Fault Zone Architecture Research Articles

Fault zone architecture and hydraulic properties characterization using earth tide analysis

Fault zone architecture and hydraulic properties, which determine subsurface fluid flow and storage, have received long-term research attention from hydrogeologists. Current field experimental techniques and numerical simulation methods are limited by the scale and complexity of the investigations and are inadequate for deep subsurface fault zones. Estimating the internal rock properties and structures of fracture zones requires a combination of methods from many different fields. In recent years, groundwater tidal response methods have been widely used to characterize subsurface hydrogeomechanical properties because of their advantages of being in situ and low cost. In this study, tidal analysis was employed for a well within the Blue Ridge Fault Zone, and we integrated the fracture zone properties revealed by a single well. Well W-03 revealed an aquifer with an overall permeability of ∼ 10–14 m2 and hydraulic diffusivity of 0.24 m2⋅s−1. The fault damage zone had a permeability of 10–14 ∼ 10–13 m2 while the host rock had a permeability of 10–16 ∼ 10–14 m2. In addition, we found that the heterogeneity of specific storage affects the assessment of poroelasticity parameters, we offer a method to estimate these parameters without relying on specific storage, yielding porosity estimates ranging from 0.05 to 0.06, aligning with field test results. Furthermore, the estimated dip and strike of the fault zone were also consistent with the outcrop and borehole observations.

Normal fault architecture, evolution, and deformation mechanisms in basalts, Húsavik, Iceland: Impact on fluid flow in geothermal reservoirs and seismicity

Faults within layered basaltic sequences significantly influence hydrothermal fluid flow in shallow geothermal reservoirs and potentially during CO2 sequestration and storage. Nevertheless, their characterization regarding fault zone architecture, fluid flow, deformation mechanisms, and seismic potential remains underdeveloped. This study addresses this gap by integrating structural and microstructural observations with X-ray diffraction analyses of exposed normal-transtensional faults associated with the seismically active Húsavík-Flatey Fault in the Tjörnes Fracture Zone, Northern Iceland. Our findings demonstrate that the evolution of basalt-hosted normal-transtensional faults progresses through distinct stages: (1) low-displacement fault propagation from pre-existing cooling joints; (2) fault linkage via dilational jogs; (3) damage zone/fault core growth through brecciation and cataclastic processes; (4) shear localization along sharp slip surfaces; and (5) smearing of volcaniclastic interbeds along the principal fault plane. Evidence of shear localization, truncated clasts, and hydrothermal breccias/veins suggests repeated seismic slip events facilitated by overpressured fluids. Conversely, the presence of clay-rich foliated cataclasite indicates aseismic slips during interseismic periods. Slip along fault jogs, bends, geometric irregularities, and orientation changes causes the dilatant opening of the fault planes and extensional horsetail fractures at fault tips. These structures create main tabular zones for lateral movement of hydrothermal fluids parallel to the fault strike in shallow geothermal reservoirs situated in active extensional-transtensional tectonic settings. In addition, the dilational jogs and the intersection of horsetail veins with the hosting faults may define linear zones of high structural permeability and intense localized fluid flow parallel to the σ2 paleostress orientation and finally mineral precipitation. The results of this study can be utilized to improve models of geothermal fluid flow for enhanced recovery in basaltic reservoirs and assess seismic risk in basaltic faults.

Crustal Electrical Anisotropic Structure of the Altyn Tagh Fault in the Subei Area, NW China: Implications for Fault Zone Architecture

Crustal Electrical Anisotropic Structure of the Altyn Tagh Fault in the Subei Area, NW China: Implications for Fault Zone Architecture

Seismotectonics and landslides of the NE border of the Calabrian Arc (Southern Italy)

