The exploitation of georesources and subsurface storage may lead to various hazards including subsidence and induced seismicity. The need for à priori knowledge of fault-rock properties in subsurface exploitation is clearly illustrated by the Groningen gas field, where gas extraction has led to induced seismicity on a complex network of pre-existing faults. Despite the advanced state of knowledge on the Groningen field in terms of depositional facies, host rock composition, and fault distribution at the reservoir level, kinematic definitions and structural observations of these faults are missing at all observational scales. In particular, the internal structure of fault zones and composition of fault rocks require significantly more understanding. In this study, we present an integrated dataset that synthesizes fault gouge (micro-)structural observations and experimental properties from samples of one Groningen reservoir fault, complemented by meso-scale interpretations derived from reflection seismics and well data. We identified two genetically different types of fault rocks at the reservoir level. Quartzofeldspathic gouges with interstitial clays result from cataclastic grain size reduction followed by pressure solution-related grain indentations, which infers time-dependent healing by pressure solution as well as cementation. Clay-supported gouges were formed by lithology mixing and deformation is accommodated by slip on phyllosilicate foliation planes. We infer that fault reactivation in clay-rich fault cores promotes aseismic slip, whereas reactivation of cemented quartzofeldspathic faults may involve cataclasis outside the pre-existing fault core, promoting seismic slip. Our core-scale and microscale observations provide quantitative data on the intrinsic properties of fault rocks and their distribution in fault zones in the Groningen Rotliegend reservoir. Together with the reservoir scale structural fault interpretations and inferences from mechanical experiments, they provide a geological context for the present-day observed induced seismicity.
Granular analogue materials are widely used to simulate brittle upper-crustal deformation in scaled physical models. However, conventional materials such as dry sand and wet clay may not sufficiently reproduce the complexity of segmented fault systems and associated secondary structures. This study investigates the mechanical behaviour of dry sand-clay mixtures and evaluates their influence on fault nucleation, growth, and segmentation in extensional analogue models. Four sand-clay mixtures containing 2.5%, 5%, 10%, and 15% clay by weight were characterised using ring-shear tests to determine density, cohesion, and frictional properties. Pure sand and pure clay were also analysed for comparison. The results show that increasing clay content progressively modifies the mechanical response of the mixtures, producing a transition from sand-dominated to clay-dominated behaviour. Mixtures containing 2.5% and 5% clay exhibit increased cohesion while maintaining friction angles comparable to pure sand, whereas the 10% mixture displays transitional behaviour and the 15% mixture shows predominantly compaction-dominated behaviour. The mechanical characterisation was complemented by a series of oblique-rift analogue experiments designed to evaluate the influence of material cohesion on fault-system development. Compared with pure sand models, the cohesive mixtures promoted earlier fault nucleation, enhanced fault segmentation, and the development of more abundant secondary fault networks. Despite these differences, all experiments produced comparable first-order rift architectures, indicating that cohesion primarily influenced fault localisation and structural complexity rather than overall basin geometry. The results demonstrate that modest increases in cohesion significantly affect fault growth and segmentation in analogue models. Among the materials tested, the 2.5% and 5% mixtures provided the most effective balance between frictional behaviour and structural resolution, enabling improved representation of brittle fault architectures in extensional settings.
We unravel the spatio-temporal variations in paleostress and paleoshortening orientations from the leading-edge of the Ramgarh thrust (RT) sheet, the roof thrust of the lower Lesser Himalayan duplex, in the Darjeeling Himalaya by integrating dynamic, kinematic inversion of fault-slips and minimum finite penetrative strain. Shear fracture intensity, orientation, and slip-sense spatially vary within the RT sheet. The heterogeneous fault-slips (n=208) comprise fourteen homogeneous subsets tracking incremental deformation through pre-, syn-, and post-folding stages. Kinematic inversion detected the earliest layer-parallel shortening (LPS). Thus, this approach is more insightful than dynamic inversion in reconstructing progressive deformation in similar thrust sheets with prolonged deformation history. The shallow crustal deformation is partitioned into thrust (∼36%), strike-slip (∼24%), and normal (∼40%) components by outcrop-scale fault-slips, with thrust faults dominating near the RT zone. Spatio-temporal variations in paleoshortening directions is recorded. Only 16% of the fault-slips are consistent with far-field convergence-related stress orientations, indicating not all fractures are late-formed. Local stresses are interpreted to predominate over far-field stresses in generating ∼84% of the fault-slips. Penetrative strain is pre-folding with consistent subvertical maximum shortening axis (Z) recording early plastic deformation. In contrast, ∼37% of the fault-slips record subvertical Z, ∼15% ∼E-W subhorizontal Z, ∼16% ∼N-S subhorizontal Z, and ∼32% plunging Z. Thus, minor-faults preserve incremental shortening stages better than estimated minimum finite strain ellipsoids and are critical in reconstructing detailed kinematics within leading-edge of thrust sheets. This study demonstrates the necessity of deciphering the relative timing of various fracture sets prior to paleostress and paleoshortening analyses.
