Fault Rocks Research Articles

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.

Relationship between hydraulic conductivity and occurrence of fault rocks in Yangsan fault zone, Korea

Relationship between hydraulic conductivity and occurrence of fault rocks in Yangsan fault zone, Korea

Methods and Case Studies of Ultra‐deep Fault Seal Evaluation

AbstractFault seals are significant for petroleum exploration and production. This study summarizes the fault sealing impacting factors, including lithological juxtaposition, mud smearing, fault rocks and the fault plane stress states, as well as evaluation methods like Allan maps and Shale Gouge Ratio (SGR). The seal evaluation for a wrench fault focuses on its particular structural features. The evaluation methods were applied to the Jinma–Yazihe structure and the Shunbei oilfield. The source rock is the Xujiahe Formation of the Upper Triassic, the reservoirs and caprocks being of the Shaximiao Formation of the Lower Jurassic. The fault sealing evaluations in major faults proved the reservoir formation processes in the wells Jinfo 1 (JF1) and Chuanya 609 (CY–609), based on the editions of the Allan map showing lithological juxtaposition, the calculation of SGR showing mud smear and analyses of fault stress states. The analyses of stress states were also applied to Shunbei 5 strike‐slip fault in the Shunbei area in Tarim Basin. The various sections along the fault were of different mechanical properties, such as compression and extension. Petroleum exploration has demonstrated that the extensional sections are more favorable for oil accumulation than the compressional sections. These evolutionary methods and other understandings will help in analyses of deep fault sealing.

Integrated geochemical, mineralogical, and microstructural identification of faults in tunnels and its application to TBM jamming analysis

Faults are frequently one of those adverse geological conditions that jam tunnel boring machines (TBMs). However, the identification of faults and the engineering geological analysis of the corresponding jamming accidents still lack a better understanding. In this study, a method for fault identification through integration of the microstructural, geochemical, and mineralogical characteristics of the rocks was developed. The method was applied to the TBM 2 bid section of the Central Shanxi Yellow River Diversion Project to determine the geological characteristics of the rocks using optical thin section, X-ray fluorescence, and X-ray diffraction analysis. The results of this study provide evidence for the establishment of fault-induced geochemical effects and mineral transformations. Not only were fault FT1 and the nearby fault rock successfully identified, but evidence of weakened mechanical properties of the fault rocks was also obtained. The structure and composition of the migmatitic granites were strongly altered by long-term tectonic stress fracturing and pervasive acidic fluid leaching, forming loosely fractured, poorly integrated granitic granulate-porphyroclastic rock. The fault rock is rich in clay minerals and solid graphite lubricant. The comprehensive effect of the fault rock’s characteristics, geological structure, groundwater, and ground stress led to the collapse of the weak rock within the fault zone, which was the main geological reason for the jamming accident. Finally, we proposed corresponding engineering treatment measures and subsequent excavation recommendations. The proposed method can be used for the integrated identification of faults in underground engineering and mining projects, and the geological analysis of TBM jamming accidents.

The quantitative evaluation of fault sealing during the Cenozoic in Gaoyou Depression, Subei Basin

The quantitative evaluation of fault sealing is conducted based on comprehensive analysis of mechanism and type analysis of fault seal, fault seal evaluation, and case study analysis. The results indicate that the properties of fault sealing are the interaction of fault order, net‐to‐gross ratio, fault activity, mud distribution on fault walls, fault dip, and buried depth. In addition, there are three types of fault sealing, including juxtaposition seal, fault rock seal, and cementation seal. Furthermore, the opening and closing of faults play an essential role in controlling hydrocarbon migration and accumulation. Finally, the fault sealing plays a controlling role in reservoir preservation.

