Article

Graphite as a lubricating agent in fault zones: An insight from low- to high-velocity friction experiments on a mixed graphite-quartz gouge: GRAPHITE AS A LUBRICATING AGENT IN FAULT

May 2013
Journal of Geophysical Research: Solid Earth 118(5)

DOI:10.1002/jgrb.50175

Authors:

Kiyokazu Oohashi

Yamaguchi University

Takehiro Hirose

Japan Agency for Marine-Earth Science Technology

Toshihiko Shimamoto

Institute of Geology, China Earthquake Administration

[1] Graphite is a very low friction material, often enriched within fault zones due to mechanical or chemical processes. The effects of weak minerals on the strength of faults have been examined by friction experiments on bimineralic mixtures. However, previous experiments were conducted with limited shear strains, even though applied shear strains and textural developments had already been signaled as significant factors in the weakening of faults. We therefore conducted large-displacement, low- to high-velocity friction experiments with graphite-quartz gouges, to determine how much graphite is needed to reduce frictional strength, and to examine how textures contribute to the strength reduction of a mature fault at various slip rates. We found that the coefficients of friction of the gouges decrease nonlinearly with increasing graphite fraction for any given shear strain and slip rate, decreasing first with 5–20 vol% graphite, then reaching similar frictional levels to pure graphite with 30–50 vol% graphite. The nonlinear weakening trends can be fitted by sigmoidal curves. The weakening with 10–30 vol% graphite is associated with zones of slip-localization and the development of a graphite-lubricated penetrative slip surface(s). With increasing shear strain, the relationship between strength and graphite fraction evolves abruptly from an early gentle curve to a sigmoidal curve, and the frictional strength drops significantly even with small amounts of graphite (~10 vol%). Our results highlight the importance of shear strain and textural developments on weak faults, not only with respect to graphite, but also other fault lubricants such as the phyllosilicates.

Frictional strength, electrical conductivity, and microstructure of calcite–graphite mixtures sheared at a subseismic slip rate

Article

Oct 2023
TECTONOPHYSICS

High-pressure Physical Behavior of Minerals and Rocks Mineralogy, Petrology and Geochemistry

Book

Full-text available

Feb 2023

The high-pressure physical behavior of minerals and rocks is crucial in exploring the transport process and its surface response of deep Earth interior’s material at high temperature and high-pressure conditions. In the long history of formation and evolution for Earth and Planet, most of Earth materials are in a state of high-temperature and high-pressure environments. High-temperature and high-pressure experimental and theoretical calculations modelings are the most efficient methods in disclosing the deep understandings for the cause and origin of crust, mantle and core for the Earth and other terrestrial Planet. And thus, in order to further elucidate the complex physiochemical process in the deep interior of Earth and other terrestrial Planet, the high-pressure physical behavior of minerals and rocks plays a vital role among available geophysical methods. Some important high-pressure physical behavior for many physical parameters of minerals and rocks including electrical conductivity, elasticity, equation of state, rheology, deformation, et al. have been performed under controlled temperature, pressure, oxygen fugacity, water content and chemical composition in the recent several years. High-temperature and high-pressure experimental and theoretical calculations modelings for minerals and rocks have long been an essential pathway to understanding high-pressure physical behavior. Notably, in order to efficiently model the material composition and transport conduction of deep interior for the Earth and other terrestrial Planet, some high-pressure physical parameter measurements and their theoretical modelings (e.g. electrical conductivity, elasticity, equation of state, rheology, deformation, et al.) have been successfully made great progress in the recent years. Almost all of minerals and rocks for the cycle layer in the deep interior Earth (crust, mantle and core) are well concerned. Some in-situ high-pressure experimental techniques and measurement methods, such as autoclave, piston-cylinder, multi-anvil press and diamond anvil cell have been advanced and wide adopted in different geological tectonic conditions. Besides, some new techniques including high-pressure spectroscopy (infrared, Raman), super-computer modeling, synchrotron X-ray diffraction et al. are employed and widespread applied to the deep material science of the Earth interior. All of them greatly make it advance for the high-pressure physical behavior of minerals and rocks. Except for the high-pressure physical behavior of minerals and rocks, some newest progress in the field of mineralogy, petrology and geochemistry are also paid attention in this Research Topic. Therefore, all of these researches will be moving forward the research of solid Earth geophysics and geochemistry. This Research Topic aims to explore the high-pressure physical behavior of minerals and rocks, Mineralogy, Petrology and Geochemistry. All of these high-pressure physical behaviors for minerals and rocks including in-situ experimental measurements of high-pressure physical parameters, high-pressure spectroscopy characterizations and theoretical calculations are included at high-pressure conditions, as well as some mainstream in the solid geophysics and geochemistry including mineralogy, petrology and geochemistry in this section.

High-pressure Physical Behavior of Minerals and Rocks Mineralogy, Petrology and Geochemistry

Book

Full-text available

Feb 2023

Lidong Dai

Structural fabrics of carbon grains in a natural fault gouge reactivated by the 2008 Wenchuan earthquake

Article

Full-text available

Nov 2022

Natural fault gouges reactivated by the 2008 Wenchuan earthquake are typically rich in carbon in shallow parts of the seismogenic fault zone. Although experimental evidence indicates that amorphous carbon can be changed to graphite during seismic slips, this transformation has not yet been observed in nature. We conducted a nanoscale investigation of a carbon‐rich co‐seismic gouge from a surface rupture related to the Wenchuan earthquake using high‐resolution transmission electron microscopy. We found that all mineral grains were wrapped in amorphous carbonaceous materials with sinuate and straight graphene layer stacks. The sinuate layer was the transient material (~0.3456 nm) formed by amorphous carbon transforming to graphite; the graphene layer was graphite flakes (0.3354 nm). This means that graphitization occurred on the mineral grain surfaces (asperities) in the shallow slip zones during previous earthquake cycles, which could decrease the friction strength of the co‐seismic fault gouge and explain the dynamic weakness of the shallow parts of the Longmenshan seismogenic fault zone.

Origin of multiple principal slip zones in a fault gouge zone within granitoids

Article

Aug 2022
J STRUCT GEOL

Fault zones in crystalline rocks generally possess a relatively narrow (<50-cm-thick) fault core (i.e., a fault gouge layer/zone). However, the Geumwang Fault, a major strike-slip fault developed in granitic rocks in Korea, has a wider (∼24-m-thick) fault core with several gouge layers. Here we conduct detailed field and microanalytical investigations on the fault core of the Geumwang Fault. Results showed that some of the gouges are primary gouges in which seismic slip was localized as the principal slip zones (PSZs), whereas others are secondary gouges that formed by the injection of fluidized gouge material into fractures. Within the PSZs, the presence of amorphous carbon and graphite implies that frictional heat generated by coseismic slip triggered siderite decomposition. It was likely the consequence of gouge fluidization induced by thermal pressurization under fluid drained conditions. In addition, the gouge fluidization seems to neutralize the foliation of the PSZ. This process, together with weakening of the damage zone by long-term alteration, may facilitate subsequent seismic slip occurred along foliated breccia or the fault core boundary rather than the former neutralized PSZs, thereby widening the Geumwang Fault. We propose that coseismic fluidization of PSZ by thermal pressurization can act as a potential mechanism to widen the fault core and generate multiple gouge layers in crystalline rocks.

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

Article

Full-text available

Jun 2022

Jinyu Chen

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 (μ ss) and a logarithmic decrease in the steady-state electrical conductivity (σ ss) 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.

Diagenetic and shear-induced transitions of frictional strength of carbon-bearing faults and their implications for earthquake rupture dynamics in subduction zones

Article

Full-text available

May 2019

Subduction-related diagenetic reactions affect fault strength and are thus important for understanding earthquake rupture dynamics in subduction zones. Carbonaceous material (CM) is found worldwide in active plate-boundary and intracontinental faults, yet the effect of its transformation on frictional strength and rupture dynamics remains unknown. We conducted high-velocity friction experiments together with organochemical analyses on CM in the form of lignite, bituminous coal, anthracite and graphite. Results clearly show that an increase in CM maturity and crystallinity leads to a decrease in the peak friction coefficient (from 0.5 to 0.2). We also infer that friction applied to low-grade CM increases its maturity, but friction applied to high-grade CM reduces its maturity. These findings suggest that both diagenetic and shear-induced transformations of CM strongly affect the frictional strength of CM-bearing faults, potentially affecting the depth-dependences of frictional strength and rupture dynamics on plate-subduction faults.

Structural Disorder of Graphite and Implications for Graphite Thermometry

Article

Full-text available

Jul 2017
SED

Graphitization, or the progressive maturation of carbonaceous material, is considered an irreversible process. Thus, the degree of graphite crystallinity, or its structural order, has been calibrated as an indicator of the peak metamorphic temperatures experienced by the host rocks. However, discrepancies between temperatures indicated by graphite crystallinity versus other thermometers have been documented in deformed rocks. To examine the possibility of mechanical modifications of graphite structure and the potential impacts on graphite thermometry, we performed laboratory deformation experiments. We sheared highly crystalline graphite powder at normal stresses of 5 and 25 megapascal (MPa) and aseismic velocities of 1, 10 and 100 µm s−1. The degree of structural order both in the starting and resulting materials was analyzed by Raman microspectroscopy. Our results demonstrate structural disorder of graphite, manifested as changes in the Raman spectra. Microstructural observations show that brittle processes caused the documented mechanical modifications of the aggregate graphite crystallinity. We conclude that the calibrated graphite thermometer is ambiguous in active tectonic settings.

The influence of organic–rich shear zones on pelagic sediment deformation and seismogenesis in a subduction zone

Article

Oct 2014

We have examined the mineralogy and deformation of black shear zones containing abundant carbonaceous materials (CM) and clay minerals in bedded ribbon cherts in a Jurassic accretionary complex, central Japan. Microtextural observations indicate that pressure solution and cataclastic deformation were the primary deformation mechanisms in the cherts. Whole–rock mineral compositions were quantitatively investigated using X–ray diffraction, Raman spectroscopy, and a CHN elemental analyzer. The results show that the samples contain variable amounts of CM and clay minerals, up to 17 wt% and 60 wt%, respectively. Moreover, the clay and CM contents in the samples, including the host rock cherts, show a positive correlation represented by a single compositional trend, and this may be explained by the progressive concentration of clays and CM due to pressure solution and the removal of soluble quartz or mass transfer processes associated with deformation. Intact cherts dominated by quartz seem to provide plausible source rocks for the nucleation of seismic slip including slow slip events, while the abundant CM and clays as observed in the black shear zones may have effectively weakened and stabilized the sliding behavior. These results are important for understanding deformation processes in the Japan Trench.

Internal structures and high-velocity frictional properties of Longmenshan fault zone at Shenxigou activated during the 2008 Wenchuan earthquake

Article

Full-text available

Oct 2014

This paper reports internal structures of a wide fault zone at Shenxigou, Dujiangyan, Sichuan province, China, and high-velocity frictional properties of the fault gouge collected near the coseismic slip zone during the 2008 Wenchuan earthquake. Vertical offset and horizontal displacement at the trench site were 2.8 m (NW side up) and 4.8 m (right-lateral), respectively. The fault zone formed in Triassic sandstone, siltstone, and shale about 500 m away from the Yingxiu-Beichuan fault, a major fault in the Longmenshan fault system. A trench survey across the coseismic fault, and observations of outcrops and drill cores down to a depth of 57 m revealed that the fault zone consists of fault gouge and fault breccia of about 0.5 and 250–300 m in widths, respectively, and that the fault strikes N62°E and dips 68° to NW. Quaternary conglomerates were recovered beneath the fault in the drilling, so that the fault moved at least 55 m along the coseismic slip zone, experiencing about 18 events of similar sizes. The fault core is composed of grayish gouge (GG) and blackish gouge (BG) with very complex slip-zone structures. BG contains low-crystalline graphite of about 30 %. High-velocity friction experiments were conducted at normal stresses of 0.6–2.1 MPa and slip rates of 0.1–2.1 m/s. Both GG and BG exhibit dramatic slip weakening at constant high slip rates that can be described as an exponential decay from peak friction coefficient μ p to steady-state friction coefficient μ ss over a slip-weakening distance D c. Deformation of GG and BG is characterized by overlapped slip-zone structures and development of sharp slickenside surfaces, respectively. Comparison of our data with those reported for other outcrops indicates that the high-velocity frictional properties of the Longmenshan fault zones are quite uniform and the high-velocity weakening must have promoted dynamic rupture propagation during the Wenchuan earthquake.

