Fault Core Research Articles

The Effect of Shear Displacement and Wear on Fault Stability: Laboratory Constraints

AbstractFaults are composed of a damage zone and a fault core with wear material that has accumulated during sliding. Fault zone width and structural complexity have been suggested as key factors that dictate the stability of fault slip and the transition from stable to unstable motion. Here we address the role of fault rock wear rate and the presence of gouge on the stability of frictional sliding. We performed biaxial shear experiments on quartz‐rich bare rock surfaces having different resistances to abrasion (i.e., wear production with slip), as well as simulated quartz gouge. To characterize the influence of shear displacement on fault stability, constant velocity and velocity step experiments were performed to displacements up to 30 mm. Mechanical data are analyzed within the rate‐and‐state framework, and integrated with post‐mortem microscopic analyses of the samples. For initially bare surfaces a shear displacement threshold is required to transition from stable to unstable sliding. Stick‐slip events (laboratory earthquakes) evolve systematically as a function of fault slip. Shear displacement has a clear effect on the rate‐and‐state parameters (a‐b) and Dc. For all our tests, (a‐b) decreases with increasing shear displacement, implying enhanced velocity weakening and potential unstable behavior for larger fault slip. For high wear rates and simulated gouge, Dc decreases with increasing slip. However, for low wear rate faults, changes in Dc are negligible. Our results demonstrate that, fault stability varies systematically with fault slip and in particular that shear displacement and strain localization are dominant parameters controlling fault stability.

The Effect of Clay Content on the Dilatancy and Frictional Properties of Fault Gouge

AbstractMature fault cores are comprised of extremely fine, low permeability, clay‐bearing gouges. Saturated granular fault materials are known to dilate in response to increases in sliding velocity, resulting in significant pore pressure drops that can suppress instability. Up to now, dilatancy has been measured predominantly in clay‐poor gouges. Clay minerals have low frictional strengths and, in previous experiments, even small proportions of clay minerals were shown to affect the frictional properties of a fault. It is important, therefore, to document in detail the impact of the proportion of clay on the frictional behavior and dilatancy of fault rocks. In this work, a suite of triaxial deformation experiments elucidated the frictional behavior of saturated, synthetic quartz‐clay (kaolinite) fault gouges at effective normal stresses of 60, 25, and 10 MPa. Upon a 10‐fold velocity increase, gouges of all clay‐quartz contents displayed measurable dilatancy with clay‐poor samples yielding comparable changes to previous studies. Peak dilation did not occur in the pure quartz gouges, but rather in gouges containing 10 to 40 wt% clay. The clay content of the simulated gouges was found to control the gouge frictional strength and the stability of slip. A transition occurred at ∼40 wt% clay from strong, unstably sliding quartz‐dominated gouges to weak but stably sliding clay‐dominated gouges. These results indicate that in a low permeability, clay‐rich fault zone, the increases in pore volume could generate pore‐fluid pressure transients, contributing to the arrest of earthquake nucleation or potentially the promotion of sustained slow slip.

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.

Fault damage zone and its effect on deep shale gas: Insights from 3D seismic interpretation in the southern Sichuan Basin, China

Fault damage zones affect the preservation and enrichment of shale gas. Most fault damage zone studies are based on directly observed outcrops. In this study, we provide an example of fault damage zone analysis using 3D seismic data of deep shales buried more than 3500 m in the Sichuan Basin, China. The 3D seismic variance attribute and the cumulative frequency method were used to define the background value and then estimate the damage zone widths of different sizes of faults. We found that the damage zone widths range from 80 m to 1570 m, and trend to stabilize with the increase in displacements. The scattering caused by the relay zone can be interpreted as segment linkage mode and damage superposition related to subsidiary faults. The damage intensity decays following a power law with distance from the fault core, in agreement with field observations and the distribution of seismic variance. Damage zone widths display a significant asymmetry across the fault, and the hanging wall damage zone is generally wider than the footwall damage zone. We discuss the impact of the fault damage zone on shale gas preservation, which is related to the sealing of the direct caprock. It appears that the seismic variance attribute is an effective tool for subsurface characterization of fault damage zones and can help identify the location of deep unconventional energy.