Evidence of Early Permian extension during the post-Variscan evolution of the central Southern Alps (N Italy)

In the central Southern Alps (N Italy) some well-preserved Permian extensional structures, that exceptionally escaped the Alpine deformation, have been recently identified. Their analysis offers important insights for the interpretation of the post-Variscan tectonics affecting the Southern Alps during the Permian. We describe here a previously unknown fault system related to the development of the Early Permian Orobic Basin, where large exposures of the Variscan basement preserve their original tectonic contacts with the overlying Lower Permian cover. The fault system consists of Low-Angle Normal Faults (LANFs) accompanied by High-Angle Normal faults (HANFs), these last entirely developed within the volcanic, volcaniclastic and terrigenous deposits. The studied structures occurring in the upper part of the Gerola Valley, share several features with Early Permian normal faults already recognized in other areas of the central Southern Alps. The low-angle normal fault planes are characterized by a continuous layer of coarse-grained fault breccias, locally impregnated by tourmalinites. The fault rocks have been invariably observed along the fault surface at the basement-cover contact all across the study area. This newly identified fault system with its characteristic combination of low- and high-angle normal faults suggests a tectonic regime characterized by pure extension in the central Southern Alps, rather than by a transtensional regime during the Early Permian. The provided data give new insights into the Early Permian geodynamic scenario, which is discussed in light of the transition from the Pangea B to the Pangea A configuration.

Elastic Contrast, Rupture Directivity, and Damage Asymmetry in an Anisotropic Bimaterial Strike‐Slip Fault at Middle Crustal Depths

Mature faults with large cumulative slip often separate rocks with dissimilar elastic properties and show asymmetric damage distribution. Elastic contrast across such bimaterial faults can significantly modify various aspects of earthquake rupture dynamics, including normal stress variations, rupture propagation direction, distribution of ground motions, and evolution of off-fault damage. Thus, analyzing elastic contrasts of bimaterial faults is important for understanding earthquake physics and related hazard potential. The effect of elastic contrast between isotropic materials on rupture dynamics is relatively well studied. However, most fault rocks are elastically anisotropic, and little is known about how the anisotropy affects rupture dynamics. We examine microstructures of the Sandhill Corner shear zone, which separates quartzofeldspathic rock and micaceous schist with wider and narrower damage zones, respectively. This shear zone is part of the Norumbega fault system, a Paleozoic, large-displacement, seismogenic, strike-slip fault system exhumed from middle crustal depths. We calculate elastic properties and seismic wave speeds of elastically anisotropic rocks from each unit having different proportions of mica grains aligned sub-parallel to the fault. Our findings show that the horizontally polarized shear wave propagating parallel to the bimaterial fault (with fault-normal particle motion) is the slowest owing to the fault-normal compliance and therefore may be important in determining the elastic contrast that affects rupture dynamics in anisotropic media. Following results from subshear rupture propagation models in isotropic media, our results are consistent with ruptures preferentially propagated in the slip direction of the schist, which has the slower horizontal shear wave and larger fault-normal compliance.

High Stress Deformation and Short‐Term Thermal Pulse Preserved in Pyroxene Microstructures From Exhumed Lower Crustal Seismogenic Faults (Lofoten, Norway)

AbstractEarthquake rupture in strong, anhydrous lower continental crust requires high brittle failure stresses unless high pore fluid pressures are present. Several mechanisms proposed to generate high stresses at depth imply transient loading driven by a spectrum of stress changes, ranging from highly localized stress amplifications to crustal‐scale stress transfers. High transient stresses up to GPa magnitude are proposed by field and modeling studies, but the evidence for transient prerupture loading is often difficult to extract from the geological record due to overprinting by coseismic damage and slip. However, the local preservation of deformation microstructures indicative of crystal‐plastic and brittle deformation associated with the seismic cycle in the lower crust offers the opportunity to constrain the progression of deformation before, during and after rupture, including stress and temperature evolution. Here, detailed study of pyroxene microstructures characterizes the short‐term evolution of high‐stress deformation and temperature changes experienced before and during lower crustal earthquake rupture. Pyroxenes are sampled from pseudotachylyte‐bearing faults and damage zones of lower crustal earthquakes recorded in the exhumed granulite facies terrane of Lofoten, northern Norway. The progressive sequence of microstructures indicates localized high‐stress (at the GPa level) prerupture loading accommodated by low‐temperature plasticity, followed by coseismic pulverization‐style fragmentation and subsequent grain growth triggered by the short‐term heat pulse associated with frictional sliding. Thus, up to GPa‐level transient high stress (both differential and shear) leading to earthquake nucleation in the dry lower crust can occur in nature, and be preserved in the fault rock microstructure.