Potential Role of Volcanic Glass‐Smectite Mixtures in Slow Earthquakes in Shallow Subduction Zones: Insights From Low‐ to High‐Velocity Friction Experiments

Article

Full-text available

Jul 2023

Volcanic glass and its mixture with smectite are commonly observed in shallow parts of subduction zones. As volcanic glass layers often act as glide planes in submarine landslides, and because its alteration product, smectite, is one of the frictionally weakest geological materials, the frictional characteristics of volcanic glass‐smectite mixtures are important for fault slip behavior in shallow parts of subduction zones. We performed a series of friction experiments on volcanic glass‐smectite mixtures with different smectite contents from 0% to 100% at various velocity conditions from 10 μm/s to 1 m/s under an effective normal stress of 5 MPa and pore pressure of 10 MPa. In general, apparent friction coefficients negatively depend on the smectite content at any velocity tested. We found that samples with smectite contents of 15%–30% showed a drastic slip‐weakening behavior at intermediate velocities of 1–3 mm/s. Finite element method modeling shows that thermal pressurization does not contribute to the observed weakening behavior. The critical nucleation length estimated from the slip‐weakening behavior is approximately 1–10 km, which is large enough to prevent the slip to accelerate to seismic slip velocity. Therefore, gouges with minor amount of clay, such as subducting volcanic ash layers, may contribute to the occurrence of the slow earthquakes at shallow depths in subduction zones.

Increased biomass and carbon burial 2 billion years ago triggered mountain building

Article

Full-text available

Nov 2021

The geological record following the c. 2.3 billion years old Great Oxidation Event includes evidence for anomalously high burial of organic carbon and the emergence of widespread mountain building. Both carbon burial and orogeny occurred globally over the period 2.1 to 1.8 billion years ago. Prolific cyanobacteria were preserved as peak black shale sedimentation and abundant graphite. In numerous orogens, the exceptionally carbonaceous sediments were strongly deformed by thrusting, folding, and shearing. Here an assessment of the timing of Palaeoproterozoic carbon burial and peak deformation/metamorphism in 20 orogens shows that orogeny consistently occurred less than 200 million years after sedimentation, in a time frame comparable to that of orogens through the Phanerozoic. This implies that the high carbon burial played a critical role in reducing frictional strength and lubricating compressive deformation, which allowed crustal thickening to build Palaeoproterozoic mountain belts. Further, this episode left a legacy of weakening and deformation in 2 billion year-old crust which has supported subsequent orogenies up to the building of the Himalayas today. The link between Palaeoproterozoic biomass and long-term deformation of the Earth’s crust demonstrates the integral relationship between biosphere and lithosphere. High burial of organic carbon in sediments around 2 billion years ago acted to enhance crustal deformation and led to intensified mountain building both in the Paleoproterozoic and since, suggests an assessment of the timing of carbon burial and deformation

Graphitic material in fault zones: Implications for fault strength and carbon cycle

Article

May 2019
EARTH-SCI REV

The study discusses the presence, formation and destruction of graphitic material in fault rocks of exhumed fault zones. Because of the low strength, the presence of lubricating graphitic material along fault zones has important implications for understanding tectonic movements in various crustal levels. Fault zones are permeable for ascending and descending fluids and represent, therefore, effective pathways of fluids between deep lithospheric levels and Earth's surface, and this plumbing system is part of the carbon exchange system of the global carbon cycle between deep lithosphere and atmosphere. Processes of formation, structure and microfabrics of the graphitic material and the implications for the global carbon cycle in natural fault zones are still poorly understood. This paper gives an overview on the range of the origin of graphitic material along fault zones (e.g. organic vs. carbonatic vs. fluid origin) and its physical formation and destruction mechanisms. The presence of graphitic carbon permits: (1) to recognize faults with graphitic lubricants during faulting and allow assess, therefore, crustal strength over various temporal and spatial scales, (2) how carbon-bearing material is moving through the fault zone hence recording the complex structural history, (3) how carbon represents a monitor of fluid transport through fault zones, and (4) how graphitic material allow to pinpoint peak temperature conditions of the faulting process. The data implies also carbon transfer between depth and surface, which contributes to the global carbon cycle, but to a hitherto unknown extent. The presence of graphitic carbon in fault rocks has also implications on fault mechanics, engineering geology, nuclear waste repositories and assessment of seismic hazard.

Carbonaceous Materials in the Fault Zone of the Longmenshan Fault Belt: 1. Signatures within the Deep Wenchuan Earthquake Fault Zone and Their Implications

Article

Full-text available

Sep 2018

Graphitization of carbonaceous materials (CM) has been experimentally demonstrated as potential evidence of seismic slip within a fault gouge. The southern segment of the Longmenshan fault, a CM-rich-gouge fault, accommodated coseismic slip during the 2008 Mw 7.9 Wenchuan earthquake and potentially preserves a record of processes that occurred on the fault during the slip event. Here, we present a multi-technique characterization of CM within the active fault zone of the Longmenshan fault from the Wenchuan earthquake Fault Scientific Drilling-1. By contrast with field observations, graphite is pervasively and only distributed in the gouge zone, while heterogeneously crystallized CM are present in the surrounding breccia. The composite dataset that is presented, which includes the localized graphite layer along the 2008 Wenchuan earthquake principal slip zone, demonstrates that graphite is widely distributed within the active fault zone. The widespread occurrence of graphite, a seismic slip indicator, reveals that surface rupturing events commonly occur along the Longmenshan fault and are characteristic of this tectonically active region.

High-velocity frictional properties of Alpine Fault rocks: Mechanical data, microstructural analysis, and implications for rupture propagation

Article

Full-text available

Apr 2017
J STRUCT GEOL

The Alpine Fault in New Zealand is a major plate-bounding structure that typically slips in ∼M8 earthquakes every c. 330 years. To investigate the near-surface, high-velocity frictional behavior of surface- and borehole-derived Alpine Fault gouges and cataclasites, twenty-one rotary shear experiments were conducted at 1 MPa normal stress and 1 m/s equivalent slip velocity under both room-dry and water-saturated (wet) conditions. In the room-dry experiments, the peak friction coefficient (μp = τp/σn) of Alpine Fault cataclasites and fault gouges was consistently high (mean μp = 0.67 ± 0.07). In the wet experiments, the fault gouge peak friction coefficients were lower (mean μp = 0.20 ± 0.12) than the cataclasite peak friction coefficients (mean μp = 0.64 ± 0.04). All fault rocks exhibited very low steady-state friction coefficients (μss) (room-dry experiments mean μss = 0.16 ± 0.05; wet experiments mean μss = 0.09 ± 0.04). Of all the experiments performed, six experiments conducted on wet smectite-bearing principal slip zone (PSZ) fault gouges yielded the lowest peak friction coefficients (μp = 0.10–0.20), the lowest steady-state friction coefficients (μss = 0.03–0.09), and, commonly, the lowest specific fracture energy values (EG = 0.01–0.69 MJ/m2). Microstructures produced during room-dry and wet experiments on a smectite-bearing PSZ fault gouge were compared with microstructures in the same material recovered from the Deep Fault Drilling Project (DFDP-1) drill cores. The near-absence of localized shear bands with a strong crystallographic preferred orientation in the natural samples most resembles microstructures formed during wet experiments. Mechanical data and microstructural observations suggest that Alpine Fault ruptures propagate preferentially through water-saturated smectite-bearing fault gouges that exhibit low peak and steady-state friction coefficients.

Friction properties and deformation mechanisms of halite(-mica) gouges from low to high sliding velocities

Article

Nov 2016
EARTH PLANET SC LETT

The evolution of friction as a function of slip rate is important in understanding earthquake nucleation and propagation. Many laboratory experiments investigating friction of fault rocks are either conducted in the low velocity regime ( – ) or in the high velocity regime (0.01–1 m s⁻¹). Here, we report on the evolution of friction and corresponding operating deformation mechanisms in analog gouges deformed from low to high slip rates, bridging the gap between these low and high velocity regimes. We used halite and halite–muscovite gouges to simulate processes, governing friction, active in upper crustal quartzitic fault rocks, at conditions accessible in the laboratory. The gouges were deformed over a 7 orders of magnitude range of slip rate ( –1 m s⁻¹) using a low-to-high velocity rotary shear apparatus, using a normal stress of 5 MPa and room-dry humidity. Microstructural analysis was conducted to study the deformation mechanisms. Four frictional regimes as a function of slip rate could be recognized from the mechanical data, showing a transitional regime and stable sliding ( –10⁻⁶ m s⁻¹), unstable sliding and weakening ( –10⁻³ m s⁻¹), hardening ( –10⁻¹ m s⁻¹) and strong weakening ( –1 m s⁻¹). Each of the four regimes can be associated with a distinct microstructure, reflecting a transition from mainly brittle deformation accompanied by pressure solution healing to temperature activated deformation mechanisms. Additionally, the frictional response of a sliding gouge to a sudden acceleration of slip rate to seismic velocities was investigated. These showed an initial strengthening, the amount of which depended on the friction level at which the step was made, followed by strong slip weakening.

Micro-scale composition and texture of fault rocks from SAFOD core: evidence for deformation processes and fluid-rock reactions

Article

Full-text available

Jan 2013

Applicability of slip tendency for understanding long-term fault activity: a case study of active faults in northeastern Japan

Article

May 2015

The assessment of fault activity is necessary to mitigate against damaging earthquakes. To adequately assess long-term fault activity, a new technique that complements chronological investigations is required. Here we examine the applicability of the slip tendency (ST) method, based on physical models, to assess fault activity in Japan. We computed the ST using a friction coefficient of 0.6 for the faults, a common friction coefficient for rocks, and the stress state estimated from earthquake focal mechanisms in north-eastern Japan. Calculated ST values are high for the most active faults (e.g., the Senya segment) and are low for inactive fault (e.g., the Sakunami—Yashikidaira segment). Therefore, we propose that the ST method can be applied in assessments of fault activity. However, the ST method sometimes underestimates fault activity, and we have explored the necessary input parameters to obtain reliable results. To use the ST method, the determination of these parameters is critical for the robust assessment of fault activity based on physical models.

Tracing of a deep fault zone in the Olkhon region by the self-potential method

Conference Paper

Full-text available

Aug 2023

N.O. Kozhevnikov

Surface geological mapping and remote sensing are principal tools when studying the geology of the Olkhon region. However, the possibilities of these techniques, especially in extrapolating their results to depth, are limited. Obviously, to study the structure of the Olkhon region, it is necessary to involve geophysical methods enabling three-dimensional geological mapping. The article presents and discusses the results of applying the self-potential (SP) method in studies of the Chernorud-Barakchin deep fault zone. The zone is distinguished by intense negative SP anomalies, which are produced by a geobattery formed by steeply dipping electronic conductors with electrically interconnected graphite mineralization and surrounding ion-conductive rocks. The anomalies trace near-to-vertical shear planes along which movements occur having resulted in the formation of the Chernorud graben.