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.

Structures evolution along strike-slip fault zones: The role of rheology revealed by PIV analysis of analog modeling

This study investigates how rheology can influence the nucleation and evolution of strike-slip structures and, consequently, the structural architecture. To that end, we carried out a series of analog experiments using materials with different rheological behavior. The materials' properties, such as the angle of internal friction, cohesion, and density, were measured. The Particle Image Velocimetry (PIV) technique was applied to investigate the strain pattern on the surface, the amplitude of damage zones and fault core, the activeness/inactiveness of the structures over the evolution of strike-slip fault, and the earliest nucleation stage of fractures. Two sets of experiments were performed: i) single-layer experiments constituted by rheologically homogeneous material (quartz sand, dry clay, plaster powder, and a mixture of quartz sand and plaster powder), and ii) multilayer experiment using a heterogeneous stratified sequence of materials with different rheology. Except for the plaster powder model, during the first increments of deformation, the strain is widely distributed to give rise to a deforming zone with varying widths, where subsidiary structures nucleate. As deformation evolves, the deformation zone progressively narrows as the maximum strain concentrates along the main fault. Nucleation, development, and relative chronology of structures, such as R, R', P, Y-fractures, pull-apart basins, and positive and negative flower structures, varied according to the material used. The structural architecture of the dry clay and plaster powder models are more similar, with a greater quantity of smaller fractures than the other models. P-shears and pull-apart basins are also present. The quartz sand and mixture models developed a more complex damage zone and a higher topographic relief. The results emphasize that rheology is a controlling factor in the deformation zone architecture. Rheology controls fracture and fault arrangement, structure relative chronology, and the localization of contractional and extensional domains over all the stages of development of strike-slip tectonics.

Characterization of two-phase flow in a faulted sandstone

Fault zones present in some underground CO2 storage formations can impact on injectivity, storage capacity, and the path taken by migrating CO2, including possible leakage to overlying formations and the escape of a plume beyond the area of a demarcated storage lease licence. A thorough understanding of the 2-phase flow in fault zones is therefore essential to de-risk any storage location where faults could be present. Many previous studies have addressed top-seal risks from fault reactivation, but the influence of intra-reservoir faults on flow within the target storage formations has received less attention. Studies of intraformational faults in sandstones have typically used plugs cored from outcrops or from subsurface boreholes, where important aspects of the geological history such as the stress conditions during faulting, and the degree of post-faulting compaction and cementation may be poorly constrained. Moreover, only intact fault rocks can be cored and re-loaded under stress with this approach, so that it is very difficult to establish the interacting effects or rock fractures and cataclastic gouge in a single experiment using borehole/outcrop samples.This study investigates newly created fault rock in sandstone that has been deformed under controlled conditions and without the effects of secondary cementation. We used a large direct shear rig to deform a large, water-saturated Bentheimer sandstone block to sufficient displacement (40mm) and under enough overburden pressure (25 MPa) to create a zone of cataclastic gouge several mm thick bounded by a damage zone of fractured rock. The sheared block was cored at an inclined angle while deep frozen to extract an oriented plug containing a complete and intact section of fault rock that could be reloaded under hydrostatic conditions in an x-ray transparent core holder. The faulted plug of Bentheimer was used for running steady-state drainage and imbibition 2-phase flow experiments. The fluid saturation and its spatial distribution in 3D inside the faulted plug were dynamically monitored during all stages of the experiment in a medical x-ray CT scanner.Analysis of the flow patterns revealed from the X-ray CT images collected over time enables us to characterize in detail, the influence of recently formed fault core and damage zones on fluid flow both during drainage as gas breaks across the fault zone, and as water re-invades during imbibition. The fault core acts as a semi-permeable membrane that is only permeable to the wetting phase (water) under pressure differentials less than the capillary entry pressure (Pce) of the membrane. During drainage stages, the high Pce of the fault core led to backfilling of the upstream end of the core and almost to compartmentalization of the sample. The fracture network that exists in the damage zone played a critical role in keeping the upstream side of the fault core connected to the downstream side. During imbibition stages, the high Pce of the fault core led to significant gas trapping at the upstream side of the fault core adjacent to the fault core-damage zone interface. While the fault core formed a barrier to flow, the fracture network can provide bypass pathways for gas flow. In this particular sample bypassing through fractures was sufficient such that for a particular value of average gas saturation the flow rate was faster in the faulted rock than in the unfaulted reservoir rock.