Faults and shear zones parallel to bedding are fundamental in sedimentary successions but are generally more difficult to recognize in the field than faults cutting across strata. Micro-scale indicators such as striations or fibrous veins may be absent or weathered, requiring larger-scale markers for reliable identification. We present a new field-based classification of kinematic markers for recognizing bedding-parallel fault at outcrops. Six structural scenarios are distinguished and grouped into two categories: (1) offset primary and preexisting structures, including disrupted primary structures, disrupted dilational fractures, and deformed cover sediments at the base of beds; and (2) characteristic fault patterns associated with bedding-parallel slip, including duplex structures, offset meso-scale faults, and synthetic shear fractures. This framework consolidates previously scattered criteria and provides a systematic approach for identifying bedding-parallel fault and slip sense. The classification has broad implications. For field geologists, it reduces errors in stratigraphic correlation and structural mapping. For exploration geoscientists, it highlights potential fluid migration pathways influencing mineralization and hydrocarbon accumulation. For engineering geologists, it distinguishes tectonic bedding-parallel fault from landslide failure planes, improving hazard assessment. By providing a practical and versatile tool for detecting bedding-parallel fault, this study advances both academic and applied structural geology in developing field-based methods for understanding folds and faults.
Tectono-metamorphic mélanges are critical to understanding orogenic belts, yet their formation processes remain controversial. The Mogutai (MGT) massif in the Dunhuang orogenic belt, NW China, provides an ideal window into this issue. Metamorphic rocks in the MGT mélange belt occur as coherent thrust sheets or exhibit "blocks-in-matrix" fabrics. Petrological, mineral chemical, and geochronological data indicate that granulites in the western part of the belt record peak P-T conditions of 12.8-16.1 kbar and 710-820 °C, with metamorphic ages of 418-419 Ma. Integration with published data reveals that the MGT mélange belt comprises metamorphic rocks formed at various depths and times, and exhumed along different paths. Detailed mapping and structural analysis indicate that rocks in the belt were assembled during two levels of tectonic mixing. First, an imbricated thrust system stacked thrust sheets of different origins of diverse P-T histories. Second, within individual thrust sheets, blocks-in-matrix fabrics developed, incorporating blocks with diverse P-T histories within a shared matrix. The entire system was subsequently overprinted by strike-slip deformation. Field observations and analytical data demonstrate that coherent thrust sheets, blocks, and matrices formed independently and were assembled in the Mogutai mélange belt over a prolonged period from the Ordovician to the Carboniferous. Permian-Triassic sinistral strike-slip further modified the belt. It can be concluded that the tectono-metamorphic mélange belt can undergo a prolonged formation process and assembled progressively by emplacement of rock bodies of various P-T-t conditions (“metamorphism-first” model), instead of a “assembly-first” model that all rock bodies in the mélange belt were assembled first then underwent unified evolutionary history . It suggests that different rock bodies from tectono-metamorphic mélange belt cannot represent a united P-T-t evolutionary history of an orogenic belt. Each rock body may only record its own unique evolution.
The deformation behavior of volcanic rocks is governed by their microstructure (porosity, pore geometry, orientation) and external conditions such as pressure, temperature, and strain rate. At low confining pressures, porous rocks fail by shear fracturing, whereas higher pressures promote ductile processes, including cataclastic flow or compaction band formation. While these processes are well established in sedimentary rocks, compaction localization in volcanic rocks remains poorly understood. Here, we investigate how preferred pore orientation influences the mechanical behavior, compaction band development, and permeability of lava deformed in the ductile regime. We focus on a porous lava from Volvic, France, for which 3D imaging reveals a pronounced pore preferred orientation. Cylindrical samples were cored with the pore major axes parallel (VBY) and perpendicular (VBZ) to the maximum principal stress and permeability measurement direction. Triaxial deformation experiments show that VBY samples require significantly higher stress to initiate inelastic deformation. Post-deformation imaging reveals oblique compaction bands in VBY samples and diffuse, sub-perpendicular bands in VBZ samples, with microcracking being more abundant in VBZ samples, indicating that compaction localization is strongly influenced by pore preferred orientation. Despite similar porosities, initial permeability in VBY samples is an order of magnitude higher than in VBZ samples. Upon deformation, microcracking in VBZ samples reduces flow-path tortuosity and leads to an order-of-magnitude permeability increase, whereas permeability in VBY samples increases negligibly. Our findings demonstrate that preferred pore orientation affects strain localization, mechanical behavior, and fluid transport in lava, with implications for volcanic risk assessment and geothermal energy extraction.