Macro to micro fault zone characteristics, fracture systems and porosity in basement hosted unconformity-type uranium deposits of the Athabasca region (Saskatchewan, Canada)

Thesis

Full-text available

Dec 2021

Maher Abdelrazek

The Athabasca region in western Canada is a metallogenic province hosting numerous uranium deposits that contain among the highest grades as known. The uranium mineralization is associated with the unconformity between the Archean basement and the overlying undeformed Athabasca sandstones. Some of the known economic deposits are located within basement faults that developed along inherited ductile shear zones. Development of these brittle fault zones is usually characterized by breccia, damage zones, and frequently by a vertical offset at the unconformity. Reactivated inherited basement structures allowed the development of structural traps in which the mineralization could be precipitated. The typical known model for these basement-hosted deposits, or “Ingress deposits”, implies that basin-derived fluids enriched in uranium flowing along basement faults. Interaction between these fluids and basement rocks leads to uranium precipitation in the structures. In a mineralizing system in which structural control is prominent, deformation zones may have been acted either as fluid conduits or barrier regarding the fluid circulations. Numerous studies mention such structures and their role played in uranium mineralization process in the basement, in the Athabasca region, in the Thelon and in the McArthur basin (Australia), with however lack of knowledge about the exact mechanisms and the conditions related to the development of the fertile structures. Distinction is not made between structures that may act as a fluid conduit or as fluid screen, which strongly limits the knowledge of the role played by different episodes of deformation and alteration into the setup of a fertile porosity network. This thesis focuses on the study of basement structures, U-mineralization and their alteration halo at different scales and allows to highlight the lead role played by the preexisting ductile fabric in the development of a favorable system for uranium-bearing fluid flow. Basement shear zones testify about a long fluid history (hydrothermal alteration, silicification, dissolution), and while reactivated later under brittle conditions made favorable environment for fluid flow beneath the Athabasca basin. Reactivation in the brittle domain leads to the development of a fractured corridors that are characterized by the reworking of ductile fabric into open fractures, accompanied with development of tensile jogs, which are suitable for mineral precipitation. The mineralization was emplaced lately within such structures that form a favorable porosity network within these strongly deformed and altered intervals. The present work allowed to define a structural signature that is common for two different prospects located at two opposite sides of the Athabasca basin. This signature can be qualified as a “structural footprint” is based on the overprinting of ductile shear zones by a network micro-fracture, in strongly deformed and altered basement rocks. The results also showed the importance of the structural approach at different scales (from project-scale, to drill core sample scale, and microstructures analysis) in the understanding of which mechanisms are critical in the emplacement of a favorable porosity network related with uranium mineralization.

Macro et micro caractéristiques des zones de failles, systèmes de fracture et porosité des gisements d'uranium de socle de l'Athabasca (Saskatchewan, Canada)

Thesis

Full-text available

Dec 2021

Maher Abdelrazek

La région d’Athabasca au Canada concentre les gisements d’uranium de haute teneur les plus riches connus. Les minéralisations uranifères sont associées à la discordance entre le socle archéen à paléoprotérozoïque et les grès non-déformés de l’Athabasca. Une partie des gisements économiques connus sont situés au niveau de failles de socle qui se développent le long d’anciennes zones de cisaillement. Le développement de ces failles se traduit par la présence de brèches, zones de fracturation, et parfois d’un rejet vertical de la discordance. La réactivation des structures de socle héritées a permis la formation de pièges structuraux dans lesquels précipite la minéralisation. Le modèle de formation de ces gisements de socle, ou modèle « Ingress », propose la circulation de saumures de bassin chargées en uranium dissous le long des failles de socle. L’interaction entre ces fluides et les roches du socle entraine la précipitation de l’uranium dans les structures. Dans un système minéralisateur où le contrôle structural est fort, les zones de déformation peuvent agir comme chemin préférentiel ou écran vis-à-vis de la circulation fluide. De nombreux travaux décrivent ces structures et leurs rôles dans la mise en place des minéralisations d’uranium de socle dans l’Athabasca, le Thelon et le bassin de McArthur en Australie, mais les conditions et mécanismes de développement de ces structures sont peu contraints. La distinction n’est pas faite entre les différents types de structures dans la mise en place des minéralisations (drain ou piège), ce qui limite la compréhension de l’impact des différents épisodes de déformation et d’altération dans la mise en place d’un réseau de porosité fertile. Ces travaux de thèse, basés sur l’étude des structures de socle et des altérations associées à différentes échelles, ont mis en avant l’importance de l’héritage ductile dans le développement d’un système propice à la circulation de fluides et à la mise en place de minéralisations d’uranium. Les zones de cisaillement ductile témoignant d’une longue histoire fluide (altération hydrothermale, silicification, dissolution), et qui réactivées ultérieurement dans le domaine cassant sont des environnements favorables à la circulation de fluides sous le bassin. Des réactivations cassantes vont entrainer le développement d’une zone de fracturation et permettre, selon l’orientation des structures par rapport au champ de déformation, l’ouverture des plans de foliation. Le développement de ces fentes de tension ou « tensile jogs » créée l’espace adéquat à un remplissage minéral. La minéralisation se mets en place tardivement dans ces structures qui forment un réseau de porosité adéquat au sein de ces intervalles très déformés et altérés. Les travaux présentés ont permis de définir une signature structurale commune à deux gisements situés de part et d’autre du bassin d’Athabasca. Cette signature que l’on peut qualifier « d’empreinte structurale » repose sur la superposition de déformations cisaillantes ductiles et d’une zone de micro-fracturation, dans des roches de socle fortement déformé altéré. Les résultats obtenus ont aussi montré l’importance de l’approche structurale à différentes échelles (depuis l’échelle du projet, de l’observation des carottes jusqu’à celle des microstructures) dans la compréhension des mécanismes critiques pour la mise en place d’un réseau de porosité en relation avec des minéralisations d’uranium.

A single extensional, diffuse, ductile fault zone in the Goriganga section, western Himalaya: Part of the upper South Tibetan Detachment System?

Article

Sep 2022
TECTONOPHYSICS

The Trans-Himadri Detachment Fault (T-HDF) of the Kumaon region of western Himalaya, a north-easterly steeply dipping discrete brittle/semi-brittle fault, had previously been regarded as a strand of the South Tibetan Detachment System (STDS). Yet, elsewhere in the Himalaya, the STDS is defined as a diffuse, ductile low-angle normal-sense shear zone. This aspect of the STDS remained conjectural and was even refuted by various workers in the context of the Goriganga valley in the Kumaon. We have systematically documented the STDS as a low-angle, northerly dipping, ductile, diffuse shear zone, with a true thickness of >5 km in this transect. In the present scenario, the shear zone is located only within the psammo-pelitic basal part of the Tethyan Sedimentary Sequence (TSS), and bypassed the rock units of the Greater Himalayan Sequence (GHS). Multiple grain-statistical parameters of mylonitized micaceous quartzites point to an anastomosing architecture which displays a single precursory damage zone and a wider core for the entire STDS shear zone. The ⁴⁰Ar*/³⁹Ar geochronology and oxygen isotope analysis of the micas in mylonites demonstrated that the STDS evolved in weak lithologies, between c.14–11 Ma, at ~300–350 °C ambient temperature, at or immediately below the brittle-plastic transition depth in the quartzo-feldspathic continental crust. Consideration of similar range of optimum ages of cessation of the extensional activity and already published timings of the end of brittle motion of the STDS in adjacent regions suggests that this wide diffuse shear zone is a deeper counterpart of the brittle upper STDS. This is in sharp contrast to the exposures of the detachment elsewhere in the Himalaya, where diffuse zone demonstrably constitutes the lower branch of this detachment. This finding has potential implications on overall understanding of the deformation related to the STDS and late Miocene geodynamic evolution of the GHS in this region.

The Memory of a Fault Gouge: An Example from the Simplon Fault Zone (Central Alps)

Article

Full-text available

Jun 2022

A fault gouge forms at the core of the fault as the result of a slip in the upper brittle crust. Therefore, the deformation mechanisms and conditions under which the fault gouge was formed can document the stages of fault movement in the crust. We carried out a microstructural analysis on a fault gouge from a hanging-wall branch fault of the Simplon Fault Zone, a major low-angle normal fault in the Alps. We use thin-section analysis, together with backscattered electron imaging and X-ray diffractometry (XRD), to show that a multistage history from ductile to brittle deformation, together with a continuous exhumation history from high to low temperature, took place within the fault gouge. Because of the predominance of pressure solution and veining, we associated a large part of the deformation in the fault gouge with viscous-frictional behaviour that occurred at the brittle-ductile transition. Phyllosilicates and graphite likely caused fault lubrication that we suggested played a role in the formation of this major low-angle normal fault.

Properties of fault zones and their influences on rainfall-induced landslides, examples from Alborz and Zagros ranges

Article

Full-text available

Mar 2022
ENVIRON EARTH SCI

Mohsen Ehteshami-Moinabadi

Rainfall-induced landslides are among the most fatal and destructive geological hazards in the mountainous regions where active faulting is a major geological process. So far, many studies have been conducted on the various factors that influence or trigger rainfall-induced landslides; however, lesser attention has been paid to the proximity of rainfall-induced landslides to major faults. First time, this paper is going to discuss the concepts of fault damage zone and fault zone processes that interact and influence the occurrence of rainfall-induced landslides. This is done by introducing the concept of fault damage zone and its architecture and then presenting several examples from high mountains of Alborz and Zagros in which fault zones interact proactively with mass movement processes. Fault zone processes control the rainfall-induced landslides by three ways: (1) increasing fracture density near major faults that produces more debris and reduces rock strength, (2) weak minerals growing in fault zones in which clay minerals grow and (3) topographic features produced by active faulting. Presented examples show how active faulting can influence localization of rainfall-induced landslides. Intensely weathered rocks in the fault zone are a major source of sediment flux combined with the water to form mudflows. Additionally, fault mechanism and movement direction of fault blocks influence on internal geometry and attitude of minor structures is fault zone that controls properties of rock materials in the fault zone and therefore influence on slope strength. The interaction and connectivity among tectonic faults, river systems and erosion is a complex feature that should be considered in modeling landslides in the mountainous regions.

Title: Hydro‐mechanical measurements of sheared crystalline rock fractures with applications for EGS Collab experiments 1 and 2

Article

Full-text available

Feb 2022

We present hydro‐mechanical measurements that characterize shear on natural fractures in schist, amphibolite, and rhyolite specimens from the enhanced geothermal system (EGS) Collab Project's Experiment 1 and 2 sites (E1 and E2) at the Sanford Underground Research Facility. We employed a triaxial direct shear method augmented with X‐ray imaging to perform hydroshearing (injection‐induced shearing) and mechanical shearing on naturally fractured specimens at in situ stress conditions. Measurements included fracture permeability, strength, stress‐dependent aperture, shear dilation, and frictional strength. Results reveal that in situ natural fractures must be permeable, weak, and shear‐oriented to be hydrosheared, and only a subset of the observable in situ fractures were suitable. When sheared, the fracture permeability typically increased by a factor of 10 or more and this increase was retained over time. However, shear slip did not always result in permeability increase. High phyllosilicate content associated with exceptionally weak fractures exhibited poor or even decreased permeability after stimulation. These measurements in combination with site data were used to conduct a slip‐tendency analysis for different fracture sets, and we selected the top candidate natural fractures for hydroshearing at the EGS Collab sites. We also found that the lower in situ shear stress and stronger fractures at the E2 site make hydroshearing more challenging than at the E1 site. Overall, the methods and analysis used in our work can be applied to any geothermal project to identify in situ joint sets that are best suited for hydroshearing, which in turn can help to optimize well placement and energy production.

Optically Stimulated Luminescence Signal Resetting of Quartz Gouge During Subseismic to Seismic Frictional Sliding: A Case Study Using Granite‐Derived Quartz

Article

Full-text available

Oct 2020

Attempts to determine the age of faulting recorded by fault rocks have been made using K–Ar, fission‐track, electron spin resonance, and luminescence dating methods. Here we report the unambiguous reduction (resetting) of optically stimulated luminescence (OSL) signals of quartz gouge after friction experiments and estimate the seismological and geological conditions required for OSL signal resetting in natural fault zones. In the experiments, we used quartz sand with a particle size of <150 μm and equivalent dose of 31.5 ± 16.6 Gy. Friction experiments were conducted with a rotary‐shear, high‐velocity friction apparatus at slip rates (V) of 200 μm s−1 to 1.3 m s−1, normal stress of 1.0 MPa, and displacement of 10 m. In the experiments conducted under dry conditions, the OSL signal starts to decrease from V = 0.25 m s−1 and becomes near zero at V ≥ 0.65 m s−1. OSL signal resetting is also observed in the experiment sheared at 1.3 m s−1 under water‐added conditions. At V = 0.25 and 0.40 m s−1, partial resetting occurs, which is characterized by coexistence of particles with and without an OSL signal. OSL signal intensity shows a strong correlation with applied power density and frictional heat during high‐velocity friction, and the signal exponentially decreases with increasing power density and temperature. The power density required for partial and complete OSL signal resetting is ~0.17 and 0.6 MW m−2, respectively. Assuming a co‐seismic fault slip rate of 0.6 m s−1, the depths required for partial and complete resetting are expected to be ≥11 and ≥42 m.