Numerical modelling of spatially and temporally distributed on-fault induced seismicity: implication for seismic hazards

Induced seismicity is strongly related to various engineering projects that cause anthropogenic in-situ stress change at a great depth. Hence, there is a need to estimate and mitigate the associated risks. In the past, various simulation methods have been developed and applied to induced seismicity analysis, but there is still a fundamental difference between simulation results and field observations in terms of the spatial distribution of seismic events and its frequency. The present study aims to develop a method to simulate spatially distributed on-fault seismicity whilst reproducing a complex stress state in the fault zone. Hence, an equivalent continuum model is constructed, based on a discrete fracture network within a fault damage zone, by employing the crack tensor theory. A fault core is simulated at the center of the model as a discontinuous plane. Using the model, a heterogeneous stress state with stress anomalies in the fault zone is first simulated by applying tractions on the model outer boundaries. Subsequently, the effective normal stress on the fault plane is decreased in a stepwise manner to induce slip. The simulation result is validated in terms of the b-value and other seismic source parameters, hence demonstrating that the model can reproduce spatially and temporally distributed on-fault seismicity. Further analysis on the parameters shows the variation of frequency-magnitude distribution before the occurrence of large seismic events. This variation is found to be consistent with field observations, thus suggesting the potential use of this simulation method in evaluating the risk for seismic hazards in various engineering projects.

Numerical Investigation of Water Inflow Characteristics in a Deep-Buried Tunnel Crossing Two Overlapped Intersecting Faults

Because fault core zones and damage zones overlap, when a tunnel crosses the intersecting faults the groundwater flow characteristics of the tunnel-surrounding rock will be different compared to that from a single fault. By using the theory of “Three-district zoning of faults”, an improved Darcy–Brinkman numerical model for a tunnel crossing the intersecting faults was established in this work. Based on the relative vertical positions between the tunnel axis and the intersection center of faults, the underground water seepage field was analyzed at steady-state by solving the improved Darcy–Brinkman equation for the host rock zone and the fault zone. The simulation results show that the flow field around the tunnel is almost unaffected by the relative positions but is mainly dependent on the relative heights. Specifically, the relative position variation of the fault intersection to the tunnel axis has little effect on the pore pressure. In terms of flow velocity, regardless of the relative positions of the fault intersection and the tunnel, the maximum value of flow velocity almost occurs near the bottom of the tunnel excavation face and consistently displays high values within a small distance ahead of the excavation face, and then decreases quickly as the distance increases. Furthermore, the flow velocity changes minimally in the host rock. It will likely encounter the maximum water inflow rate when the tunnel excavation face passes through the intersection. The numerical simulation results can provide a practical reference for predicting water inflow into deep-buried tunnels passing through overlapped intersecting faults.

Fracture development and controlling factors at metamorphic buried-hill reservoirs of Bozhong 19-6 gas field in Bohai Bay, East China

Fractures of multi-scales and multi-origins are primary storage space and effective seepage channels for metamorphic buried-hill reservoirs. They not only allow communication between various pores to enhance storage and seepage capacity, but also are essential for high yields. Fracture development and controlling factors at metamorphic buried-hill reservoirs of Bozhong 19-6 Gas Field were investigated based on imaging log, core data, experiments, e.g., thin sections and scanning electron microscope, and outcrop description. Results show that structural fractures, weathering fractures and dissolution fractures were developed in the metamorphic buried-hill reservoirs in the study area, among which structural fractures and weathering fractures are the most popular ones. However, fracture types varied obviously among different structure positions. Lithology, fault, weathering crust and ancient landform are primary factors affecting fracture distribution. Fractures were prone to be developed at lithology with high bright mineral contents. Faults were characterized by a dual structure including fault core and damage zone. Fracture density followed a decreasing trend with increasing distance from faults. Damage zone could be identified when fracture density was consistent with regional fracture density. The width of the damage zone was determined by factors such as fault scale and structure location. Well-connected weathering fractures were popular at the leached unit, with structural fractures of secondary importance. The unweathered unit was dominated by structural fractures with low density.