ABSTRACT The north-eastern border of CA represents the accretionary system developed during its continuous collision with Apulian block. The tardive oblique tectonic component is highlighted by pervasive NW-SE crustal transpressive left-lateral strike slip faults developed from the Upper Miocene to Quaternary tectonic stages. The seismotectonic and landslides Main Map represents the update of the main faults, the style of their arrangement and kinematics, the seismotectonic features and landslide area distribution with risk implications. Seismotectonics allows us to recognise a vertical crustal zonation with the distribution of earthquakes and their focal mechanism solution. The prevalent modality of ruptures depicts a mainly transcurrent and compressive solution in mid-lower crust portions respectively. This data, together with the historical seismicity analysis of the area, lead to consider the NW-SE strike-slip faults improved on the Main Map, as recent and tectonically active. The areal distribution of landslide and susceptibility of the area is due to the interaction between seismotectonic peculiarity, lithological typologies and fault zone architectures. Landslides and flood phenomena of this sector involve many municipalities, major and minor infrastructures and economic and social activities. The landslides and the faults have been identified, mapped and classified, originally at detail scale and, then, represented at 1:50,000 scale in the Main Map, included as supplementary material. The geo-structural and geomorphological data were analysed in a geographic information system allowing data management and implementation. The work presents an updated knowledge framework of risk conditions of the study zone, available to plan and reduce the fundamental elements that determine the landslide and seismic risks in this region.

Strike‐slip fault zone architecture and its effect on fluid migration in deep‐seated strata: Insights from the Central Tarim Basin

AbstractThe internal fault architecture is crucial in assessing the significance of faults in fluid migration. The development of overlapping zones between segments and subsidiary structures is characteristic of a strike–slip faults. However, their internal architectures and roles in fluid migration are still poorly understood. The Tarim Basin's recently identified strike–slip faults imply that the petroleum resource is hosted in caves that were formed by subsequent dissolution after the formation of the fault zones in carbonate rocks, indicating that the internal fault architecture may be closely linked to the accumulation of petroleum. We investigated the architecture of the strike–slip fault zone using field, geochemical, seismic and well‐logging data. The results revealed that the strike–slip faults contain flower‐like structures in their vertical profiles and an en échelon and ‘X’ conjugate pattern in their horizontal slices. The fault core may become more complex because of the flower structure as fault breccia, slip surfaces, hydrothermal veins, dissolved pores and caves develop, and the damage zone contains multiple stages of fractures with high dip angles. Compared with ‘X’ pattern conjugate faults, NE‐trending strike–slip faults have a more developed and connected fault zone. The fault core acts as a fast conduit for fluid transport and experiences significant elemental losses, and the elemental variations in the damage zone may relate in long‐term and relatively lower‐level fluid–rock interactions. Three fault zone architecture models were created, namely, a releasing bend, a restraining bend and a single segment, and their controlling impacts on fluid migration were addressed accordingly. Our findings imply that fluid migration and accumulation are more favourable at the releasing bend than at the restraining bend and single segment.

A multi-method investigation of the permeability structure of brittle fault zones with ductile precursors in crystalline rock

Brittle faults and fault zones are among the most hydraulically active elements in predominantly impermeable crystalline host rock. They pose a significant challenge to underground infrastructure like nuclear waste repositories. Brittle fault zones frequently occur along pre-existing ductile shear zones as they introduce weakness planes in the rock.Four brittle fault zones of ductile origin were analyzed in the Rotondo Granite at the “BedrettoLab” in the Swiss Central Alps. Scanline mapping, rock sampling and permeability measurements using three different methods provide detailed insights into the heterogeneous fault zone architecture and hydrogeology. Average intact rock permeability is in the range of 10−19 to 10−17 m2. Fluid flow is channeled into single open or partially mineralized fractures of, at point-scale, up to 10−14 m2, demonstrated by selective gas probe permeameter measurements and borehole hydraulic testing. Reduced permeabilities have been measured in close proximity to these permeable features, indicating alteration of and around the fracture walls.