Deformation fabrics and strain localization mechanisms in graphitic carbon-bearing rocks from the Ailaoshan-Red River strike-slip fault zone

Article

Full-text available

Aug 2020
J STRUCT GEOL

Graphitic carbon-bearing rocks generally occur in low-to high-grade metamorphic units. In many brittle faults, graphitic carbon is often associated with gouge or low-grade metamorphic rocks whereas in ductile faults, graphitic carbon commonly occurs in marble, schist or gneiss. Carbonaceous material gradually transforms from an amorphous into an ordered crystalline structure by increasing thermal metamorphism. The degree of graphitization is believed to be a reliable indicator of peak temperature conditions in the metamorphic rock. In this contribution, based on detailed field observations, the variably deformed and metamorphosed graphitic gneisses to phyllites, located within the footwall and hangingwall unit of the Cenozoic Ailaoshan-Red River strike-slip shear zone are studied. According to lithological features and temperatures determined by Raman spectra of carbonaceous material, these graphitic rocks and deformation fabrics are divided into three types. Type I is represented by medium-grade metamorphism and strongly deformed rocks with an average temperature of 509 °C and a maximum temperature of 604 °C. Type II is affected by low-grade metamorphism and deformed rocks with an average temperature of 420 °C. Type III is affected by lower-grade metamorphism and occurs in weakly deformed/undeformed rocks with an average temperature of 350 °C. Slip-localized micro-shear zones and laterally continuous or discontinuous slip-planes constituted by graphitic carbon aggregates are developed in Types I and II. The electron back-scattered diffraction (EBSD) lattice preferred orientation (LPO) patterns of graphitic carbon grains were firstly observed in comparison with LPO patterns of quartz and switch from basal , rhomb to prism slip systems, which indicate increasing deformation temperatures. Comparison of quartz and graphite LPOs indicate that graphite LPOs show higher temperature conditions. According to the graphitic slip-planes, micro-shear zones with grain-size reduction and mylonitic foliation constituted by graphitic carbon minerals, we also propose that the development of fine-grained amorphous carbon plays an important role in rheological weakening of the whole rock during progressive ductile shearing.

Attenuation of High-Frequency P and S Waves in the Crust of Central and Western Tien Shan

Article

Full-text available

Sep 2020
PURE APPL GEOPHYS

In order to better understand the attenuation characteristics in the crust of the central and western Tien Shan orogenic belt, we investigate both Qp and Qs values by applying the extended coda-normalization method. We estimate the frequency-dependent attenuation of both P and S waves in the frequency band of 1.0–20.0 Hz using data from local networks. The average frequency relations of Qp and Qs have been derived as Qp = (61 ± 9) × f(1.21±0.08) and Qs = (77 ± 6) × f(1.11±0.04) by fitting a power-law frequency dependence model for the study region. The low Q0 and high η values indicate that the central and western Tien Shan is a tectonically and seismically active region. We also find lateral variations of both Qp,s and Qs/Qp values, reflecting complex tectonic structures in the study area. In general, the relatively high-attenuation areas corresponding to tectonically active regions are found beneath mountainous ranges in the central Tien Shan whereas the relatively low-attenuation areas are associated with stable regions, such as Fergana Basin and Issyk-Kul Lake. Besides, regions in the central Tien Shan with Qs/Qp > 1 are likely partially saturated with fluids or rich in scattering heterogeneities, whereas regions in the western Tien Shan display Qs/Qp < 1, possibly suggesting almost complete fluid saturation.

Frictional properties of simulated shale-coal fault gouges: Implications for induced seismicity in source rocks below Europe's largest gas field

Article

May 2020
INT J COAL GEOL

We report 21 frictional sliding experiments performed on simulated fault gouges prepared from shale-coal mixtures. Our aim was to investigate the effects of local coal seam smearing on the frictional properties and induced seismogenic potential of faults cutting the Upper Carboniferous source rocks underlying the Groningen gas reservoir (Netherlands). We used shale/siltstone core recovered from beneath the Groningen reservoir plus Polish bituminous coal of similar age and origin to coals locally present in the Groningen source rocks. We performed friction experiments in velocity stepping, constant velocity, slide-hold-slide (SHS) and slide-unload-slide (SUS) modes, under near in-situ conditions of 100 °C and 40 MPa effective normal stress, employing sliding velocities of 0.1–100 μm/s and a variety of pore fluids. Samples with 0–50 vol% coal showed friction coefficients ~0.45, with minor slip weakening. Samples with ≥50 vol% coal showed marked slip-weakening from peak friction values of ~0.47 to ~0.3, regardless of experimental conditions, presumably reflecting strain localization in weak coal-rich shear bands, possibly accompanied by changes in coal molecular structure. However, re-sliding experiments (SUS) showed that slip-weakening is limited to small initial displacements (2–3 mm), and does not occur during slip reactivation. At (near) steady state, almost all experiments performed at in-situ stress, pore water pressure (15 MPa) and temperature conditions exhibited stable, velocity strengthening behaviour, regardless of coal content. By contrast, under dry and gas-saturated (CH4, Argon) conditions, or using water at 1 atm, 50:50 (vol%) shale-coal mixtures showed velocity-weakening and even stick-slip. Our results imply that faults in the Groningen Carboniferous shale-siltstone sequence are not prone to induced earthquake nucleation at in-situ conditions, even when coal-bearing or coal-enriched by smearing. However, the mechanisms controlling coal friction remain unclear at the sliding velocities studied, and the evolution of coal friction at seismic slip velocities remains unknown.

Frictional slip weakening and shear-enhanced crystallinity in simulated coal fault gouges at subseismic slip rates

Preprint

Full-text available

Apr 2020

Abstract. Previous studies show that organic-rich fault patches may play an important role in promoting unstable fault slip. However, the frictional properties of rock materials with near 100 % organic content, e.g. coal, and the controlling microscale mechanisms, remain unclear. Here, we report seven velocity stepping (VS) and one slide-hold-slide (SHS) friction experiments performed on simulated fault gouges prepared from bituminous coal, collected from the upper Silesian Basin of Poland. These experiments were performed at 25–45 MPa effective normal stress and 100 °C, employing sliding velocities of 0.1–100 μm s−1, using a conventional triaxial apparatus plus direct shear assembly. All samples showed marked slip weakening behaviour at shear displacements beyond ~ 1–2 mm, from a peak friction coefficient approaching ~ 0.5 to (near) steady state values of ~ 0.3, regardless of effective normal stress or whether vacuum dry flooded with distilled (DI) water at 15 MPa pore fluid pressure. Analysis of both unsheared and sheared samples by means of microstructural observation, micro-area X-ray diffraction (XRD) and Raman spectroscopy suggests that the marked slip weakening behaviour can be attributed to the development of R-, B- and Y- shear bands, with internal shear-enhanced coal crystallinity development. The SHS experiment performed showed a transient peak healing (restrengthening) effect that increased with the logarithm of hold time at a linearized rate of ~ 0.006. We also determined the rate-dependence of steady state friction for all VS samples using a full rate and state friction approach. This showed a transition from velocity strengthening to velocity weakening at slip velocities > 1 μm s−1 in the coal sample under vacuum dry conditions, but at > 10 μm s−1 in coal samples exposed to DI water at 15 MPa pore pressure. This may be controlled by competition between dilatant granular flow and compaction enhanced by presence of water. Together with our previous work on frictional properties of coal-shale mixtures, our results imply that the presence of a weak, coal-dominated patch on faults that cut or smear-out coal seams may promote unstable, seismogenic slip behaviour, though the importance of this in enhancing either induced or natural seismicity depends on local conditions.

A regional Alpine graphite décollement level beneath the NW Pannonian Basin

Article

Full-text available

Sep 2019

The so-called Transdanubian Conductivity Anomaly (TCA) of the Hungarian part of the NW Pannonian Basin has been well known for more than five decades. The exceptionally low resistivity (i.e. 1–2 Ωm) zone has a very large areal extent (on the order a few thousand km2) and it is an entirely subsurface anomaly occurring at depth between circa 3–15 km, with no corresponding outcrops. Various geological explanations of this enigmatic crustal-scale geophysical anomaly range from invoking sub-horizontal Alpine nappe contacts to sub-vertical dikes with graphite and/or saline fluid content. Only one possible analogue outcrop area was considered for the high conductivity anomaly so far, namely the Drauzug/Gailtal area of the Eastern Alps in Austria, some 300 km to the West from the TCA area. Previous attempts to find correspondence between the TCA and prominent seismic reflectors seen on 2D seismic reflection profiles were based on data acquired by research institutions. This study systematically correlates, for the first time, the TCA with 2D industry seismic reflection data in the same area. Our new results show a very strong correlation between the subsurface extent and location of the TCA with various sub-horizontally oriented Cretaceous Alpine nappe surfaces. In addition, we draw on the latest structural correlation of the Alpine nappe stack of the Transdanubian Range with its proper tectonic counterpart in the Eastern Alps.At the southern edge of the Upper Austroalpine units in northern Styria, in the Veitsch Nappe of the Greywacke Zone, numerous graphite localities are known historically. These laterally extensive graphite units in NW Styria formed as the result of greenschist-grade metamorphism of a Carboniferous coal sequence during the Cretaceous. For the first time, we describe here one well penetration of possibly age-equivalent graphitic units in NW Hungary. Correlation of the magnetotelluric anomaly with the distinct reflection seismic signature suggests that the same Palaeozoic graphitebearing Upper Austroalpine units should be present at 3–15 km depth in our study area.Therefore we propose that the best explanation for the observed extent and geometry of the TCA is the presence of graphite in subhorizontal, tectonically thinned detachment surfaces at the base of the Upper Austroalpine nappe edifice of NW Hungary

Role of Weak Materials in Earthquake Rupture Dynamics

Article

Full-text available

Apr 2019

Weak materials in seismic slip zones are important in studies of earthquake mechanics. For instance, the exceptionally large slip during the 2011 Tohoku-Oki earthquake has been attributed to the presence of smectite in the fault zone. However, weak fault rocks cannot accumulate large amounts of elastic strain, which is thought to counter their ability to enhance seismic rupture. It is well known that if the permeability of a weak fault is low enough to allow friction-induced thermal pressurization of interstitial fluid, the fault strength decreases dramatically. However, whether intrinsic weakness of fault material or thermal pressurization more efficiently produces large slip on faults bearing weak materials has not been determined. To investigate the role of weak materials in earthquake rupture dynamics, we conducted friction experiments and dynamic rupture simulations using pure smectite and pure graphite to represent weak fault materials. Even when we assumed no thermal pressurization, simulated faults in both media were able to trigger large slip because their extremely low friction was insufficient to arrest the inertial motion of rupture propagating along the fault. We used similar rupture simulations to investigate the cause of the huge slip near the trench during the 2011 Tohoku-Oki earthquake and demonstrated that it can be attributed to thermal pressurization, although our findings suggest that the presence of smectite in the plate-boundary fault may also be required.