Fracture Energy and Breakdown Work During Earthquakes

Large seismogenic faults consist of approximately meter-thick fault cores surrounded by hundreds-of-meters-thick damage zones. Earthquakes are generated by rupture propagation and slip within fault cores and dissipate the stored elastic strain energy in fracture and frictional processes in the fault zone and in radiated seismic waves. Understanding this energy partitioning is fundamental in earthquake mechanics to explain fault dynamic weakening and causative rupture processes operating over different spatial and temporal scales. The energy dissipated in the earthquake rupture propagation along a fault is called fracture energy or breakdown work. Here we review fracture energy estimates from seismological, modeling, geological, and experimental studies and show that fracture energy scales with fault slip. We conclude that although material-dependent constant fracture energies are important at the microscale for fracturing grains of the fault zone, they are negligible with respect to the macroscale processes governing rupture propagation on natural faults. ▪ Earthquake ruptures propagate on geological faults and dissipate energy in fracture and frictional processes from micro- (less than a millimeter) to macroscale (centimeters to kilometers). ▪ The energy dissipated in earthquake rupture propagation is called fracture energy ( G) or breakdown work ( Wb) and scales with coseismic slip. ▪ For earthquake ruptures in natural faults, the estimates of G and Wbare consistent with a macroscale description of causative processes. ▪ The energy budget of an earthquake remains controversial, and contributions from different disciplines are required to unravel this issue.

Induced Seismicity by Groundwater Extraction at the Dead Sea Fault, Jordan

AbstractThe earthquake sequence, with a maximum earthquake magnitude of MW 3.8, that occurred during January–February 2022 at the northern Dead Sea fault, is shown to be induced by extensive groundwater abstraction in Wadi Al‐Arab basin. Wadi Al‐Arab basin, which is bordered in the west by the Dead Sea fault, has been overexploited by extensive groundwater abstraction causing significant drawdowns. Relative earthquake relocation indicates an elongated S‐N sequence subparallel to the Dead Sea fault. We simulate the three‐dimensional hydraulic head changes in the past 40 years at Wadi al Arab basin. Results show that the drawdowns at the Dead Sea fault wells reached a value greater than 180 m. We use these results to further model the poroelastic effects of the drawdown on the stability of the Dead Sea fault using a typical fault architecture including fault core surrounded by damage zone. Upward groundwater drainage through the permeable damage zone leads to compaction and strengthening. Failure on the Dead Sea fault is expected to occur on the impermeable fault core or at the protholith where weakening is expected. Groundwater abstraction in Wadi Al‐Arab basin cause changes of a few MPa in the Coulmb Failure Stress (ΔCFS) and trigger seismicity in these sections. This is the second location along the Dead Sea fault where groundwater abstraction was shown to recently induce earthquakes. With growing demand for water and long lasting droughts in the Middle East, seismicity induced by groundwater abstraction might reoccur in the near future.

Fluid-rock interaction control on fault reactivation: A review of the Montmell-Vallès Fault System, central Catalan Coastal Ranges (NE Iberia)

