition 405), EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-11611, https://doi.org/10.5194/egusphere-egu25-11611, 2025.
The Enping 17 sag (EP17) is located in the northern part of the Pearl River Mouth Basin, and the Cenozoic boundary faults with complex-trending have been developed. The genetic mechanism of the boundary faults is still unclear, which limits the understanding of the structural evolution of the sag in the study area. Using high-quality three-dimensional seismic data, through seismic interpretation, throw-distance (T-x) plots, physical experiment and tectonic evolution sections, the geometric characteristics, activity and genesis of boundary faults are analyzed, and the reactivation mechanism of pre-existing faults and the evolution process of sags are discussed. The results show that the EP17 boundary fault is curved, and there are differences between the north and south sides: the north is dominated by NE-trending faults, accompanied by nearly ENE-trending faults, with slope-flat type and four-stage rolling anticline on the seismic profile; the south is a near NS-trending fault with shovel-type and two-stage rolling anticline. The selective reactivation of NE-trending and near NS-trending pre-existing faults in the Cenozoic controlled the evolution of the sag. The Enping 17 sag has undergone multiphase extension, and the extension direction rotates from NW-SE to N-S clockwise. The early Eocene NE-trending and near NS-trending pre-existing faults are reactivated at the same time, forming a strong extension zone at the fault tips, resulting in a rapid link between the north and south faults. These group of faults continued to be active during the Middle Eocene, and a new ENE-trending fault was formed in the north. The NE-and ENE-trending faults in the northern part of the Late Eocene continued to move, controlling the migration of the sedimentary center to the northern sag.
How to cite: Zhang, Z., Liu, H., Peng, G., and Long, Z.: Reactivation mechanism of pre-existing faults in multiphase extensional setting: A case study of Enping 17 sag, Pearl River Mouth Basin, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-2194, https://doi.org/10.5194/egusphere-egu25-2194, 2025.
Granite emplacement is often assisted by shear zones. These shear zones may be synmagmatic and can lead to the development of various structural features, ranging from complex fabric to faults and their interactive networks. The Neoarchean Closepet Granite (2.56-2.51 Ga) of Eastern Dharwar Craton, India is replete with faults and fractures of various orientations, which are used to decipher the regional brittle tectonics. Detailed analysis of strike-slip and high-angle oblique-slip faults using the Right Dihedron and Rotational Optimization methods reveals an overall E–W oriented horizontal regional compressive stress. The area is further subdivided into smaller domains to observe variations in the local stress field. Results indicate a distinct deformation pattern across the region. The northern portion is influenced by ENE-WSW directed regional compression, while the southern portion is shaped by ESE-WNW directed regional compression. Fault orientations and their kinematics across the pluton indicate that the faults are associated with a large-scale Riedel shear along the pluton boundary. In the north, deformation is compatible with an NW-SE sinistral shear zone, while in the south, fault patterns are consistent with an NNE-SSW dextral shear zone. This variation in shear is attributed to the overall geometry of the pluton boundary. We also interpret that, during the emplacement and the post-emplacement period, the rheological boundary between the granite and the surrounding host rock, evolved into a shear zone, facilitating the development of faults across the Closepet pluton.
How to cite: Das, G. and Mondal, T. K.: Paleostress Field Reconstruction from the strike-slip faults of Neoarchean Closepet Granite (Eastern Dharwar Craton, South India): Its implications in understanding Precambrian Tectonics, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-12263, https://doi.org/10.5194/egusphere-egu25-12263, 2025.
The geology of northern Oman is distinctive because of the emplacement of the massive Semail Ophiolite onto the stable Arabia platform in the late Cretaceous followed by the later development of the Jebal Akhdar and Saih Haitat domes. East of Muscat, the Wadi Kabir Fault forms an important and long-lived structure at the northern edge of the Saih Hatat dome. In the Bandar Jissah area, Triassic carbonates occur in the footwall of the NNE-dipping Wadi Kabir Fault while rocks of the Semail Ophiolite, newly discovered rocks of the metamorphic sole, and a sequence of Paleogene-Eocene sedimentary rocks crop out in the footwall. Previous workers posit that regional extension commenced via early low-angle detachment faulting (‘Banurama detachment’) that was followed by higher-angle normal faulting along the Wadi Kabir and associated faults which developed as basin-bounding structures for the Paleocene Bandar Jissah rift basin. Folds in the hanging wall cover sequence are interpreted as the product of rollover during extension and basin formation.
Our detailed mapping as well as structural and kinematic analysis illustrates that folds in the hanging wall are contractional structures that formed due to tectonic inversion along the Wadi Kabir and other faults. The overall shortening is modest (~10%) and primarily confined to the hanging wall rocks, consistent with buttressing against mechanically rigid rocks in the footwall of the Wadi Kabir Fault. The low-angle ‘Banurama detachment’, which places Paleogene-Eocene sedimentary rocks over Triassic carbonates, records south-directed thrust movement (not north-directed extensional slip) and with contractional slip past the null point. This structure appears to be an emergent fault in which the reactivated cover strata above the Wadi Kabir fault were thrust southward over the ground surface and shed sediment from the growing hanging wall anticline.