A study on the effects of fault architecture on fluid circulation in the Gediz Graben by the finite volume method

It is widely recognized that in geothermal fields, meteoric water infiltrates deep into the subsurface of the earth and then travels through cracks and fractures, returning to the surface as it becomes heated. The patterns of fluid flow are primarily determined by the interaction between forces driven by gravity and pressure gradients. The ultimate forms of fluid flow patterns are primarily determined by the anisotropies of permeability associated with fault zones. In this study, a series of numerical simulations utilizing the finite volume approach were conducted to investigate the effects of fault zone architecture on fluid flow patterns and temperature distributions. Four distinct types of fault zone architecture were created in the simulations, including localized barrier, combined conduit-barrier, localized conduit, and distributed conduit. The results revealed that fault zone architecture has only a minor effect on fluid flow velocities and temperature distributions, except in cases along faults with very high permeabilities. The simulations suggest that this type of 2-D numerical modeling can be easily applied and utilized in other faulted geothermal systems.

Architecture of intraplate strike-slip fault zones in the Yanchang Formation, Southern Ordos Basin, China: Characterization and implications for their control on hydrocarbon enrichment

Commercial discoveries in the Ordos, Tarim, and Sichuan Basins of China have aroused widespread interest in strike-slip fault-controlled reservoirs. The productivity of different wells, however, can vary within one strike-slip fault zone, suggesting that variability in fault zone architecture controls hydrocarbon enrichment. To date, very few studies have finely and quantitatively explored fault zone architecture in the southern Ordos Basin, inhibiting hydrocarbon development. With the goal of understanding the architecture variability and scale of strike-slip fault zones in the subsurface, we carried out a refined and quantitative study of faults in the Jinghe Oilfield in the southern Ordos Basin by integrating outcrops, wellbore cores, well logs, and 3D seismic data. We quantitatively constrained the subsurface damage zone boundaries according to the slope changes in the cumulative CFI (comprehensive fracture index log) curves at logging scale and the cumulative FSI (fault shape index seismic attribute) curves at seismic scale. We proposed a model highlighting the effects of fault segmentation, architectural configuration, and damage zone asymmetry on the variability of fault zone architecture. Results showed that strike-slip faults can be divided into transtensional, pure strike-slip, and transpressional segments along the fault strike, with transtensional and pure strike-slip segments dominant in the Jinghe Oilfield. These segments exhibited six types of geometric structures, forming complex architectures with multi-fault cores and multi-damage zones surrounding the main and secondary faults. Each segment was further complicated by three configurations of gouge, breccia, and damage zones along the fault dip. The damage zone asymmetry further increased the architecture variability, with the fracture density and damage zone width of the hanging wall being greater than those of the footwall. Quantitative analysis showed that the fault zone width was the greatest for transtensional segments, intermediate for transpressional segments, and the lowest for pure strike-slip segments. And there was a positive linear correlation between the fault core width and the fault zone width (including the fault core and damage zone). Fault segmentation and damage zone asymmetry affected the hydrocarbon enrichment along the fault strike and dip within one strike-slip fault zone. We conclude that damage zones in transtensional segments, particularly in hanging walls, are primary targets for hydrocarbon development in the southern Ordos Basin.

How pore pressure changes affect stress and strain in fault zones with complex internal structure – Insights from hydromechanical modelling

Faults not only affect the hydraulic regime and the local stress state, but are also prone to reactivation by pore pressure changes, potentially leading to induced seismicity, among others. In the present study, a generic finite element model with a detailed fault zone description including damage zones and a fault core with highly strained rock and fault core lenses of host rock is used to study the stress and strain evolution due to pore pressure changes. Special focus is on the temporal evolution of the effective stresses within the fault zone and the strain partitioning between its various subunits. The coupled hydromechanical simulations comprise five different scenarios covering pore pressure increase due to fluid injection and pore pressure decrease due to fluid production for permeable and semipermeable fault zones as well as different distances between the fault core lenses. Modelling results show that the variable hydromechanical properties of the different fault zone rocks lead to spatially and temporarily highly heterogeneous stress and strain patterns within the fault zone and its surrounding. In particular, substantial stress rotations of up to 45° and differently in the various subunits of the fault zone are found in response to pore pressure changes. Plastic straining as an indication for potential fault reactivation is observed particularly in the injection scenarios. Models provide a template for modifications with respect to fault zone architecture and hydromechanical properties. They can also serve as starting point to define bulk fault zone properties for reservoir-scale simulations which cannot capture the complexities of a fault zone regarding geometry and material distribution.