Structural inheritance is a key factor controlling the tectonic evolution of the central Catalan Coastal Ranges. Up to two periods of tectonic inversion (one positive during the Paleogene and the other negative during the Neogene) affected a previously well-developed Mesozoic extensional basin system and characterized the Cenozoic evolution of the area. In this scenario, tectonic fault inversion is often observed along the Montmell-Vallès Fault System. Fault reactivation shows differences along strike from NE to SW and appears decoupled from surface to depth due to its kinked-planar geometry and the change of fault dip from >60° to 30° in depth. The ability of the Mesozoic faults to be reactivated appears also influenced by changes in the mechanical properties of the inherited fault zone. Whereas the deeper and less dipping panels of the major faults are reactivated in the entire zone (contractional during the Paleogene and extensional during the late Oligocene-Neogene), the upper and highly dipping parts of the faults only show local reactivations. The observations indicate that fault dip, the indirect role of hosting lithologies (granites and siliciclastic metasediments versus carbonate rocks) on fault rocks, the indirect role of mineral precipitation and cementation product of fluid circulation, and the direct role of the mechanical properties of the resulting fault rocks (gouge versus cemented breccias) significantly control the fault reactivation. Upper crust low angle fault segments are easily reactivated during contractional deformation but not during the extensional one. Conversely, the segments with a high-angle dip are more easily reactivated during the extensional deformation but not during the contractional one. In the study area, this resulted in the formation of footwall short-cuts developed during the Paleogene compression and extensional short-cuts occurred during the Neogene extension. On the other hand, reactivation is effective in areas where granites and siliciclastic metasediments characterize the host-rock, and non-cohesive fault gouge forms the pre-existent fault core. Instead, fault reactivation appears restricted or even prevented where the host-rock includes thick carbonate successions, and the pre-existent damage zone is formed by highly cemented and cohesive breccias.

Fault permeability from stochastic modeling of clay smears

AbstractIn normally consolidated, shallow (depth <~3 km) siliciclastic sequences, faults develop clay smears. Existing models include the dependence of permeability on the clay fraction, but improved predictions of fault permeability should account for uncertainty and anisotropy. We introduce PREDICT, a methodology that computes probability distributions for the directional components (dip-normal, strike-parallel, and dip-parallel) of the fault permeability tensor from statistical samples for a set of geological variables. These variables, which include geometrical, compositional, and mechanical properties, allow multiple discretizations of the fault core to be populated with sand and clay smears, which can be used to upscale the permeability to a coarser scale (e.g., suitable for reservoir modeling). We validated our implementation with experimental data and applied PREDICT to several stratigraphic sequences. We show that fault permeability is controlled by the clay smear configuration and, crucially, that it typically exhibits multimodal probability distributions due to the existence of holes. The latter is a unique feature of our algorithm, which can be used to build fault permeability scenarios to manage and mitigate risk in subsurface applications.

Naturally Fractured Carbonate Reservoir Characterization: A Case Study of a Mature High-Pour Point Oil Field in Hungary

An integrated technical study was conducted for a field development project in west Hungary. This study offers a better solution for estimating petrophysical properties and fracture facies vertically along the well and laterally for 3D static and dynamic models of naturally fractured reservoirs in carbonate rocks. More than 30 wells with 40 years of production history were used in order to build reliable static and dynamic models. The fracture class/facies plays an essential role in the spatial distribution of petrophysical properties during 3D reservoir modeling. It was defined by integrating conventional logs, image logs, drilling parameters, and production or well test data. Three fracture facies are defined as macrofracture (including permeable subseismic fault), microfracture, and host rock. Subsequently, the fracture class’s spatial distribution is guided by seismic attributes of fault likelihood combined with the geological concept of the fault and damage zone. As a result, the established fracture classes along the wells are validated by static and dynamic subsurface data. A spherical self-organizing map (SOM) was also implemented for predicting the open-fracture location in wells having limited subsurface data. Moreover, fracture lateral distribution follows the distribution of the fault zone of the fault core, high damage zone, low damage zone, and host rock. The higher the fault displacement, the wider the damage zone and fault core formed. Macrofractures and microfractures frequently appear around the fault core and high damage zone. While only microfractures are dominantly present in the low damage zones, in contrast, the unfractured class is dominantly distributed in the host-rock area. Also, the lithologies are considered in distributing the fracture class because the rock mechanic properties and the number of fractures are strongly controlled by rock compositions. Once the fracture class is distributed, porosity, permeability, and water saturation are modeled in the three-dimensional (3D) geocellular model. Finally, this fracture class also plays a role as a rock typing for reservoir simulation. The saturation height model is built using the fracture class distribution resulting in the initialization, history-matching process, and production forecast from 20 wells showing excellent quality. As a novelty, this study offers a better understanding of fracture distribution and accelerates the history-matching process with a more confident result of the production forecast. In the absence of advanced technologies, like image logs and production logging (PLT) measurements, this study still effectively assists us in recognizing the fracture presence and its quality in both well-depth interval and 3D spatial space, and successfully guided us in proposing new infill drilling with strong confidence and delivering on the high end of expected results.