Carbonaceous Materials in the Fault Zone of the Longmenshan Fault Belt: 2. Characterization of Fault Gouge from Deep Drilling and Implications for Fault Maturity

Article

Full-text available

Sep 2018

In recent works on the determination of graphitization of carbonaceous materials (CM) within the principal slip zone (PSZ) of the Longmenshan fault (China), we demonstrated that the formation of graphite, resulted from strain and frictional heating, could be evidence of past seismic slip. Here we utilize Raman Spectroscopy of CM (RSCM) on the CM-bearing gouges in the fault zone of the Longmenshan fault belt, at the borehole depth of 760 m (FZ760) from the Wenchuan earthquake Fault Scientific Drilling project-1 (WFSD-1), to quantitatively characterize CM and further retrieve ancient fault deformation information in the active fault. RSCM shows that graphitization of CM is intense in the fault core with respect to the damage zone, with the graphitized carbon resembling those observed on experimentally formed graphite that was frictionally generated. Importantly, compared to the recognized active fault zone of the Longmenshan fault, the RSCM of measured CM-rich gouge shows a higher degree of graphitization, likely derived from high-temperature-perturbation faulting events. It implies that FZ760 accommodated numerous single-event displacement and/or at higher normal stresses and/or in the absence of pore fluid and/or along a more localized slip surface(s). Because graphite is a well-known lubricant, we surmise that the presence of the higher degree graphitized CM within FZ760 will reduce the fault strength and inefficiently accumulate tectonic stress during the seismic cycle at the current depth, and further infer a plausible mechanism for fault propagation at the borehole depth of 590 m during the Mw 7.9 Wenchuan earthquake.

Microstructures in landslides in Northwest China – Implications for creeping displacements?

Article

Nov 2017
J STRUCT GEOL

Microstructures, mineralogical composition and texture of selected landslide samples from three landslides in the southern part of the Gansu Province (China) were examined with optical microscopy, transmission electron microscopy (TEM), x-ray diffraction (XRD) and synchrotron x-ray diffraction measurements. Common sheet silicates are chlorite, illite, muscovite, kaolinite, pyrophyllite and dickite. Other minerals are quartz, calcite, dolomite and albite. In one sample, graphite and amorphous carbon were detected by TEM-EDX analyses and TEM high-angle annular dark-field images. The occurrence of graphite and pyrophyllite with very low friction coefficients in the gouge material of the Suoertou and Xieliupo landslides is particularly significant for reducing the frictional strength of the landslides. It is proposed that the landslides underwent comparable deformation processes as fault zones. The low friction coefficients provide strong evidence that slow-moving landsliding is controlled by the presence of weak minerals. In addition, TEM observations document that grain size reduction in clayey slip zone material was produced mainly by mechanical abrasion. For calcite and quartz, grain size reduction was attributed to both pressure solution and cataclasis. Therefore, besides landslide composition, the occurrence of ultrafine-grained slip zone material may also contribute to weakening processes of landslides. TEM images of slip-zone samples show both locally aligned clay particles, as well as kinked and folded sheet silicates, which are widely disseminated in the whole matrix. Small, newly formed clay particles have random orientations. Based on synchrotron x-ray diffraction measurements, the degree of preferred orientation of constituent sheet silicates in local shear zones of the Suoertou and Duang-He-Ba landslide is strong. This work is the first reported observation of well-oriented clay fabrics in landslides.

Transformation of slip modes of fault with spatially inhomogeneous structure

Conference Paper

Nov 2016

We study the principal possibility of a controllable change of the fault slip mode in slider experiments. It is shown that a slight change of the material composition and/or the rheology of a local segment of the interblock contact may lead to an essential change of slip mode parameters. One of the ways to change the rheology of a local interface segment without changing its material composition is to damp it. The efficiency of the slip mode transformation by liquid injection is determined by the material composition of a local segment exposed to the injection.

Deep crustal expressions of exhumed strike-slip fault systems: Shear zone initiation on rheological boundaries

Article

Sep 2016
EARTH-SCI REV

The formation of major exhumed strike-slip faults represents one of the most important dynamic processes affecting the evolution of the Earth's lithosphere. Detailed models of the potential initiation, their properties and architecture of orogen-scale exhumed strike-slip faults, which are often subparallel to mountain ranges, are rare. The initiation of strike-slip faults is at depth, where temperature-controlled rheological weakening mechanisms play the essential role localizing future strike-slip faults. In this review study, we highlight that in pluton- and metamorphic core complex (MCC)-controlled tectonic settings, as end-members, the initiation of strike-slip faults occurs by rheological weakening along hot-to-cold contacts deep within the crust and mantle lithosphere, respectively. These endmember processes are potential mechanisms for the initiation of orogen-scale exhumed strike-slip faults at depth result in a specific thermal and structural architecture. Similar processes guide the overall displacement and ultimately the exhumation at such deep levels. These types of exhumed strike-slip dominated fault zones expose a wide variety of mylonitic, cataclastic and non-cohesive fault rocks on the surface, which were formed at different structural levels of the crust during various stages of faulting and exhumation. Exhumation of mylonitic rocks is, therefore, a common feature of such reverse oblique-slip strike-slip faults, implying major transtensive and/or transpressive processes accompany pure strike-slip motion during exhumation. A major aspect of many exhumed strike-slip faults is their lateral thermal gradient induced by the lateral juxtaposition of hot and cold levels of the crust controlling relevant properties of such fault zones, and thus the overall fault architecture (e.g., fault core, damage zone, shear lenses, fault rocks) and its thermal structure. These properties of the overall fault architecture include strength of fault rocks, permeability and porosity, the hydrological regime, as well as the nature and origin of circulating hydrothermal fluids.

Hydraulic properties of samples retrieved from the Wenchuan earthquake Fault Scientific Drilling Project Hole‐1 (WFSD‐1) and the surface rupture zone: Implications for coseismic slip weakening and fault healing

Article

Full-text available

Jun 2016
GEOCHEM GEOPHY GEOSY

In this study, we report the hydraulic properties of samples recovered from the first borehole of the Wenchuan earthquake Fault Scientific Drilling and from outcrops associated with the surface rupture zone of the 2008 Wenchuan earthquake. Compositional and microstructural analyses have also been performed on selected samples. Using the pore pressure oscillation method, the permeability measurements show that (1) fault gouge samples have low permeabilities, decreasing from 2 × 10−18 m2 at an effective pressure (Pe) of 10 MPa (equivalent to an in situ depth of 600 m) to 9 × 10−21 m2 at 155 MPa. (2) Intact and cemented samples are impermeable with permeabilities less than 2 × 10−20 m2 at 10 MPa. (3) Fractured samples have variable permeabilities, ranging from 3 × 10−15 to 1 × 10−20 m2 at 10 MPa, and are most insensitive to changes in the effective pressure. (4) Granitic cataclasites have a moderate permeability at low pressure (i.e., 10−16 to 10−17 m2 at 10 MPa); which decreases rapidly with increasing Pe. Hydraulic conduction of the fault is believed to be influenced by the permeability of the fractures developed, which is controlled by the density, aperture, and/or connectivity of the fractures. Microstructural and compositional analyses of the samples indicate that the fault zone heals through chemically mediated fracture closure related to mineral precipitation, possibly assisted by pressure solution of stressed fracture asperities. Although other weakening mechanisms remain possible, our laboratory measurements combined with numerical modeling reveal that thermal/thermochemical pressurization, perhaps leading to gouge fluidization, played an important role in the dynamic weakening of the Wenchuan earthquake, at least in the study area.

A rotary-shear low to high-velocity friction apparatus in Beijing to study rock friction at plate to seismic slip rates

Article

Oct 2014

This paper reviews 19 apparatuses having high-velocity capabilities, describes a rotary-shear low to high-velocity friction apparatus installed at Institute of Geology, China Earthquake Administration, and reports results from velocity-jump tests on Pingxi fault gouge to illustrate technical problems in conducting velocity-stepping tests at high velocities. The apparatus is capable of producing plate to seismic velocities (44 mm/a to 2.1 m/s for specimens of 40 mm in diameter), using a 22 kW servomotor with a gear/belt system having three velocity ranges. A speed range can be changed by 103 or 106 by using five electromagnetic clutches without stopping the motor. Two cam clutches allow fivefold velocity steps, and the motor speed can be increased from zero to 1,500 rpm in 0.1–0.2 s by changing the controlling voltage. A unique feature of the apparatus is a large specimen chamber where different specimen assemblies can be installed easily. In addition to a standard specimen assembly for friction experiments, two pressure vessels were made for pore pressures to 70 MPa; one at room temperature and the other at temperatures to 500 °C. Velocity step tests are needed to see if the framework of rate-and-state friction is applicable or not at high velocities. We report results from velocity jump tests from 1.4 mm/s to 1.4 m/s on yellowish gouge from a Pingxi fault zone, located at the northeastern part of the Longmenshan fault system that caused the 2008 Wenchuan earthquake. An instantaneous increase in friction followed by dramatic slip weakening was observed for the yellowish gouge with smooth sliding surfaces of host rock, but no instantaneous response was recognized for the same gouge with roughened sliding surfaces. Instantaneous and transient frictional properties upon velocity steps cannot be separated easily at high velocities, and technical improvements for velocity step tests are suggested.

Effect of a change in the state of stress on the likelihood of inland fault failure during the Mw 6.6 Iwaki earthquake resulting from the Mw 9.0 2011 Tohoku earthquake, Japan

Article

Sep 2015
TECTONOPHYSICS

A Mw 6.6 earthquake occurred on 11 April 2011 in the Iwaki area, northeastern Honshu arc, inland Japan (hereafter, the 2011 Iwaki earthquake). This event occurred just one month after the 2011 Mw 9.0 off the Pacific coast of Tohoku earthquake (11 March 2011; hereafter, the 2011 Tohoku earthquake). The 2011 Iwaki earthquake was produced by near-simultaneous slip on two faults (the Itozawa and Yunodake faults). Here, we examine the failure on the Itozawa and Yunodake faults with respect to the change in the state of stress in the Iwaki area produced by the 2011 Tohoku earthquake. Furthermore, we quantitatively evaluate the excess fluid pressure. Our analysis reveals that the antecedent state of stress played an important role in the occurrence of the 2011 Iwaki earthquake. We demonstrate the importance of excess fluid pressure in terms of the initiation of the 2011 Iwaki earthquake. Three end-member models of failure were identified based on our modeling with different friction coefficients. (1) High friction coefficients (>. 0.75), in which case triggering of the Iwaki earthquake would have required fluid pressure levels greater than those of σ3. These pressures should have been regulated by fault-valve action, and evidence for such action should appear in the seismic record. (2) Low friction coefficients (<. 0.4), in which case triggering of the Iwaki earthquake would have occurred in the absence of excess fluid pressure, although this is inconsistent with the relatively high competence of the crust in the area as inferred from the velocity structure derived from tomographic data. (3) Moderate friction coefficients (0.4-0.75), which would occur at moderate levels of excess fluid pressure, which are reasonable given the calculated magnitudes of crustal stress.

Shear‐Enhanced Electrical Conductivity of Synthetic Quartz‐Graphite Gouges: Implications for Electromagnetic Observations in Carbonaceous Shear Zones

Article

Full-text available

Nov 2023

Graphite is considered as a material that promotes fault weakening and electrical conductivity (σ) enhancement at fault zones. We studied how shear deformation may affect the evolution of friction and electrical conductivity of synthetic quartz (Qz)‐graphite (Gr) mixtures and, more importantly, whether the σ of the mixtures present visible changes at the beginning of the simulated fault slip. Long‐displacement friction experiments were performed on 1.2–2.3 mm‐thick gouge specimens of varied Gr volume fraction (XGr = 0–100 vol.%) under identical normal stress (2 or 5 MPa), slip rate (∼1.0 mm/s), and N2‐flushing conditions. The experimental results suggested that the σ of the specimens with ≥4.6 vol.% XGr abruptly increased under limited shear displacement. With continued shear, the steady‐state electrical conductivity (σss) increased by more than seven orders of magnitude when XGr > 3.4 vol.%, while the steady‐state frictional coefficient remained high (0.54–0.80) except for the specimens with XGr > 13.6 vol.%. The post‐mortem microstructures revealed that the high σss observed in the intermediate Gr content specimens (3.4–13.6 vol.%) is associated with an ad‐hoc fabric (graphite–cortex clasts) present in the principal slip zone. For high Gr content, excess Gr flakes fill the pores and help develop mechanically lubricated surfaces. We propose that low Gr content (i.e., as low as 3.4 vol.%) can cause high conductivity anomalies in natural shear zones. Overall, the findings suggest that the initiation of slips within carbonaceous shear zones can be detected by identifying unusual temporal signals using electromagnetic stations.