Influence of rheologically weak layers on fault architecture: insights from analogue models in the context of the Northern Alpine Foreland Basin

We present a series of analogue models inspired by the geology of the Zürcher Weinland region in the Northern Alpine Foreland Basin of Switzerland to explore the influence of rheological weak, i.e. (partially) ductile layers on the 3D evolution of tectonic deformation. Our model series test the impact of varying weak layer thickness and rheology, as well as different kinematics of an underlying “basal fault”. Model analysis focuses on deformation in the weak layer overburden and, uniquely, within the weak layer itself. We find that for low to moderate basal fault displacements, the above-mentioned parameters strongly influence the degree of coupling between the basal fault and the weak layer overburden. Coupling between the basal fault and overburden decreases by reducing the strength of the weak layer, or by increasing the weak layer’s thickness. As a result, basal fault displacement is less readily transferred through the weak layer, leading to a different structural style in the overburden. By contrast, increasing the amount, or rate, of basal fault slip enhances coupling and leads to a more similar structural style between basal fault and overburden. Moreover, dip-slip displacement on the basal fault is more readily transferred to the overburden than strike-slip displacement of the same magnitude. Our model results compare fairly well to natural examples in the Northern Alpine Foreland Basin, explaining various structural features. These comparisons suggest that rheological weak layers such as the Jurassic Opalinus Clay have exerted a stronger control on fault zone architecture than is commonly inferred, potentially resulting in vertical fault segmentation and variations in structural style. Furthermore, the novel addition of internal marker intervals to the weak layer in our models reveals how complex viscous flow within these layers can accommodate basal fault slip. Our model results demonstrate the complex links between fault kinematics, mechanics and 3D geometries, and can be used for interpreting structures in the Alpine Foreland, as well as in other settings with similar weak layers and basal faults driving deformation in the system.

Shear zone evolution and the path of earthquake rupture

Abstract. Crustal shear zones generate earthquakes, which are at present unpredictable, but advances in mechanistic understanding of the earthquake cycle offer hope for future advances in earthquake forecasting. Studies of fault zone architecture have the potential to reveal the controls on fault rupture, locking, and reloading that control the temporal and spatial patterns of earthquakes. The Pofadder Shear Zone exposed in the Orange River in South Africa is an ancient, exhumed, paleoseismogenic continental transform which preserves the architecture of the earthquake source near the base of the seismogenic zone. To investigate the controls on earthquake rupture geometries in the seismogenic crust, we produced a high-resolution geologic map of the shear zone core mylonite zone. The core consists of ∼ 1–200 cm, pinch-and-swell layers of mylonites of variable mineralogic composition, reflecting the diversity of regional rock types which were dragged into the shear zone. Our map displays centimetric layers of a unique black ultramylonite along some mylonite interfaces, locally adding to thick composite layers suggesting reactivation or bifurcation. We present a set of criteria for identifying recrystallised pseudotachylytes (preserved earthquake frictional melts) and show that the black ultramylonite is a recrystallised pseudotachylyte, with its distribution representing a map of ancient earthquake rupture surfaces. Pseudotachylytes are most abundant on interfaces between the strongest wall rocks. We find that the geometry of lithologic interfaces which hosted earthquakes differs from interfaces lacking pseudotachylyte at wavelengths of ≳ 10 m. We argue that the pinch-and-swell structure of the mylonitic layering, enhanced by viscosity contrasts between layers of different mineralogy, is expected to generate spatially heterogeneous stress during viscous creep in the shear zone, which dictated the path of earthquake ruptures. The condition of rheologically layered materials causing heterogeneous stresses should be reasonably expected in any major shear zone, is enhanced by creep, and represents the pre-seismic background conditions through which earthquakes nucleate and propagate. This has implications for patterns of earthquake recurrence and explains why some potential geologic surfaces are favored for earthquake rupture over others.