3D numerical modelling and simulation of the impact of fault zones on fluid flow in sandstones of the Rio do Peixe Basin, NE Brazil

Deformation bands are usually responsible for a reduction in permeability perpendicular to the structure planes of up to three orders of magnitude, while the fault core represent a reduction of up to seven orders of magnitude in cross-fault permeability, imposing large anisotropies to fluid flow. As deformation bands occur distributed along the damage zone, they impact not only the across-fault flow but also the along-fault flow. The fault core is usually represented using fault-transmissibility multipliers (TMs) along the fault planes, using well-established workflows. However, there is a lack of methods to represent fault damage zones in any direction and grid-cell sizes. In this context, we proposed new methods to: (1) estimate the deformation intensity in damage zones; (2) calculate their most representative value within the cell domain; and (3) calculate the equivalent permeability of a cell containing oblique deformation bands. The workflow is applied to a 3D numerical model for the Santa Helena High in the Rio do Peixe Basin, NE Brazil. We performed streamline simulations in four models to evaluate the impact of fault damage zones and the fault core in fluid flow. Our models show that the fault core and damage zone negatively affected the performance of the reservoir. Supplementary material : A description of the method developed to estimate the deformation intensity in damage zones and of the method developed to calculate the equivalent permeability are available at https://doi.org/10.6084/m9.figshare.c.6251469

Comparing intrarift and border fault structure in the Malawi Rift: Implications for normal fault growth

Early stages of normal fault growth are seldom described using field observations of active normal faults. Here we first estimate the displacements of active border and intrarift faults in the Zomba Graben in the low extension (< 10 %) Malawi Rift, and then quantify micro-to macroscale fault damage and mineralisation associated with their surface exposures. The 22 km long Mlunguzi and 39 km long Chingale Step intrarift faults have fault zones 4–52 m wide. In contrast, we estimate the fault zone of the 51 km long Zomba border fault is 32–118 m wide. Calcite and clay alteration is limited to the fault damage zones and fault cores, and the extent and intensity of fault damage and mineral alteration is greater on the Zomba border fault compared to the intrarift faults. Relative to global compilations, normal faults in the Zomba Graben have lengthened quickly while developing narrow fault zones, given their displacement. The minor damage on these long, low-displacement normal faults may reflect the influence of lithology, negligible fault healing, and/or activation of pre-existing weaknesses.

The heterogeneity of petrophysical and elastic properties in carbonate rocks controlled by strike-slip fault: A case study from yangjikan outcrop in the tarim basin

The petrophysical and elastic properties of the carbonate rocks controlled by strike-slip fault are still poor understood due to their significant complexities and strong heterogeneities, causing challenges for the exploitation and development of these carbonate reservoirs. To better comprehend the heterogeneity of petrophysical and elastic properties of the fault-controlled carbonate rocks, a small carbonate strike slip fault zone is selected, which is located in Yangjikan section in the Tarim basin. We combine the geostatistical, microscopic observation, petrophysical and ultrasonic analyses to characterize and understand the complexities and heterogeneity of petrophysical and elastic properties with different distance to the main fault core. The results show significant complexities and heterogeneities of petrophysical and elastic properties, and the variations of rocks drilled from the fault core are relatively drastic compared to those from the damage zone. The equivalent pore aspect ratios of the rocks are calculated from the differential effective medium theory, the values in the fault core are much higher compared to those from the damage zone, which can discriminate the pore network architectures and can represent reference for determination of the boundary of fractured damage zone. The controlling factors on the heterogeneity of the petrophysical and elastic properties for the selected fault zone are discussed, which mainly include the influence of fault structure position on fracture development, fluid selective filling on effective fracture and pore development, and the sequence on pore development. The results can contribute to the recognition and prediction of the petrophysical and elastic properties and hydrocarbon exploration in carbonate strike-slip fault zone in the Tarim Basin, and can provide reference for the construction of integrate multiscale heterogeneities models.