An In Situ Decarbonation Kinetic Study of Calcite Using Synchrotron Radiation XRD

ABSTRACT In situ decarbonation kinetics of calcite were investigated at high temperatures, up to 900 °C, using synchrotron radiation powder X-ray diffraction. The sequence of X-ray diffraction spectra reveals that calcite begins to thermally decompose into lime (CaO) and CO2 at 800 °C and ambient pressure. The decarbonation degree gradually increases with temperature, and calcite completely transforms into lime at 900 °C. The kinetic analysis of the isothermal data using an Avrami model involving nucleation and growth yields the values for the decarbonation rate and reaction order. Our results indicate that the decarbonation rate increases from 2.89 × 10–4s–1 to 3.48 × 10–3s–1 with elevated temperature from 840 to 880 °C, showing a positive temperature dependence on the reaction rate. The calculated Avrami exponent (n) values between 1.35 and 2.38 suggest that the thermal decomposition of calcite should be mainly dominated by homogeneous nucleation and CO2 diffusion-controlled growth. In natural carbonate fault rocks, the decarbonation of CaCO3 caused by frictional heating may be strengthened by the action of high shear velocity. In addition, the resulting ultrafine powder and CO2 pressurization can remarkably reduce the friction coefficient between two fault planes, which further leads to carbonate fault weakening. The yielding result will be conductive to better understanding the role of decarbonation of calcite in some active fault zones.

Pseudotachylyte-Mylonites Record of Transient Creep From Inter-Seismic Ductile to Co-Seismic Rupture

Transient creep during an earthquake cycle is very important to understand the rheology of fault and deformation mechanisms in the brittle–plastic transition zone. In this paper, we analyzed the microstructures of samples for mylonites, pseudotachylyte, and cataclasite under optical microscope, SEM, and EBSD, which were collected from the Red River fault in southwest of China, where we uncovered a series of ductile to brittle deformed rocks which recorded transient creep related to earthquakes. The results show that mylonites formed at the inter-seismic creep were overprinted by pseudotachylyte and cataclasite which were produced during co-seismic rupture, and cracks in cataclasite were healed during the post-seismic relaxation. Based on the analysis of the microstructures and deformation mechanism of fault rocks, we propose the oscillation deformation model to explain transient creep of the brittle–plastic transition zone during the seismic cycle in the Red River fault.

The Effect of Quartz Particle Size on the Frictional and Electrically Conductive Properties of Mixed Graphite–Quartz Gouges

Mature faults usually contain fault rocks with a wide range of mineral grain sizes. Despite the importance of mineral grain sizes in affecting fault slip behaviors, little is known about the potential mechanism(s). To better understand this problem, electrical conductivity measurements on synthetic carbon-bearing gouges were conducted along a fault-parallel direction under progressive fault slip. All experiments were carried out under a slip rate of 1 mm/s, a normal stress of 2 MPa, ambient temperature, and a pure N2 atmosphere. The specimens that were used were mixtures of identical 6 wt% graphite (Gr) powders and 94 wt% quartz (Qz) particles with five different particle sizes (#100–12500 mesh). As Gr has a low friction coefficient and high electrical conductivity, the approach in this study may provide a favorable opportunity to examine the relation between the evolutions of friction and shear textures. The experimental results indicated that the reduction in Qz particle sizes causes gradual segregation of the Gr powders in the skeletal frame formed by granular Qz particles, resulting in the decreased interconnectivity of the anastomosing Gr-film networks and the destruction of Gr-lubricated slip surface(s). Then, it eventually manifests as an increase in the steady-state frictional coefficient (μss) and a logarithmic decrease in the steady-state electrical conductivity (σss) for Gr-bearing specimens. Furthermore, the Gr-bearing gouges containing >3 μm Qz particles first develop foliated layers, and subsequent Gr films penetrate around the boundary of the Qz particles to form conductive interconnected networks during a progressive fault slip. These experimental results implied that carbonaceous materials (CMs) represented by Gr may complicate the frictional properties of fine-grained fault gouges in mature faults.