Along-strike architectural variability of an exhumed crustal-scale seismogenic fault (Bolfin Fault Zone, Atacama Fault System, Chile)

Fault zone architecture and its internal structural variability play a pivotal role in earthquake mechanics, by controlling, for instance, the nucleation, propagation and arrest of individual seismic ruptures and the evolution in space and time of foreshock and aftershock seismic sequences. Nevertheless, the along-strike architectural variability of crustal-scale seismogenic sources over regional distances is still poorly investigated. Here, we describe the architectural variability of the >40-km-long exhumed, seismogenic Bolfin Fault Zone (BFZ) of the intra-arc Atacama Fault System (Northern Chile). The BFZ cuts through plutonic rocks of the Mesozoic Coastal Cordillera and was seismically active at 5–7 km depth and ≤ 300 °C in a fluid-rich environment. The BFZ includes multiple altered fault core strands, consisting of chlorite-rich cataclasites-ultracataclasites and pseudotachylytes, surrounded by chlorite-rich protobreccias to protocataclasites over a zone up to 60-m-thick. These fault rocks are embedded within a low-strain damage zone, up to 150-m-thick, which includes strongly altered volumes of dilatational hydrothermal breccias and clusters of epidote-rich fault-vein networks at the linkage of the BFZ with subsidiary faults. The strong hydrothermal alteration of rocks along both the fault core and the damage zone attests to an extensive percolation of fluids across all the elements of the structural network during the activity of the entire fault zone. In particular, we interpret the epidote-rich fault-vein networks and associated breccias as an exhumed example of upper-crustal fluid-driven earthquake swarms, similar to the presently active intra-arc Liquiñe-Ofqui Fault System (Southern Andean Volcanic Zone, Chile).

Complex structural fault system and distributed deformation across the Big Bend of the Red River fault, Yunnan, China

To understand why the 130 km-long restraining bend segment (Big Bend) of the Red River fault (RRF) in Yunnan lacks historical ruptures and has a low geologic slip-rate, deep seismic profiles, relocated earthquakes and GPS-measured velocities are integrated to investigate the fault zone architecture and deformation associated with this segment. The Middle to Late Cenozoic southward extrusion of the Chuan-Dian Fragment, along with the northeastward blocking of the Yunxian Convex, have curved the RRF and its sub-parallel faults towards the southwest, forming the Big Bend. Branch faults split off the Big Bend RRF at depths between the upper lithosphere-mantle and the lower middle crust; splaying upward and sideward, they either connect/reactivate preexisting faults or become blind faults in the middle to upper crust, resulting in seismically active Lancang, Blind Wuliangshan, Qujiang and similar faults on both sides of the Big Bend. Together these faults form a complex fault system that shows a trans-crustal flower structure with a ∼ 350 km width at the surface. Across this fault system GPS-measured dextral-shearing at ∼8 mm/a and horizontal-shortening at ∼3.5 mm/a represent the deep deformation rate of the Big Bend RRF under the flower structure. 85% ± of the deep shearing and almost all of the deep shortening are partitioned onto branch and subsidiary faults bounding and within the flower structure, and only 15% ± of the deep shearing to the Big Bend RRF in the brittle crust, due to the complex fault system architecture and the existence of a large-scale asperity on the main RRF in the brittle crust. Across this fault system the deformation partitioning mode is nearly distributed. Active faulting and prehistoric surface rupture suggest that the Big Bend RRF has the capability to produce large earthquakes at recurrence intervals of millenniums, much longer than the local written earthquake history of 300–500 years.

Fault zone architecture and lithology-dependent deformation mechanisms of the Himalayan frontal fold-thrust belt: Insights from the Nahan Thrust, India