These structures require an interval of shortening/transpression in northern Oman that post-dates rift basin formation and deposition of mid-Eocene marine sediments in the Seeb Formation. The Wadi Kabir Fault also has localized zones of listwaenite preserved in its damage zone that is derived from ophiolitic rocks in the hanging wall. Collectively, the Wadi Kabir Fault is a long-lived structure that’s experienced multiple episodes of both extensional and contractional slip since the Cretaceous.
How to cite: Bailey, C. and Andreas, S.: Cenozoic tectonic inversion, buttressing, and emergent faulting along the Wadi Kabir Fault, northern Oman, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-14151, https://doi.org/10.5194/egusphere-egu25-14151, 2025.
Posters virtual: Tue, 29 Apr, 14:00–15:45 | vPoster spot 2
EGU25-6417 | Posters virtual | VPS28
Architecture of a mud diapir-like structure: insights from ocean-bottom-node seismic dataTue, 29 Apr, 14:00–15:45 (CEST) | vP2.9
We present high-resolution ocean bottom node (OBN) seismic data of the Dongfang 1-1 structure in the Yinggehai Basin of the South China Sea, which hosts China's largest offshore gas reservoir. The OBN seismic data reveals more continuous and detailed reflections compared to conventional seismic data, highlighting the internal structure and formation mechanism of a diapir-like structure. The seismic images show a tapered conical structure characterized by a concentric distribution of fractures, with a significant increase in fracture intensity and connectivity towards the center. These fractures, particularly the sub-vertical ones, are interpreted as natural hydraulic fractures formed due to intense overpressurization in the Lower Miocene strata, with formation pressure coefficients up to 2.2. The fractures are believed to have originated from thermogenic hydrocarbon gas generation and inorganic CO2 production. The throughgoing fractures that traverse the entire Neogene succession, including the thick Upper Miocene sealing mudrocks, provide crucial pathways for deep gas-bearing fluids to charge the Pliocene sandstone reservoir. The study underscores the importance of natural hydraulic fractures in bypassing thick sealing sequences and conduiting fluids in deep overpressured environments. Moreover, our results may provide guidance for accurate geological interpretations of mud diapir-like structures in conventional seismic images in many other sedimentary basins.
How to cite: Meng, Q., Yang, B., Guo, Z., and Hao, F.: Architecture of a mud diapir-like structure: insights from ocean-bottom-node seismic data, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-6417, https://doi.org/10.5194/egusphere-egu25-6417, 2025.
EGU25-9784 | ECS | Posters virtual | VPS28
Stress Distribution and Fracture Development Along the Altyn Tagh Fault:Insights from 3D Discrete Element ModelingTue, 29 Apr, 14:00–15:45 (CEST) | vP2.25
The slip behavior of the Altyn Tagh Fault (ATF) plays a key role in improving our understanding of the tectonic deformation processes shaping the Tibetan Plateau. In this investigation, a three-dimensional (3D) model representing the central segment of the ATF was constructed using discrete element numerical simulations to examine the main damage zones and stress distribution in the Akato Tagh Bend, AKsay Bend, and Xorkoli segments. The simulation results were then cross-referenced with fault orientation measurements from the northern Qaidam Basin and focal mechanism solutions (FMS) to assess their precision and reliability. The results indicate that the stress environment is stable in the linear strike-slip Xorkoli segment, whereas the stress distribution in the Akato Tagh Bend and AKsay Bend segments exhibits significant heterogeneity, with alternating regions of high and low stress. On the concave side of these bends, compressive stress accumulates, fostering the formation of local thrust faults or folds along the fault plane. Conversely, on the convex side, tensile stress dominates, promoting the development of normal faults or extensional fractures. In the restraining bend region, tensile stress remains horizontal, though its orientation shifts considerably as fault displacement increases. The bend segments also show significant variations in shear stress, which can lead to the creation of secondary fault features like Riedel shears. The intensity and distribution of shear stress are influenced by the curvature and bending angle of the fault, with larger bending angles in the Akato Tagh Bend producing more pronounced shear stress concentrations. Fractures are primarily concentrated at the fault tips, along fault intersections, and within the fault plane, with the fault damage zone being notably wider in the Akato Tagh Bend and AKsay Bend segments. As fault displacement increases, the width of the damage zone and fracture density initially increase rapidly before reaching a plateau. Moreover, the primary damage zone develops earlier in the restraining Akato Tagh Bend and AKsay Bend segments compared to the linear strike-slip Xorkoli segment, which absorbs more strain before the principal displacement zone forms. Therefore, the Akato Tagh Bend exhibits the highest fracture intensity, followed by the AKsay Bend and Xorkoli segment. These findings offer significant insights into the slip behavior and stress distribution along the ATF and enhance our understanding of the tectonic processes in the Tibetan Plateau.
How to cite: Chen, Z., Li, J., and Liu, G.: Stress Distribution and Fracture Development Along the Altyn Tagh Fault:Insights from 3D Discrete Element Modeling, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-9784, https://doi.org/10.5194/egusphere-egu25-9784, 2025.