Lithological and structural anatomy of an unusually well exposed strike-slip fault pressure ridge in the Nanjieshan, western China

We combine field observations, drone photogrammetry, and petrographic and XRD analysis to document the internal lithological and structural variability of an exceptionally well-exposed strike-slip fault pressure ridge, in the northern Tibetan foreland. The ridge exposes alternating dm-m thick discolored zones of fault gouge, breccia, and cataclasite likely reflecting the original Riedel shear array and progressive widening of the fault zone as comminution, fluid pressures, mineral alteration, and strain hardening and weakening processes evolved and migrated vertically and laterally within the core. The principal fabric defines an internal foliation fan, and sinistral, oblique-thrust faults bound the ridge supporting a transpressive squeeze-up origin. Outcrop- and micro-scale grain size reduction, gouge formation, and calcite precipitation suggest that thermal pressurization, mechanical lubrication, fault-valve fluid cycling, and periodic gouge zone lock-up may have been important factors controlling fault slip behavior and seismic energy release. Local variations in the distribution and thicknesses of brittle fault rocks, strain intensity, alteration mineralogy and fault core fabrics are a sobering reminder that a single 2-dimensional exposure across any regional fault system will fail to capture the complex petrological, mineralogical, mechanical and rheological heterogeneity that is typical of an evolving intraplate strike-slip fault system, especially where it reactivates an older fault/shear zone.

Observational and theoretical evidence for frictional-viscous flow at shallow crustal levels

Along the Hikurangi Subduction Margin, accretionary prism uplift has exposed the Hungaroa fault zone, an inactive thrust developed within the Middle to Late Eocene Wanstead Formation. Within the ~33 m-wide fault core, deformation of the smectitic, calcareous mudstone matrix produced a penetrative foliation that locally wraps around clasts. Deformation occurred at temperatures constrained by syntectonic calcite vein clumped isotope thermometry, which yielded a narrow range of Δ 47 values between 0.445 ± 0.024‰ and 0.482 ± 0.013‰, corresponding to a mean calcite precipitation temperature of 82 −12 +13 °C. Optical and scanning electron microscopy analyses reveal that calcite underwent: dissolution along stylolites and clast, vein, and microlithon margins; precipitation in foliation-parallel and foliation-perpendicular extension veins; and precipitation in hybrid veins and strain fringes. Maximum differential stress estimates obtained from calcite twin densities (44.1 ± 13.9 to 96.6 ± 20.8 MPa) are consistent with those sustainable by a cohesionless fault at ~3 km depth with a friction coefficient in the range measured for two calcareous mudstones (μ = 0.38 to 0.50) and a micrite clast (μ = 0.61 and 0.64). Marlstone clasts within the foliated calcareous mudstone matrix contain mutually cross-cutting shear fractures and extension veins with crack-seal textures, providing evidence for temporal fluctuations in shear strength resulting from pore fluid overpressure transients. At strain rates imposed during laboratory experiments, frictional sliding involves granular flow processes. Yet, calcite microstructures indicate that diffusive mass transfer played an important role in accommodating deformation. We model the fault zone rheology assuming diffusion-controlled frictional-viscous flow, with deformation at strain rates γ ˙ ≤ 10 −9 s −1 able to have taken place at very low shear stresses ( τ < 10 MPa) given sufficiently short diffusion distances ( d < 0.1 mm), even in the absence of pore fluid overpressures. However, if grain-scale and fracture-scale processes change the diffusion distance, fault zones deforming via frictional-viscous flow can exhibit temporally variable strain rates. Thus, our results suggest that the shallow (up-dip) limit of the seismogenic zone is not a simple function of temperature in fault zones governed by a frictional-viscous flow rheology. • Displacement occurred via distributed deformation across a 33 m-wide fault core. • Fault core rocks experienced spatiotemporal variations in stress and strength. • Frictional-viscous flow can explain the weakness of shallow crustal thrust faults. • Frictional-viscous flow can occur at temporally variable strain rates.