Intraplate fault systems frequently exhibit a kinematic paradox wherein deep instrumental seismicity indicates strike-slip deformation, while shallow Quaternary records are dominated by reverse faulting. Prevailing paradigms routinely attribute this discrepancy to a temporal evolution of the regional stress field, while overlooking depth-dependent structural and rheological controls. To address this, we investigate the southeastern Korean Peninsula by integrating a comprehensive Quaternary fault-slip dataset with contemporary stress tensors inverted from deep focal mechanisms. Our results confirm a deep, contemporary strike-slip stress field driven by E-W maximum horizontal stress, contrasting sharply with the dominant reverse-faulting regime recorded at the surface. By applying the deep contemporary stress tensor to shallow fault geometries via normalized slip tendency (TsN) analysis, we show that only 6.4% of the regional fault network is optimally oriented for reactivation. Crucially, structural cross-sections reveal a systematic steepening of fault dips with depth along the Yangsan and Ulsan systems, transitioning from shallow, low-angle reverse splays to steep, sub-vertical strike-slip faults. These findings challenge the temporal regime switch hypothesis. Instead, we demonstrate that this kinematic discrepancy is the result of profound up-dip kinematic and 3D strain partitioning inherent to transpressional positive flower structures. Deep, inherited master faults accommodate strike-slip motion, while localized stress deflections force shallow reverse reactivation. Consequently, relying exclusively on shallow paleoseismic trenching fundamentally mischaracterizes the large-magnitude strike-slip potential of deep fault cores. Future intraplate seismic hazard models must consider a unified 3D framework that accounts for structurally deflected, depth-partitioned rupture kinematics.
Anisotropy of magnetic susceptibility (AMS) is an important tool for understanding the magnetic fabric of rocks, such as depositional conditions or information about strain ellipsoid. This contribution presents AMS results and magnetic mineralogy analysis of sedimentary units from Central Precordillera (Agua Hedionda anticline) and Eastern Precordillera (Las Salinas Norte, Sur Wedge anticlines), located in the central Andes of Argentina.The AMS ellipsoids do not preserve a sedimentary magnetic fabric, suggesting that it has been overprinted by tectonic deformation. The AMS ellipsoid presents a clockwise deviation of the maximum susceptibility axes (K1) by an average of 14–34° from the strike of the bedding plane and a pattern of deviations of the minimum axes (K3) from the normals to the stratification planes. These features are interpreted as the product of non-coaxial deformation associated with interstratal shear under a transpressional regime. Integration of the AMS results with the structural characteristics of the study area provides a comprehensive model linking interstratal shear and transpressional deformation to the development of AMS fabrics. This approach indicates that the folds formed through flexural-slip mechanisms and allows the reconstruction of the relative timing of the structures involved. Accordingly, the Agua Hedionda and Las Salinas Norte anticlines exhibit AMS patterns consistent with a stage of non-coaxial deformation that postdated structural uplift. In contrast, the structural geometry of the Las Salinas Sur Wedge anticline, together with the observed AMS fabrics, suggests that folding developed synchronously with deformation during the regional transpressional event.
Previous studies have shown that syn- and post-orogenic deformation typically reactivates inherited rheologically weak zones, such as pre-existing regional nappe contacts or weak sedimentary layers. In contrast, partitioning of strain has been less quantified by observational studies analysing the internal structure of these weak zones. A good example to study such deformation is provided by the SE Dinarides Mountains of Europe, where deformation is localised within stratigraphic layers composed of deep-water sediments that were repeatedly (re-)activated by deformation episodes associated with changes in tectonic regimes. Here, Cretaceous–Paleocene foredeep turbidites presently exposed in the footwall of the East Bosnian-Durmitor thrust have been affected by successive stages of deformation with different kinematics or tectonic styles during the switch from Cretaceous–early Oligocene thrusting to Oligocene–Miocene extension and post-9 Ma transpression. To understand the mechanisms of strain partitioning, a field kinematic study was performed in Montenegro and the SE part of Bosnia and Herzegovina. The results demonstrate that although reactivation took place within this weak stratigraphic horizon, different styles of deformation are observed at higher resolution, controlled by structural inheritance and geodynamic evolution. While the thrusting (re-)activation was distributed and partitioned by different mechanisms along the orogenic strike, the subsequent extension was localised mostly in the southeast, controlled by slab tearing and detachment. Strike-slip structures crosscutting the northwestern parts of the studied area developed during the last stages of tectonic plate convergence, controlled by a more external localisation of transpressional indentation in the orogenic system. The overall analysis allows a better understanding of the regional-scale strain distribution based on deformation within rheologically weak zones, and provides a novel structural solution for the SE Dinarides.