Brittle cataclastic process of fault rocks based on a large-displacement direct shear model realized with DEM

Fault rocks in the brittle regime are often the result of large-displacement shearing. Traditional direct shear tests that perform small displacements can only study one episode of the whole faulting process. Ring shear apparatuses that can achieve large-displacement shearing are mostly used to study incohesive soil or friction granular materials. The objective of this paper is to study the brittle cataclastic process of fault rocks from the damage of intact rock to the frictional movement of the incohesive cataclastic rocks. A large-displacement direct shear model with a constant contact area of fault walls based on the particle-based discrete element method is proposed to simulate the structural evolution and mechanical behavior of dry brittle fault rocks. The results show that the crushing and rounding of irregular fragments are responsible for the reduction in the friction coefficient. The sharp fall is due to the crushing of rock fragments, while a moderate decline indicates grain rotation in the postpeak stress-displacement curve. This model helps to understand the cataclasis of brittle fault rocks.

Drilling and Grouting Works for Pressurised Groundwater Conditions of the Semmering Base Tunnel

The Semmering Base Tunnel (SBT), with a total length of 27.3 km, is one of the leading construction projects of the Baltic-Adriatic Railway Network. The tunnel connects the two federal provinces of Lower Austria and Styria and cuts through the eastern part of the Alps. The construction lot SBT 1.1-Tunnel Gloggnitz is characterized by a complex geological- and hydro-geological architecture containing alternating competent geological structures, major fault zones, and corresponding geological transition zones. There are three main water-bearing formations at the construction lot SBT 1.1: (a) Karst-prone blocky limestone with an initial water pressure of 10 bar. The discontinuities form a highly permeable interconnected joint network with a significant storage coefficient of the groundwater table. (b) Massive to blocky dolomite with an initial water pressure of 25 bar. The systematic discontinuities and disturbed zones of subsidiary structures with karst form a permeable, interconnected joint network. (c) The transition zone of the limestone to base structure marks the most challenging area for consolidation- and sealing grouting. Weak fault rocks characterize transition zones with intense fracturing. However, as subjected to high groundwater pressure of 10 bar, these zones are associated with the potential of flowing ground conditions. Based on the overall project requirements, specific drilling- and grouting methods and materials for pre-excavation grouting have been established and successfully implemented in the construction process. The innovations include a Standpipe-Packer substituting a conventional steel standpipe, a specifically cased drilling system with grouting inserts to prevent erosion within the borehole and allow for defined grouting. In addition, this Grouting-Pipe system replaces standard tube-à-manchettes and controls the flushing while drilling with preventers. Finally, a combined cement-polyurethane grout mix (Hybrid Grout) was implemented to stabilize the grout. The implementation of these measures will be discussed in detail, and their benefits to the construction process will be highlighted.

Numerical Simulation of Water–Silt Inrush Hazard of Fault Rock: A Three-Phase Flow Model

Numerical Simulation of Water–Silt Inrush Hazard of Fault Rock: A Three-Phase Flow Model