Shear-enhanced electrical conductivity of synthetic quartz-graphite gouges: Implications for electromagnetic observations in carbonaceous shear zones

Preprint

Apr 2023

Graphite is considered as a material that promotes fault weakening and electrical conductivity (σ) enhancement of fault zones. We studied how shear deformation may affect the evolution of friction and electrical conductivity of synthetic quartz (Qz)-graphite (Gr) mixtures and, more importantly, whether the σ of the mixtures present visible changes at the beginning of the simulated fault slip resembling the preslip of an earthquake. Long-displacement friction experiments were performed on 1.2–2.3 mm-thick gouge specimens of varied Gr volume fraction (XGr = 0–100 vol.%) under identical normal stress (2 or 5 MPa), slip rate (~1.0 mm/s), and N2-flushing conditions. The experimental results suggested that the σ of the specimens with ≥4.6 vol.% XGr abruptly increased under limited shear displacement. With continued shear, the steady-state electrical conductivity (σss) increased by more than 7 orders of magnitude when XGr > 3.4 vol.%, while the steady-state frictional coefficient remained high (0.54–0.80) except for the specimens with XG > 13.6 vol.%. The post-mortem microstructures revealed that the high σss observed in the intermediate Gr content specimens (3.4–13.6 vol.%) is associated with an ad-hoc fabric (graphite-cortex clasts) present in the principal slip zone. Whereas for high Gr content, excess Gr flakes fill the pores and help develop mechanically lubricated surfaces. We propose that low Gr content (as low as 3.4 vol.%) can cause high conductivity anomalies in natural shear zones. Moreover, electromagnetic stations distributed along the strike of carbonaceous shear zones aim to detect the pre-slip phases of earthquakes in nucleation zones.

Investigations and new insights on earthquake mechanics from fault slip experiments

Article

Apr 2022
EARTH-SCI REV

Earthquakes occur mainly on active faults. Fault slip is closely related to seismicity and is thus widely discussed in Geosciences, Seismology, and Engineering. Slip experiment is a necessary and powerful tool to explore the physical processes and mechanisms of pre-earthquake, earthquake, and aftershock. This work reviews the experiment study of fault slip from field experiments, laboratory experiments, and numerical experiments, which helps to provide a clear understanding of earthquake mechanics. We show that there are five main influencing factors in the study of fault and earthquake: stress, velocity, material, fluid, and temperature. Around these factors, the process of shear failure and rupture nucleation, the weakening and strengthening of fault, the characteristics of slip behavior, and the signal characteristics of slip are discussed. These works do not only have the potential to advance the understanding of the earthquake mechanism, but also to guide the prediction and control of earthquakes disasters. Furthermore, there are still several issues that need to be better discussed, such as the influence of stress disturbance on fault stability, the scale effect between natural faults and laboratory faults, and the role of roughness in the friction and slip characteristics of faults. Moreover, it is also necessary to consider the earthquake precursors of multiple signals such as seismic velocity, electrical signal, and magnetic signal, as well as effectively capture them in combination with artificial intelligence technology. It will be new breakthroughs in the prediction of earthquakes.

Evolution of Fault Zones and Its Rheology断層帯と断層レオロジーの進化

Article

Full-text available

Aug 2020

Knowledge of the strength of faults in the continental upper crust is critical to our understanding of crustal stress states, coseismic faulting, and lithospheric deformation. In this paper, we investigate time- and displacement-dependent fault-zone weakening (softening) over geological time caused by the hydrothermal alteration of rock, the development of faulting-related structure and fabric, and changes in the relevant deformation mechanisms. In the shallow portion of the continental seismogenic zone (< 5 km), hydrothermal alteration induced by comminution and fluid flow along fault zones progressively enriches weak phyllosilicates. The development of phyllosilicate-aligned fabric with increasing shear strain leads to an effective weakening with increasing cumulative fault displacement. In the deep portion of the seismogenic zone (> 5 km), frictional–viscous flow occurs in combination with friction contributed by phyllosilicates and the dissolution–precipitation of clasts after the introduction of water, phyllosilicates and anastomosing fabrics all increasing with greater fault displacement. In addition, the water weakening of quartz and feldspar is an important softening process in the deeper portion of the seismogenic zone (> 10 km). The smoothing of fault-zone topography by the shearing of irregularities and asperities, as well as the thickening of the fault zone, leads to a reduction over time in the bulk frictional resistance of a fault as displacement increases. These time- and displacement-dependent weakening processes of fault zones give rise to diverse strength and stress states of the crust depending on its maturity and may provide clues to reconciling the stress–heat flow paradox of crustal faults.

Frictional slip weakening and shear-enhanced crystallinity in simulated coal fault gouges at slow slip rates

Article

Full-text available

Jul 2020
SE

Previous studies show that organic-rich fault patches may play an important role in promoting unstable fault slip. However, the frictional properties of rock materials with nearly 100 % organic content, e.g., coal, and the controlling microscale mechanisms remain unclear. Here, we report seven velocity stepping (VS) experiments and one slide–hold–slide (SHS) friction experiment performed on simulated fault gouges prepared from bituminous coal collected from the upper Silesian Basin of Poland. These experiments were performed at 25–45 MPa effective normal stress and 100 ∘C, employing sliding velocities of 0.1–100 µm s−1 and using a conventional triaxial apparatus plus direct shear assembly. All samples showed marked slip-weakening behavior at shear displacements beyond ∼ 1–2 mm, from a peak friction coefficient approaching ∼0.5 to (nearly) steady-state values of ∼0.3, regardless of effective normal stress or whether vacuum-dry or flooded with distilled (DI) water at 15 MPa pore fluid pressure. Analysis of both unsheared and sheared samples by means of microstructural observation, micro-area X-ray diffraction (XRD) and Raman spectroscopy suggests that the marked slip-weakening behavior can be attributed to the development of R-, B- and Y-shear bands, with internal shear-enhanced coal crystallinity development. The SHS experiment performed showed a transient peak healing (restrengthening) effect that increased with the logarithm of hold time at a linearized rate of ∼0.006. We also determined the rate dependence of steady-state friction for all VS samples using a full rate and state friction approach. This showed a transition from velocity strengthening to velocity weakening at slip velocities >1 µm s−1 in the coal sample under vacuum-dry conditions but at >10 µm s−1 in coal samples exposed to DI water at 15 MPa pore pressure. The observed behavior may be controlled by competition between dilatant granular flow and compaction enhanced by the presence of water. Together with our previous work on the frictional properties of coal–shale mixtures, our results imply that the presence of a weak, coal-dominated patch on faults that cut or smear out coal seams may promote unstable, seismogenic slip behavior, though the importance of this in enhancing either induced or natural seismicity depends on local conditions.

Structural records and mechanical characteristics of seismic slip along an active fault crosscutting unconsolidated Quaternary sediments: Suryum fault, SE Korea

Article

Jan 2020

The Suryum fault is an active fault in southeast Korea crosscutting the Quaternary sediment deposit. We conducted outcrop-scale to microscale observations of the fault slip zone and performed shear experiments on the fault gouge at subseismic (3–30 µm/s) and seismic slip rates (0.53 m/s). The gouge is rich in clay minerals (> 58%), particularly in expandable clay minerals (40%). In the gouge zone, a very narrow (20–150 µm thick) principal slip zone (PSZ), identified by the strong alignment of clay minerals, was developed. At several places along the fault, the gouge is observed to have been injected into the Quaternary wallrock sediments. The development of the narrow PSZ is incompatible with the velocity-strengthening behavior of the Suryum fault gouge observed in the shear tests, and it may indicate that seismic rupture could be propagated along the PSZ in the clay-rich gouge. Slip localization into the PSZ was presumably possible because of significant dynamic fault weakening due to thermal pressurization (or buildup of pore pressure caused by frictional heating) in the low-permeability clay gouge during fast slip. The gouge injections along the Suryum fault may be the geological record of gouge fluidization caused by thermal pressurization. This idea is supported by observations of very low friction of the Suryum fault gouge when sheared experimentally at the seismic slip rate and of the gouge injections in the sheared gouge. The dynamic weakness of the Suryum fault implies that large displacement and low-frequency ground motion at near-surface depths would be possible when it is reactivated.

Critical Displacement of Earthquake-Triggered Catastrophic Landslides

Conference Paper

May 2017

The critical displacement is referred to as a threshold for slope failure, compared with the calculated permanent displacement under seismic load using Newmark displacement analysis. The critical displacement, usually obtained from laboratory shear tests under low and constant shear rates, is defined as the coseismic displacement beyond which strength of sliding surface reach residual values. The typical value ranges a few centimeters, depending on the frictional characteristics of sheared materials. However, this definition of the critical displacement might be oversimplified since the strength of sliding surface is velocity-dependent. Therefore, we collected the shear tests results of different materials under different shear velocities to evaluate the velocity-displacement dependency. Besides, we redefine the critical displacement of catastrophic landslide (Dcr) as the accumulated permanent displacement before rapid slide occurred. The influence of the strength and seismic parameters on the newly defined critical displacement is assessed using Newmark displacement analysis incorporating velocity-displacement dependent friction law. The dip angle of sliding surface is assumed as 15°. The synthetic seismic load is simplified as sinusoidal wave with peak ground acceleration of 600 gal. Different seismic frequencies of 0.5, 1.2, 2.0 Hz are used to evaluate the influence of frequency on Dcr. The results show that the range of Dcr is much higher than few centimeters, and Dcr is highly related to frictional characteristics of sheared materials, especially within the slip-weakening distance. Moreover, Dcr is also influenced by frequency rather than peak ground motion acceleration of the sinusoidal wave. This study highlights that the initiation of landslide are extremely complex, which can be function of frictional law, seismic frequency, and geometry of sliding plane. The reasonableness of the velocity-displacement dependent friction law and the representative of the seismic wave should be considered for evaluating the initiation of catastrophic, rapid moving landslide.

The effect of water on strain localization in calcite fault gouge sheared at seismic slip rates

Article

Feb 2017
J STRUCT GEOL

Strain localization during coseismic slip in fault gouges is a critical mechanical process that has implications for understanding frictional heating, the earthquake energy budget and the evolution of fault rock microstructure. To assess the nature of strain localization during shearing of calcite fault gouges, high-velocity ( ) rotary-shear experiments at normal stresses of 3–20 MPa were conducted under room-dry and wet conditions on synthetic calcite gouges containing dolomite gouge strain markers. When sheared at 1 m/s, the room-dry gouges showed a prolonged strengthening phase prior to dynamic weakening, whereas the wet gouges weakened nearly instantaneously. Microstructural analysis revealed that a thin (<600 μm) high-strain layer and through-going principal slip surface (PSS) developed after several centimeters of slip in both dry and wet gouges, and that strain localization at 1 m/s occurred progressively and rapidly. The strain accommodated in the bulk gouge layer did not change significantly with increasing displacement indicating that, once formed, the high-strain layer and PSS accommodated most of the displacement. Thus, a substantial strain gradient is present in the gouge layer. In water-dampened gouges, localization likely occurs during and after dynamic weakening. Our results suggest that natural fault zones in limestone are more prone to rapid dynamic weakening if water is present in the granular slipping zones.

Raman spectra of carbonaceous materials in a fault zone in the Longmenshan thrust belt, China; comparisons with those of sedimentary and metamorphic rocks

Article

Jan 2017
TECTONOPHYSICS

We analyzed micro-Raman spectra of carbonaceous materials (CM) in natural and experimentally deformed fault rocks from Longmenshan fault zone that caused the 2008 Wenchuan earthquake, to characterize degree of disordering of CM in a fault zone. Raman spectral parameters for 12 samples from a fault zone in Shenxigou, Sichuan, China, all show low-grade structures with no graphite. Low crystallinity and δ¹³C values (− 24‰ to − 25‰) suggest that CM in fault zone originated from host rocks (Late Triassic Xujiahe Formation). Full width at half maximum values of main spectral bands (D1 and D2), and relative intensities of two subbands (D3 and D4) of CM were variable with sample locations. However, Raman parameters of measured fault rocks fall on established trends of graphitization in sedimentary and metamorphic rocks. An empirical geothermometer gives temperatures of 160–230 °C for fault rocks in Shenxigou, and these temperatures were lower for highly sheared gouge than those for less deformed fault breccia at inner parts of the fault zone. The lower temperature and less crystallinity of CM in gouge might have been caused by the mechanical destruction of CM by severe shearing deformation, or may be due to mixing of host rocks on the footwall. CM in gouge deformed in high-velocity experiments exhibits slight changes towards graphitization characterized by reduction of D3 and D4 intensities. Thus low crystallinity of CM in natural gouge cannot be explained by our experimental results. Graphite formation during seismic fault motion is extremely local or did not occur in the study area, and the CM crystallinity from shallow to deep fault zones may be predicted as a first approximation from the graphitization trend in sedimentary and metamorphic rocks. If that case, graphite may lower the friction of shear zones at temperatures above ~ 300 °C, deeper than the lower part of seismogenic zone.