Brittle shallow crustal faults typically develop a complex fault zone architecture with distinct structural domains that display diverse microstructures, mineralogy, and deformation mechanisms. The development of such domains is typically controlled by the strength and composition of the protoliths, physical conditions of deformation, fluid ingress, and diachronous fault growth in response to stress accumulation and co-seismic slip. Herein, we studied the microstructure-mineralogy-kinematics of fault rocks in the Nahan Thrust, in the vicinity of the Main Frontal Thrust that represents a tectonically active zone in the Himalayan orogen. The Nahan Thrust is characterized by alternating red and gray gouge layers, and a single black gouge layer. Our results from electron microscopy and X-ray diffractometry indicate that the protolith of the red gouge layers is argillaceous sandstone, whereas that of the gray and black gouge layers is sandstone. Microstructures suggest an initially distributed deformation (aseismic creep), followed by a protracted brittle deformation event, and a later aseismic creep stage. The brittle stage is marked by progressive localization of stress, fracture development, cataclasis, frictional sliding, and seismic slips. The black gouge layer acted as the principal slip zone and exhibited ultrafine bands of micrometer-scale slip zones with vapor escape structures and clay clast aggregates, indicating seismic faulting and frictional heating during seismic slips. The preferential seismic rupture nucleation in the black gouge layer indicates a strong lithological dependence on seismic slip in the Nahan Thrust. We also conclude that heterogeneity within the Nahan Thrust resulted from primary lithological variations of the protoliths.

Geometry and scale property of a gold-silver-bearing vein system associated with an oblique-slip fault zone at Gasado Island, Korea

Gasado Island, on the southwestern Korean Peninsula, comprises c. 50 Ma volcanic rocks that are cut by faults and fractures hosting low-sulfidation epithermal Au-Ag mineralization. In this study, we examined the relationship between the fault-fracture networks and auriferous vein systems to better understand the role of brittle structures in controlling hydrothermal fluid flow. Geometrical and fractal analyses were performed to express and quantitatively compare the vein geometries and thickness population. The results were as follows: (1) The geometrical analysis revealed a strong relationship between Au-Ag-bearing quartz veins and NE-SW-striking, oblique sinistral-normal faults, indicating that these structures are a key control on mineralization. (2) Box-counting fractal analysis applied to veins hosted by fault zone demonstrated that the D-values (fractal dimensions) are directly proportional to vein density – indicating that damage zones control vein density. (3) The linear traverse line analysis yielded cumulative frequency plots of vein thicknesses that showed obvious changes in the slope, which are reflected in the variable D-values. The cause of this slope variability is interpreted to be the presence of hybrid veins that fill secondary fault planes, which disturbs the original linear relationship shown by cumulative thickness distribution data. This study shows that vein density and physical properties are strongly controlled by the types, or existence, of fault damage zones. Understanding fault zone architecture may assist with the prediction of locations that concentrate mineralizing fluids and may benefit the initial stage of mineral exploration.

The effects of fault-zone architecture, wall-rock competence and fluid pressure variations on hydrothermal veining and gold mineralization along the Sheba Fault, Barberton Greenstone Belt, South Africa

Hydrothermal vein systems illustrate the spatial and temporal evolution of structural permeabilities along the Sheba Fault, the first-order fault structure in the Sheba-Fairview complex of gold mines in the Archaean Barberton greenstone belt. The fault juxtaposes competent Moodies Group metapsammites against rheologically weaker metaturbidites and intercalated serpentinites of the Fig Tree and Onverwacht groups. Strain localization into the weaker lithologies results in the pronounced asymmetric zonation of the fault into a fault core and a fault-damage zone. The fault core in the weaker Fig Tree and Onverwacht lithologies is characterized by the transposition of earlier bedding and pervasive foliation development. In contrast, hydrothermal veining, alteration and mineralization are confined to competent Moodies Group rocks that form the tens of meter wide damage zone in the immediate footwall of the fault core. Vein systems of the Sheba West ore body disclose the multistage evolution of the fluid flow system along the fault. An early phase of crackle-breccia formation and associated carbonate (dolomite) alteration is regionally widespread and largely pre-dates the main phase of gold mineralization. Crackle breccias are overprinted by successive sets of subhorizontal and subvertical extension (mode I) veins, steeply inclined shear veins and moderately-dipping late-stage cataclasites. The orientation and controls of the vein sets demonstrate their formation during (1) progressive NW-SE directed subhorizontal shortening, and (2) waning fluid pressures from lithostatic values (mode I veins) to sub-lithostatic fluid pressures (mode II shear veins and cataclasites). The economic-grade mineralization of the Sheba West ore body relates to an undulation of the controlling Sheba Fault that corresponds to a releasing bend during top-to-the-NW thrusting. The vein systems along the Sheba Fault illustrate the controls of permeability structures as a result of lithological and rheological contrasts, geometric variations (fault bends) and fluid-pressure fluctuations during faulting.