The Role of Fault Rock Fabric in the Dynamics of Laboratory Faults

AbstractFault stability is inherently linked to the frictional and healing properties of fault rocks and associated fabrics. Their complex interaction controls how the stored elastic energy is dissipated, that is, through creep or seismic motion. In this work, we focus on the relevance of fault fabrics in controlling the reactivation and slip behavior of dolomite‐anhydrite analog faults. We designed a set of laboratory experiments where we first develop fault rocks characterized by different grain size reduction and localization at normal stresses of σN = 15, 35, 60, and 100 MPa and second, we reload and reactivate these fault rocks at the frictional stability transition, achieved at σN = 35 MPa by reducing the machine stiffness. If normal stress is lowered this way, reactivation occurs with relatively large stress drops and large peak‐slip velocities. Subsequent unstable behavior produces slow stick‐slip events with low stress drop and with either asymmetric or Gaussian slip velocity function depending on the inherited fault fabric. If normal stress is raised, deformation is accommodated within angular cataclasites promoting stable slip. The integration of microstructural data (showing brittle reworking of preexisting textures) with mechanical data (documenting restrengthening and dilation upon reactivation) suggests that frictional and chemically assisted healing, which is common in natural faults during the interseismic phase, can be a relevant process in developing large instabilities. We also conclude that fault rock heterogeneity (fault fabric) modulates the slip velocity function and thus the dynamics of repeating stick‐slip cycles.

Geothermal renewable energy prospects of the African continent using GIS

Geothermal energy potential is identified through the deep investigation of geological, geothermal, and geophysical information over a small area; however, this task is both expensive and complex. Geographic Information Systems (GIS) can integrate different types of data (or thematic layers) over large regions and use them to identify zones of high geothermal potential. In this study, we use GIS as a tool to construct the first regional-scale geothermal potential map of Africa from different datasets. The key objective of this study is to estimate the geothermally promising areas within Africa by integrating geological thematic layers (rock units and faults), geophysical layers (heat flow derived from aeromagnetic data and seismicity), and geothermal layers (hot springs and volcanoes) within the GIS database. A weighted overlay technique within the GIS environment is applied to these data to generate the geothermal potential map. The result shows 14 regions with a high geothermal favorability index. The geothermal potential map of Africa is useful for targeting and exploring new geothermal renewable energy sites and can reduce exploration costs and pinpoint investigation areas during preliminary geothermal studies.

Zircon Dating, Geochemistry, and Metallogenic Significance of Early Paleozoic Mafic Rocks in Northern Guangxi Province, China

Magmatic rocks, deformed structures, and tin–polymetallic deposits are widely distributed in the western Jiangnan Orogenic Belt (JOB) of northern Guangxi Province, China. Magmatism and mineralization are believed to have occurred in the Neoproterozoic stage. Herein, we report the zircon U–Pb dating results of investigations on the Ping’an Pb–Zn–Cu polymetallic veins hosted in gabbro near Ping’an Village. Zircon U–Pb dating of the host gabbro yielded ages of 853.0 ± 7.8, 837.7 ± 7.2, and 450.4 ± 6.7 Ma. The younger age represents the emplacement of gabbros, whereas the older Neoproterozoic age reflects inherited zircons from the wall rocks or source regions. The formation of gabbros (Caledonian) is related to the subduction and collisions of microplates in the western JOB, which are controlled by movements of the Yangtze and Cathaysian plates. We consider that the late Caledonian regional shearing in the western JOB resulted in the fracturing and faulting of rocks (Neoproterozoic and early Caledonian), which provided conduits for the flow of hydrothermal fluids and accommodation for the associated mineralization. Geochemistry investigations show that the Caledonian basic magmatic activity provided a certain material source for the final mineralization. We propose that the tin–polymetallic deposits in the northern Guangx Province, and Neoproterozoic cassiterite crystallization, were subjected to Caledonian shear crushing and hydrothermal transformation with copper, lead, zinc, and other metal elements based on our comprehensive analysis, providing a new ideology for understanding the geology and mineralization in this area.