The role of graphite layers in gravitational deformation of pelitic schist

Article

Apr 2016
ENG GEOL

Deep-seated landslides in pelitic schists are common in many countries, but are poorly investigated and understood. In this study we present the first detailed examination and modelling of landslide mechanisms in these materials. We found that pelitic schist commonly contains black, graphite-rich layers on a scale of millimeter to centimeter thickness that are typically weaker than neighboring layers. By examining microscopic textures in borehole samples obtained from landslide masses of pelitic schist, we find that ductile gravitational shearing commonly occurs within these weaker layers, accompanied by brittle fracture in the surrounding layers. To investigate these mechanisms, we have performed high-precision direct shear tests, using a novel back-pressured shearbox, on artificial rock samples both with and without graphite layers placed between pre-cut shear surfaces. The tests used normal stresses up to 800 kPa (equivalent to 32 m depth of burial). We found that the coefficients of friction for samples with graphite layers embedded in the artificial rock samples (0.30, representing an angle of internal friction of 16.7°) were much lower than those without graphite layers on the pre-cut surface (0.85). The shear strength of the artificial rocks with embedded layers of graphite decreased abruptly with increasing areal extent of the graphite layer along the shear surface, from which it can be inferred that the continuity of a graphite layer in natural pelitic schist has a considerable effect on shear resistance. These results suggest that even comparatively low dip angles of schistosity in pelitic schist could initiate microscopic slip along the graphite-rich layers.

Fast-moving dislocations trigger flash weakening in carbonate-bearing faults during earthquakes

Article

Full-text available

Nov 2015

Rupture fronts can cause fault displacement, reaching speeds up to several ms−1 within a few milliseconds, at any distance away from the earthquake nucleation area. In the case of silicatebearing rocks the abrupt slip acceleration results in melting at asperity contacts causing a large reduction in fault frictional strength (i.e., flash weakening). Flash weakening is also observed in experiments performed in carbonate-bearing rocks but evidence for melting is lacking. To unravel the micro-physical mechanisms associated with flash weakening in carbonates, experiments were conducted on pre-cut Carrara marble cylinders using a rotary shear apparatus at conditions relevant to earthquakes propagation. In the first 5 mm of slip the shear stress was reduced up to 30% and CO2 was released. Focused ion beam, scanning and transmission electron microscopy investigations of the slipping zones reveal the presence of calcite nanograins and amorphous carbon. We interpret the CO2 release, the formation of nanograins and amorphous carbon to be the result of a shock-like stress release associated with the migration of fast-moving dislocations. Amorphous carbon, given its low friction coefficient, is responsible for flash weakening and promotes the propagation of the seismic rupture in carbonate-bearing fault patches.

11. Friction of the Smectite Clay Montmorillonite A Review and Interpretation of Data

Chapter

Full-text available

Jan 2007

Strength of chrysotile-serpentinite gouge under hydrothermal conditions: Can it explain a weak San Andreas fault?

Article

Full-text available

Nov 1996
GEOLOGY

Chrysotile-bearing serpentinite is a constituent of the San Andreas fault zone in central and northern California. At room temperature, chrysotile gouge has a very low coefficient of friction (mu ≈ 0.2), raising the possibility that under hydrothermal conditions mu might be reduced sufficiently (to =200 °C, it is substantially stronger and essentially independent of velocity at the lowest velocities tested. We estimate that pure chrysotile gouge at hydrostatic fluid pressure and appropriate temperatures would have shear strength averaged over a depth of 14 km of 50 MPa. Thus, on the sole basis of its strength, chrysotile cannot be the cause of a weak San Andreas fault. However, chrysotile may also contribute to low fault strength by forming mineral seals that promote the development of high fluid pressures.

Frictional strengths of talc-serpentine and talc-quartz mixtures

Article

Full-text available

Jan 2011

Talc is a constituent of faults in a variety of settings, and it may be an effective weakening agent depending on its abundance and distribution within a fault. We conducted frictional strength experiments under hydrothermal conditions to determine the effect of talc on the strengths of synthetic gouges of lizardite and antigorite serpentinites and of quartz. Small amounts of talc weaken serpentinite gouges substantially more than predicted by simple weight averaging. In comparison, mixtures of quartz and talc show a linear trend of strength reduction at talc concentrations

Crystallographic controls on the frictional behavior of dry and water-saturated sheet structure minerals

Article

Full-text available

Mar 2004

We compare the frictional strengths of 17 sheet structure mineral powders, measured under dry and water-saturated conditions, to identify the factors that cause many of them to be relatively weak. The dry coefficient of friction μ ranges upward from 0.2 for graphite, leveling off at 0.8 for margarite, clintonite, gibbsite, kaolinite, and lizardite. The values of μ (dry) correlate directly with calculated (001) interlayer bond strengths of the minerals. This correlation occurs because shear becomes localized along boundary and Riedel shears and the platy minerals in them rotate into alignment with the shear planes. For those gouges with μ (dry) < 0.8, shear occurs by breaking the interlayer bonds to form new cleavage surfaces. Where μ (dry) = 0.8, consistent with Byerlee's law, the interlayer bonds are sufficiently strong that other frictional processes dominate. The transition in dry friction mechanisms corresponds to calculated surface energies of 2–3 J/m2. Adding water causes μ to decrease for every mineral tested except graphite. If the minerals are separated into groups with similar crystal structures, μ (wet) increases with increasing interlayer bond strength within each group. This relationship also holds for the swelling clay montmorillonite, whose water-saturated strength is consistent with the strengths of nonswelling clays of similar crystal structure. Water in the saturated gouges forms thin, structured films between the plate surfaces. The polar water molecules are bonded to the plate surfaces in proportion to the mineral's surface energy, and μ (wet) reflects the stresses required to shear through the water films.

Effect of clay content and mineralogy on frictional sliding behavior of simulated gouges: Binary and ternary mixtures of quartz, illite, and montmorillonite

Article

Full-text available

Mar 2010

We investigated the frictional sliding behavior of simulated quartz-clay gouges under stress conditions relevant to seismogenic depths. Conventional triaxial compression tests were conducted at 40 MPa effective normal stress on saturated saw cut samples containing binary and ternary mixtures of quartz, montmorillonite, and illite. In all cases, frictional strengths of mixtures fall between the end-members of pure quartz (strongest) and clay (weakest). The overall trend was a decrease in strength with increasing clay content. In the illite/quartz mixture the trend was nearly linear, while in the montmorillonite mixtures a sigmoidal trend with three strength regimes was noted. Microstructural observations were performed on the deformed samples to characterize the geometric attributes of shear localization within the gouge layers. Two micromechanical models were used to analyze the critical clay fractions for the two-regime transitions on the basis of clay porosity and packing of the quartz grains. The transition from regime 1 (high strength) to 2 (intermediate strength) is associated with the shift from a stress-supporting framework of quartz grains to a clay matrix embedded with disperse quartz grains, manifested by the development of P-foliation and reduction in Riedel shear angle. The transition from regime 2 (intermediate strength) to 3 (low strength) is attributed to the development of shear localization in the clay matrix, occurring only when the neighboring layers of quartz grains are separated by a critical clay thickness. Our mixture data relating strength degradation to clay content agree well with strengths of natural shear zone materials obtained from scientific deep drilling projects.

Frictional Behavior and Constitutive Modeling of Simulated Fault Gouge

Article

Full-text available

May 1990

Presents an investigation of the frictional properties and stability of frictional sliding for simulated fault gouge. In these experiments gouge layers (quartz sand) were sheared under saturated drained conditions and at constant normal stress (50-190 MPa) between either rough steel surfaces or Westerly granite surfaces in a triaxial apparatus. Porosity φ was monitored continuously during shear. Measurements indicate that granular gouge exhibits strain hardening and net compaction for shear strains γ >0.5-1.0. For γ sliding occurs at approximately constant shear stress and net compaction from one load/unload cycle to the next ceases. Dilatancy occurs at 1/3 to 1/2 the shear stress required for sliding and d2φ/dγ2 becomes negative at about the peak stress in a given loading cycle, indicating the onset of shear localization. Experiments with an initial gouge layer exhibit velocity strengthening, and initially bare granite surfaces exhibit velocity weakening. Data suggest that slip within unconsolidated granular material, such as some natural fault gouges, is inherently stable. -from Authors

Strength, porosity, and permeability development during hydrostatic and shear loading of synthetic quartz-clay fault gouge

Article

Full-text available

Mar 2008

The strength and permeability of fault zones must be quantified in order to accurately predict crustal strength and subsurface fluid migration. To this end, we performed experiments on mixtures of fine-grained quartz and kaolinite incremented at 10 wt% intervals between the two end-member components (analogues for natural fault gouge) in order to establish their strength and fluid flow properties during hydrostatic and shear loading. Hydrostatically compacted samples exhibited permeability reduction on increasing effective pressures from 5 MPa to 50 MPa, with the rate of reduction displaying strong dependency on the synthetic fault gouge composition. The permeability decreases continuously with increasing kaolinite content. Porosity exhibits a distinct minimum that evolves with increasing effective pressure according to the relative compaction of the quartz and kaolinite end-members. Porosity evolution with increasing clay content is predicted satisfactorily by a simple ideal packing model. At the highest effective pressure (50 MPa), permeability reduced log-linearly over 4 orders of magnitude with increasing clay content. Mechanically, sheared gouge samples showed a continuous reduction in frictional strength with increasing clay fraction. Permeability decreased further on shear loading after initial hydrostatic compaction to 50 MPa. This was most evident for the pure quartz end-member, with two orders of magnitude additional reduction, whereas the clay-rich samples were reduced only tenfold, mostly before a shear strain of 5. Variation of permeability with both clay content and shear deformation may be adequately described by previously published empirical predictors for fault zone permeability. Clay content has the largest effect on permeability, and shear deformation affects permeability of quartz-rich gouges more than clay-rich gouges.

Fabric induced weakness of tectonic faults

Data

Full-text available

Feb 2010

1] Mature fault zones appear to be weaker than predicted by both theory and experiment. One explanation involves the presence of weak minerals, such as talc. However, talc is only a minor constituent of most fault zones and thus the question arises: what proportion of a weak mineral is needed to satisfy weak fault models? Existing studies of fault gouges indicate that >30% of the weak phase is necessary to weaken faults ‐ a proportion not supported by observations. Here we demonstrate that weakening of fault gouges can be accomplished by as little as 4 wt% talc, provided the talc forms a critically‐aligned, through‐going layer. Observations of foliated fault rocks in mature, large‐ offset faults suggest they are produced as a consequence of ongoing fault displacement and thus our observations may provide a common explanation for weakness of mature faults. Citation: Niemeijer, A., C. Marone, and D. Elsworth (2010), Fabric induced weakness of tectonic faults, Geophys. Res. Lett., 37, L03304, doi:10.1029/2009GL041689.