Architecture of strike-slip fault zones in the central Tarim Basin and implications for their control on petroleum systems

The strike-slip fault zones (SSFZs) of the central Tarim Basin have great potential for hydrocarbon exploration and development. They can both serve as hydrocarbon migration paths and host hydrocarbons. However, the complexity of SSFZ varies significantly along the fault strike, and strike-slip fault-controlled deeply buried carbonate reservoirs are often extremely heterogeneous, increasing exploration risks. Here, we reconstructed the architecture and setting of the SSFZ of the central Tarim Basin and analyzed the complex and heterogeneous hydrocarbon system that characterizes this exploration area.The integrated seismic and well datasets show that the development of SSFZs has led to the fracturing of tight carbonates and enhancements in the karstification process, secondary porosity and permeability due to the presence of open fractures. Based on the vertical height of the fault in the section and continuity, the SSFZs in the central Tarim Basin have been classified as 1st-order or 2nd-order faults. The 1st-order faults are found in the repeatedly reactivated vertical SSFZ, which is composed of sub-vertical strike-slip faults in the Paleozoic strata and en-echelon normal faults in the shallow layer. The 2nd-order faults develop only in Paleozoic strata without reactivated features. Continuous through-going, hard-linked 1st-order faults are a more valuable target for future exploration than soft-linked 2nd-order faults because 1st-order faults control larger-scale fracture-cavity reservoirs, and their fault activity periods correspond to hydrocarbon accumulation periods. We classified damage zones as wall-, and linking-damage zones, based on their location around the fault. There are two types of linking-damage zones, extensional steps and contractional steps, which develop in the extensional and contractional quadrants of the fault segment, respectively. Linking-damage zones are associated with more structural complexity than narrow and straight wall damage zones. Fractures in the contractional steps are generally closed and have low conductance, which is not suitable for fracture-cavity trap formation and hydrocarbon migration. Because opening-mode fracture development and permeability enhancement are greatest at the extensional steps, these locations are more likely to be important foci for strike-slip fault-controlled carbonate reservoirs. Our findings, in combination with previous research, indicate that fault zones in the central Tarim Basin, act as combined conduit-barrier systems, with damage zones typically featuring highly conductive fracture networks and fault cores acting as seals, resulting in heterogeneous reservoir distributions.

Seismicity recorded in hematite fault mirrors in the Rio Grande rift

AbstractExhumed fault rocks provide a textural and chemical record of how fault zone composition and architecture control coseismic temperature rise and earthquake mechanics. We integrated field, microstructural, and hematite (U-Th)/He (He) thermochronometry analyses of exhumed minor (square-centimeter-scale surface area) hematite fault mirrors that crosscut the ca. 1400 Ma Sandia granite in two localities along the eastern flank of the central Rio Grande rift, New Mexico. We used these data to characterize fault slip textures; evaluate relationships among fault zone composition, thickness, and inferred magnitude of friction-generated heat; and document the timing of fault slip. Hematite fault mirrors are collocated with and crosscut specular hematite veins and hematite-cemented cataclasite. Observed fault mirror microstructures reflect fault reactivation and strain localization within the comparatively weaker hematite relative to the granite. The fault mirror volume of some slip surfaces exhibits polygonal, sintered hematite nanoparticles likely created during coseismic temperature rise. Individual fault mirror hematite He dates range from ca. 97 to 5 Ma, and ~80% of dates from fault mirror volume aliquots with high-temperature crystal morphologies are ca. 25–10 Ma. These aliquots have grain-size–dependent closure temperatures of ~75–108 °C. A new mean apatite He date of 13.6 ± 2.6 Ma from the Sandia granite is consistent with prior low-temperature thermochronometry data and reflects rapid, Miocene rift flank exhumation. Comparisons of thermal history models and hematite He data patterns, together with field and microstructural observations, indicate that seismicity along the fault mirrors at ~2–4 km depth was coeval with rift flank exhumation. The prevalence and distribution of high-temperature hematite grain morphologies on different slip surfaces correspond with thinner deforming zones and higher proportions of quartz and feldspar derived from the granite that impacted the bulk strength of the deforming zone. Thus, these exhumed fault mirrors illustrate how evolving fault material properties reflect but also govern coseismic temperature rise and associated dynamic weakening mechanisms on minor faults at the upper end of the seismogenic zone.