Grain-size-reducing- and mass-gaining processes in different hydrothermal fault rocks

AbstractInteractions of hydrothermal and deformation processes in active fault zones are important in light of seismic and geothermal activities. This study investigates different tectonites of the Grimselpass breccia fault, a hydrothermally active strike-slip structure in the Central Alps (Switzerland). We combine microstructural and geochemical investigations to decipher the interaction of grain-size reduction and associated dilation (volume gain) during frictional deformation with geochemical processes such as mass gain via precipitation, grain growth and healing. Three types of tectonites were investigated in the fault zone: (1) cockade-bearing tectonites, (2) cataclasites and (3) fault gouges. They differ in terms of microstructure of clasts, mass and volume gain/loss, as well as type and degree of cementation. Clast size distributions show decreasing median values from low-strain cataclasites to high-strain cataclasites. Mass/volume gains are up to 20 %, with samples of smallest clast sizes often showing the highest mass gain. In the case of cockade-bearing tectonites large volume and mass gain occur. In particular, the precipitation of SiO2-rich cements is inferred to (i) reduce permeability and (ii) induce a gain in strength of the respective fault rocks. Location, timing and degree of healing directly control the fault architecture by deactivating fault strands from the circulation of hydrothermal fluids. In this case, new rupturing events are necessary to reactivate locked fault strands. Such feedback cycles of episodic deformation, mass/volume and clogging preserved hydrothermal circulation and deformation over the last 3.4 Ma, representing a process chain for the long-term preservation of orogenic hydrothermal systems.

Crystallographic orientation mapping of lizardite serpentinite by Raman spectroscopy

Abstract. The serpentine mineral lizardite displays strong Raman anisotropy in the OH-stretching region, resulting in significant wavenumber shifts (up to ca. 14.5 cm−1) that depend on the orientation of the impinging excitation laser relative to the crystallographic axes. We quantified the relationship between crystallographic orientation and Raman wavenumber using well-characterised samples of Monte Fico lizardite by applying Raman spectroscopy and electron backscatter diffraction (EBSD) mapping on thin sections of polycrystalline samples and grain mounts of selected single crystals, as well as by a spindle stage Raman study of an oriented cylinder drilled from a single crystal. We demonstrate that the main band in the OH-stretching region undergoes a systematic shift that depends on the inclination of the c-axis of the lizardite crystal. The data are used to derive an empirical relationship between the position of this main band and the c-axis inclination of a measured lizardite crystal: y=14.5cos 4 (0.013x+0.02)+(3670±1), where y is the inclination of the c-axis with respect to the normal vector (in degrees), and x is the main band position (wavenumber in cm −1) in the OH-stretching region. This new method provides a simple and cost-effective technique for measuring and quantifying the crystallographic orientation of lizardite-bearing serpentinite fault rocks, which can be difficult to achieve using EBSD alone. In addition to the samples used to determine the above empirical relationship, we demonstrate the applicability of the technique by mapping the orientations of lizardite in a more complex sample of deformed serpentinite from Elba Island, Italy.

Solid grain migration on hydraulic properties of fault rocks in underground mining tunnel: Radial seepage experiments and verification of permeability prediction

When fault fracture zones are encountered in the mining tunnel excavation, the water-conducting pathway is easily formed by the granular structure of the broken surrounding rock, probably resulting in water inflow hazards. Therefore, the configuration and hydrological characteristics of existing faults are key factors for the safe mining tunnel construction. In this paper, one-dimensional (1D) radial seepage experiments were carried out under variable grain size distribution (GSD) and water pressure (WP). Hydraulic properties were investigated, including porosity and permeability under the 1D radial grain migration. Experimental results show that the permeability and porosity increase with time, and the occurrence process of water inflow can divide into four stages, including the rapid increase, decelerated increase, slow climbing, and stable period. After the seepage experiment, the porosity of the sample increases gradually from the top to the bottom and reaches the maximum at the lowermost part (outlet), indicating that the spatial distribution of the porosity exhibits the non-uniform property. Through comparisons of grain migration under different conditions, it is concluded that, with the decrease of GSD and the increase of WP, more severe fine grain migration can be occurred, leading to great changes in hydraulic properties. In addition, three prediction models were verified by the testing results, and Carman-Kozoeny model shows the highest accuracy. In the same predictive model, if a sample has a smaller grain migration capacity, then there is a higher prediction accuracy of the sample. By means of Carman-Kozoeny model, it is predicted that there is a higher final permeability in the position closer to the water outlet. Based on the research results, a series of strategies were proposed to prevent and control water inflow in fault rocks.