Out-of-sequence thrust (OOST) faults build wedge taper by internal deformation in subcritical active orogenic wedges, such as the frontal Himalaya, south of the Main Central thrust. Despite their importance, OOST's internal architecture and kinematics remain relatively unknown. We describe the results of the detailed study of the Geil-Khola thrust, an out-of-sequence thrust within the Ramgarh thrust sheet of the Darjeeling Himalaya. We used (a) new structural and lithological data, (b) new petrographical, microstructural and grain-size data and (c) geomorphic indices to understand the deformation characteristics of the Geil-Khola thrust. Our study revealed that frictional sliding (high P, low T) microstructures overprinted dislocation creep (high P, high T) microstructures in protomylonites and mylonites formed within 350 °C, at ∼20 km depth. This indicates that the Geil-Khola thrust shear zone was first exhumed, then reactivated during an out-of-sequence deformation event, and remained neotectonically active. This poses a seismic hazard that can trigger earthquakes like the 2015 Nepal Gorkha earthquake. We also find that reactivation of the fault zone created weak zones, concentrating numerous landslides within the Geil-Khola thrust fault zone. We postulate that the rheology of the fault-zone rocks controls slope stability, leading to frequent rainfall-triggered landslides in the region. We anticipate that our integrated approach will improve understanding of the internal architecture and deformation mechanisms of out-of-sequence thrust faults, their temporal evolution over time, and the natural hazards they pose. Such studies will help better plan the establishment of infrastructure in active thrust belts worldwide, including the Himalaya.
This integrated structural study combining field mapping, anisotropy of magnetic susceptibility (AMS), petrological analysis, and gravity modelling of the Weinsberg Composite Pluton (WCP) provides new insights into magma emplacement mechanisms during post-collisional plutonism in the high-grade Moldanubian Zone of the Central European Variscides. The results indicate that pluton emplacement was strongly controlled by evolving tectonic stress regimes during the Variscan post-collisional evolution. New structural and AMS data, together with P-T modelling and published geochronological constraints, suggest that granitoid crystallisation proceeded in multiple stages under changing regional stress fields. The WCP records a transition from regional N–S compression (∼335–325 Ma) to NW–SE transtension (∼325–300 Ma), expressed in both internal fabric evolution and structural relationships. These evolving stress regimes controlled magma ascent pathways and emplacement depths, with younger intrusions emplaced at progressively shallower crustal levels. This progressive evolution resulted in a complex structural overprint during the final stages of pluton assembly and cooling. Gravity modelling further constrains the subsurface geometry of the pluton, revealing an asymmetrical root zone and deeper structural features related to the Variscan tectonic reorganisation. Overall, the WCP preserves evidence of incremental magma emplacement over ∼30 Myr (∼332–300 Ma). The evolution of the Weinsberg Composite Pluton can be subdivided into three principal tecto-magmatic stages: (a) early post-collisional phase (∼332–327 Ma), characterised by the emplacement of the oldest Weinsberg-type granitoids (WTG I); (b) middle post-collisional phase (∼326–318 Ma), dominated by the emplacement of the volumetrically dominant WTG II in the central part of the pluton, followed by intrusion of the younger WTG III into a partially crystalline granitic mush; and (c) late post-collisional phase (∼318–300 Ma), marked by emplacement of smaller intrusive bodies, including the Mauthausen granite (∼318 Ma) and Freistadt granodiorite (∼302 Ma), into already largely solidified plutonic units.
Geothermal energy represents an important, largely untapped resource for clean energy generation. Economic viability of geothermal resource production requires both heat and permeable flow pathways. Faults and fractures are essential fluid flow pathways in many geothermal systems, but permeability is dependent on subsurface stress. Understanding subsurface stress evolution during reservoir stimulation and production is critical for site selection, well planning, and economic feasibility. Changes in fluid pressure and temperature are key drivers of stress evolution that can modify subsurface permeability. Geomechanical modeling and resolved stress analysis are used here to understand stress drivers and resulting spatial and temporal stress state variations. Two-dimensional (2D) models incorporate realistic subsurface configurations with variable lithologies, pre-existing discontinuities, and initial stress/pore pressure conditions. Imposed temperature and pore pressure changes representing injection and production drive stress evolution. Slip and dilation tendency provide a rigorous means for assessing fluid flow potential of discontinuities under dynamic stress conditions. Simulations show that: (1) initial stress and rock properties (mechanical, thermal) control the spatial/temporal stress state evolution; (2) stress state changes (orientation and magnitude) extend beyond the regions where imposed temperature and pore pressure changes take place; and (3) slip and dilation tendency track changes in fracture response to stress evolution, and provide a means to identify evolving flow paths. Tracking evolution of slip and dilation tendency of faults and fractures during the stress evolution provides a means to better understand both short- and long-term viability of geothermal energy projects.