3D fault-zone architecture across the brittle–plastic transition along the Median Tectonic Line, SW Japan: Fault-rock characterization

We determine the features and distribution of fault rocks along the Median Tectonic Line (MTL), SW Japan, to establish the 3D architecture of the fault zone across the brittle–plastic transition. Cataclasites exposed close to the lithological boundary fault (the MTL) can be divided into those formed by sinistral faulting at temperatures of ∼300 °C and those formed by dextral faulting at ∼250 °C. Mylonites distributed to the north of the cataclasites were formed by sinistral faulting and can be divided into lower-temperature mylonite (L-T mylonite) close to the MTL and higher-temperature mylonite (H-T mylonite) distant from the MTL, where deformation temperatures were lower and higher than 400 °C, respectively. Structures formed by sinistral faulting are oblique to those formed by dextral faulting, indicating that the former structures are older than the latter. Structures formed by sinistral faulting underwent deformation around the brittle–plastic transition. Thus, the MTL fault zone records deformation through a crustal section. Microstructural observations suggest that the differential stress just below the brittle–plastic transition (L-T mylonite) was ∼200 MPa and that this value may not change substantially in the deep crust (H-T mylonite).

High resolution architecture of neotectonic fault zones and post-8-Ma deformations in western Hungary: Observations and neotectonic characteristics of the fault zone at the Eastern Lake Balaton

Lake Balaton (Hungary), the largest lake in Central Europe, formed by the interplay of tectonic and external forces. Its shallow water and young soft sediments together allow to carry out ultra-high resolution reflection seismic surveys to investigate shallow tectonic structures and near surface stratigraphy at depth of ca. 0–30 m. To document neotectonics of the eastern lake basin and its onshore surroundings to the East, we have integrated new ultra-high-resolution seismic-reflection data with deeper penetrating multichannel lake and land seismic profiles, regional geological, geophysical and seismicity data, and geomorphological observations. Combined use of these different data sets provided an opportunity to understand better, how these different types and scales of structural features are linked. In our study area, late middle to late Miocene formations compose a deepening than shallowing sedimentary cycle from terrestrial clastic through offshore marl and deltaic sequence filling up the basin between ca. 8.6 to 7.5 Ma. The deltaic sequence is unconformably overlain by erosional remnants of late Pleistocene fluvial deposits and a mantle of latest Pleistocene to Holocene lake mud. Post-early Miocene deformation history involved two phases; a latest middle to early late Miocene transtension and a dominantly strike-slip regime with locally transpressional or transtensional character. The latter neotectonic phase reactivated the earlier faults and resulted in the propagation of 4 major fault zones across the complete late Miocene sequence. The resulting young faults show segmented geometry, stepovers, and connecting splays. The deformation also induced the modest but penetrative folding of the highest preserved Miocene deltaic sequence. The change in deformation style could happen during the late-stage of delta formation, at ca. 8 Ma although a slightly younger timing is not excluded. Faults imaged offshore apparently do not offset the Holocene lacustrine mud by discrete fractures, but the improved distribution map of recent seismicity and morphotectonic indices along their onshore continuations suggests that several segments of the fault pattern are still active, and might be capable of generating earthquakes. Integration of these different data provided an opportunity to understand better, how these different types and scales of structural features are linked and evolved one after another. • 4 fault zones and 4 types of folds were mapped below Lake Balaton. • Post-8-Ma neotectonic faults are the reactivation of earlier Miocene structures. • The mapped fault pattern serves as an example for the complexity of segmented fault zone. • Faults below the lake continue onland form a coherent structural pattern. • Recent fault activity is suggested by the new seismicity map.