Regional seismic data from the Yinggehai Basin reveal two sets of normal faults (NW-SE and E-W) extensively developed across the eastern basin, covering an area of more than 10,000 km2. These faults are confined to the Miocene strata but terminate at different depths. Detailed fault interpretation and fault throw analysis indicate that they propagated as syn-depositional faults. Based on their seismic characteristics, the faulted strata in the study area were divided into four main units. On the basis of fault displacement history, three types of faults were identified on the top horizons of each unit. Type 1 faults initiated around a younger horizon, subsequently propagated downwards and offset this horizon. Type 2 faults are "newly formed faults" that initiated at or around that horizon. Type 3 faults initiated around an older horizon and offset that horizon during upward propagation. Correlation between new fault initiation at key horizons and surface slope changes suggests that normal faults initially formed sub-perpendicular to the local slope direction, indicating gravity as the primary driving mechanism. Lithological variations appear to have played a key role in confining the development of these faults to the stratigraphic interval spanning from the uppermost Sanya Formation to the lowermost Huangliu Formation. As the slope topography changed, the orientation of newly formed faults was influenced by both the changing slope direction and the presence of pre-existing fabrics. This multiphase faulting ultimately resulted in the development of both sub-parallel and intersecting fault patterns in map view.
Orogenic collapse creates fundamental structural architectures that profoundly influence later tectonic evolution. We document how Late Triassic post-collisional collapse structures governed Jurassic-Cretaceous contractional deformation in the Hongseong-Imjingang Belt (HIB), central-western Korean Peninsula. Integrated structural, microstructural, and geochronological data from Deokjeok, Soya, and Ijak islands reveal NW-striking extensional shear zones (Deokjeok and Ijak) that accommodated top-to-the-northeast displacement during Late Triassic gravitational collapse (similar to 231-217 Ma). These detachment systems exhumed mid- to lower-crustal rocks while syn-extensional basins formed in their hanging walls, accompanied by prolonged magmatism and thermal overprinting. Quantitative grain-shape fabric analysis of K-feldspar porphyroclasts reveals contrasting fabric geometries: preserved extensional fabrics record constrictional geometry while overprinted domains display flattening geometry. During Late Jurassic-Early Cretaceous contraction (similar to 150-130 Ma), these extensional structures underwent positive inversion. This created basement-involved thrust systems that emplaced crystalline slivers within Mesozoic basin fill and reversed the kinematics of inherited shear zones. The Soya Shear Zone exemplifies this structural inheritance, preserving domains with original extensional kinematics adjacent to strongly inverted zones. Our findings highlight that collapse-derived extensional structures act as persistent mechanical weaknesses, critically influencing how continental margins respond to changing tectonic regimes.
Mineral phase transformations strongly influence the physical properties and mechanical behavior of rocks in the Earth's interior. However, the extent to which deformation promotes polymorphic transformations and associated microstructural damage at low temperatures is poorly known. Here, we examine deformation-induced phase transformation in protoenstatite (Mg2Si2O6) polycrystals using micro-Vickers indentation. We found that the metastable protoenstatite transforms into clinoenstatite during deformation, with the clinoenstatite fraction increasing systematically with applied load. Following indentation, the clinoenstatite fraction increases over time, converging on approximate to 60%-80% within 6 months. The transformation is spatially heterogeneous, occurring preferentially along indentation edges, and is accompanied by volumetric strain, intragranular misorientation, and localized microcracking. An apparent reduction in enstatite grain size results from the preferential transformation of larger protoenstatite grains. Although microhardness decreases with increasing clinoenstatite fraction, all hardness data follow a single depth-dependent scaling relationship consistent with the indentation size effect. We interpret the time-dependent increase in clinoenstatite as the continued post-indentation progression of a shear-induced PEn-CEn transformation, promoted by lattice strain that accumulated during indentation. The results demonstrate that shear-induced polymorphic transformation in enstatite promotes microstructural damage and strain redistribution at low temperatures, highlighting its potential role as a microphysical mechanism for deformation in pyroxene-bearing lithologies.