Fault lubrication during earthquakes

Article

Full-text available

Mar 2011
NATURE

The determination of rock friction at seismic slip rates (about 1 m/s) is of paramount importance in earthquake mechanics, as fault friction controls the stress drop, the mechanical work and the frictional heat generated during slip1. Given the difficulty in determining friction by seismological methods, elucidating constraints are derived from experimental studies. Here we review a large set of published and unpublished experiments (300) performed in rotary shear apparatus at slip rates of 0.1-2.6 m/s. The experiments indicate a significant decrease in friction (of up to one order of magnitude), which we term fault lubrication, both for cohesive (silicate-built, quartz-built and carbonate-built) rocks and non-cohesive rocks (clay-rich, anhydrite, gypsum and dolomite gouges) typical of crustal seismogenic sources. The available mechanical work and the associated temperature rise in the slipping zone trigger a number of physicochemical processes (gelification, decarbonation and dehydration reactions, me

Fault zone fabric and fault weakness

Article

Full-text available

Dec 2009
NATURE

Geological and geophysical evidence suggests that some crustal faults are weak compared to laboratory measurements of frictional strength. Explanations for fault weakness include the presence of weak minerals, high fluid pressures within the fault core and dynamic processes such as normal stress reduction, acoustic fluidization or extreme weakening at high slip velocity. Dynamic weakening mechanisms can explain some observations; however, creep and aseismic slip are thought to occur on weak faults, and quasi-static weakening mechanisms are required to initiate frictional slip on mis-oriented faults, at high angles to the tectonic stress field. Moreover, the maintenance of high fluid pressures requires specialized conditions and weak mineral phases are not present in sufficient abundance to satisfy weak fault models, so weak faults remain largely unexplained. Here we provide laboratory evidence for a brittle, frictional weakening mechanism based on common fault zone fabrics. We report on the frictional strength of intact fault rocks sheared in their in situ geometry. Samples with well-developed foliation are extremely weak compared to their powdered equivalents. Micro- and nano-structural studies show that frictional sliding occurs along very fine-grained foliations composed of phyllosilicates (talc and smectite). When the same rocks are powdered, frictional strength is high, consistent with cataclastic processes. Our data show that fault weakness can occur in cases where weak mineral phases constitute only a small percentage of the total fault rock and that low friction results from slip on a network of weak phyllosilicate-rich surfaces that define the rock fabric. The widespread documentation of foliated fault rocks along mature faults in different tectonic settings and from many different protoliths suggests that this mechanism could be a viable explanation for fault weakening in the brittle crust.

Planetary science. Weak faults--rotten cores

Article

Full-text available

Feb 2004
SCIENCE

R. E. Holdsworth

Earthquakes are triggered when crustal deformation produces displacements along fault zones. The rock in these zones is found to be weaker than adjacent material, but what are the mechanisms of this loss of strength? In his Perspective, Holdsworth discusses how recent studies of exposed ancient fault zones combined with laboratory analyses are revealing that rock properties at the grain scale can determine the large scale behavior of crustal deformation.

2. The Seismogenic Zone of Subduction Thrust Faults What We Know and Don’t Know

Chapter

Jan 2007

R. D. Hyndman

Fabric inducedweakness oftectonic faults

Article

Jan 2010

A new rotary-shear high-speed frictional testing machine: Its basic design and scope of research

Article

Jan 1994

Experimental studies of simulated gouge and their application to studies of natural fault zones

Article

Jan 1979

Fault geometry and paleo-movement of the central part of the Ushikubi fault, northern central Japan

Article

Jan 2008

The Ushikubi fault, located along the boundary between Toyama and Gifu prefectures, northern central Japan, is one of the longest active faults in Japan. We performed morphological and structural analyses based on geological survey of the central part of the Ushikubi fault. As a result, we identified the geometry and movement history of this fault. Along the Ushikubi fault, multiple fractures are branched, bent, paralleled, slanted intersection, and form a pretty complicated shear zone. Particularly in the central part of the study area, the multiple fractures form a strike-slip duplex. The geometry of the shear zone indicates it was originated from a sinistral strike-slip fault. In addition, the observation of polished sections and thin sections supports the result. In this study, the shear zone along the Ushikubi fault is called the Ushikubi shear zone. This shear zone was generated as a sinistral strike-slip fault in Late Cretaceous time, and has been inversely reactivated as the Ushikubi active fault.

The occurrence of graphite-bearing fault rocks in the Atotsugawa fault system, Japan: Origins and implications for fault creep

Article

May 2012
J STRUCT GEOL

Graphite in fault zones has received little attention even though it is a well-known solid lubricant that could affect frictional properties of faults dramatically. This paper reports the presence of abundant graphite in fault zones of the Atotsugawa fault system, central Japan. Mesoscopic and microscopic observations of fault rocks revealed two processes of carbon enrichment in fault zones. One is a pressure solution process or diffusive mass transfer in general which removes water-soluble minerals such as quartz and carbonates from rocks, resulting in the enrichment of insoluble minerals including carbon. The other process is precipitation of graphite from a high-temperature carbon-rich fluid, forming graphite filling fractures within cataclasitic fault zones. The two processes have led to the concentration, up to 12 wt% of graphite, in the Atotsugawa fault zones, compared to 0 to 3 wt% of carbonaceous materials in the host rocks. This concentration is high enough for graphite to affect frictional properties at wide range of slip rates. The presence of graphite may provide an explanation for the low resistivity, the patterns of microearthquakes and fault creep along the western part of the Atotsugawa fault system. Graphite should receive more attention as a weakening and stabilizing agent of faults.

Shear-induced graphitization of carbonaceous materials during seismic fault motion: Experiments and possible implications for fault mechanics

Article

Jun 2011
J STRUCT GEOL

Carbonaceous materials often concentrate in fault zones developed in pelitic rocks. Among carbonaceous minerals, graphite is known as a lubricant and possibly plays a key role in frictional properties of the fault. Graphite reported from slip localized zones suggests that graphitization can occur during seismogenic fault motion. Thus, we performed friction experiments on amorphous carbon and graphite to investigate how graphite forms in association with fault motion and how these carbonaceous minerals affect frictional properties of faults. Experiments were done at normal stresses of 0.5–2.8 MPa and slip rates of 50 μm/s to 1.3 m/s in atmospheres of air and N2 gas, using rotary-shear apparatuses. XRD and TEM analyses revealed that graphitization can indeed occur during seismogenic fault motion perhaps due to large shear strain, short-lived flash heating and stress concentration at asperity contacts, even at low temperatures and pressures under anoxic environments. We found large differences in steady-state friction coefficient μss between graphite (μss = 0.1) and amorphous carbon (μss = 0.54) at low slip rate. But amorphous carbon exhibits marked velocity weakening at slip rate above 10 mm/s, and its steady-state friction reduces to the same level as that of graphite at a slip rate of 1.3 m/s. Faults with amorphous carbon are not weak at low slip rates, but they can become dynamically weak to foster fault motion during the generation of large earthquakes. Enriched graphite in fault zones can lubricate at all slip rates even at great depths and should receive more attention.

Brittle deformation and graphitic cataclasites in the pilot research well KTB-VB (Oberpfalz, FRG)

Article

Jan 1990

Within the research well KTB-VB, late- to post-Variscan brittle deformation is widespread, indicating that the well is situated in a broad fault zone. Graphite-enriched foliated cataclasites within paragneisses are probably connected with late-Variscan NW trending reverse faults. These faults were succeeded by further, graphite-free, faults in post-Variscan time. During the formation of the graphitic cataclasites water-consuming retrograde metamorphism was active producing large amounts of phyllosilicates, especially at the expense of feldspar. The increase in phyllosilicates and graphite was associated with a switch of the dominant deformation mechanisms from brittle fracture to frictional sliding, crystal plasticity and diffusion-controlled processes. The cataclastic fabric suggests that the main part of the movement within the graphitic cataclasites was likely to have been aseismic rather than seismic.

Effects of clay content on the frictional strength and fluid transport property of faults

Article

Aug 2007

We examined two effects of the presence of clay in brittle deformations: the reduction in frictional strength and the impediment of across-fault fluid flow. Permeability was monitored during the sliding deformation of a gouge of various mixes of Na-montmorillonite powder and granular quartz along a 30° precut surface of Berea sandstone under 80 MPa of normal stress, 5 MPa of pore water pressure, and room temperature. The decrease in the friction coefficient of a gouge with increasing clay content was not simple, but showed a sharp drop at 50 vol.% clay content. The reduction in permeability due to deformation increased with increasing clay content from 0 to 24 vol.%, and a dramatic reduction of ~2.5 orders of magnitude occurred at 18 and 24 vol.% clay content. However, in a gouge with more than 29 vol.% clay content, deformation reduced the permeability by only 0.5 orders of magnitude. Thus the transitional clay contents at which the clay dominated the properties of fault strength and fluid transport were 50 and 29 vol.%, respectively. These values almost agree well with values obtained using a model with an equal-sized clast (quartz) framework. The clay matrix can completely fill the pores sustained by a closest-packed quartz framework when the clay content reaches 29 vol.%, whereas the content required to fill the pores of a framework that can barely sustain quartz-quartz contact is 50 vol.%.

Fluid- and reaction-assisted low-angle normal faulting: Evidence from rift-related brittle fault rocks in the Alps (Err Nappe, eastern Switzerland)

Article

Jul 1999
J STRUCT GEOL

Gianreto Manatschal

Brittle fault rocks associated with a rift-related low-angle detachment system, exposed in the Lower Austroalpine Err Nappe (eastern Switzerland), exhibit systematic changes in mineralogy, fabrics, and structures, which are reflected by the transition from an undeformed granite to a cataclasite and a phyllosilicate-rich gouge across the fault zone. Cross-cutting relationships between the brittle fault rocks and syn-kinematic quartz veins suggest that a major part of the 11km (min.) of displacement along the fault zone was accommodated in the thin, continuous gouge layer.Strain localization in the fault zone is controlled by fluid-assisted deformation which enhances break-down reactions of feldspar to phyllosilicates. An increasing proportion of phyllosilicates decreases the strength and the permeability of the fault zone and may facilitate inter-crystalline deformation mechanisms such as grain-boundary sliding in phyllosilicate-rich zones. Normal slip of the hanging wall relative to the footwall at low angles of inclination may be facilitated by weakening of the fault zone and transient high pore pressure events.

Effects of simulated clay gouges on the sliding behavior of Tennessee Sandstone

Article

Jun 1981
TECTONOPHYSICS

The effects of simulated fault gouge on the sliding behavior of Tennessee sandstone are studied experimentally with special reference to the stabilizing effect of clay minerals mixed into the gouge. About 30 specimens with gouge composed of pure clays, of homogeneously mixed clay and anhydrite, or of layered clay and anhydrite, along a 35° precut are deformed dry in a triaxial apparatus at a confining pressure of 100 MPa, with a shortening rate of about 5 · 10−4/sec, and at room temperature. Pure clay gouges exhibit only stable sliding, and the ultimate frictional strength is very low for bentonite (mont-morillonite), intermediate for chlorite and illite, and considerably higher for kaolinite. Anhydrite gouge shows violent stick-slip at 100 MPa confining pressure. When this mineral is mixed homogeneously with clays, the frictional coefficient of the mixed gouge, determined at its ultimate frictional strength, decreases monotonically with an increase in the clay content. The sliding mode changes from stick-slip to stable sliding when the frictional coefficient of the mixed clay-anhydrite gouge is lowered down below 90–95% of the coefficient of anhydrite gouge. The stabilizing effect of clay in mixed gouge is closely related to the ultimate frictional strength of pure clays; that is, the effect is conspicuous only for a mineral with low frictional strength. Only 15–20% of bentonite suppresses the violent stick-slip of anhydrite gouge. In contrast, violent stick-slip occurs even if the gouge contains as much as 75% of kaolinite. The behavior of illite and chlorite is intermediate between that of kaolinite and bentonite. Bentonite—anhydrite two-layer gouge exhibits stable sliding even when the bentonite content is only 5%. Thus, the presence of a thin, clay-rich layer in a fault zone stabilizes the behavior much more effectively than do the clay minerals mixed homogeneously with the gouge. This result brings out the mechanical significance of internal structures of a fault zone in understanding the effects of intrafault materials on the fault motion. Based on the present experimental results incorporated with some other experimental data, it is argued that although the stabilizing effect of montmorillonite and vermiculite is indeed remarkable at room temperature, the effect should be much less pronounced at elevated temperatures, due perhaps to the dewatering of the clays. In most geological environments where shallow earthquakes occur, the stabilizing effect of clays is probably not so conspicuous as to completely suppress the unstable motion of a fault.

Discussion: The drained residual strength of cohesive soils

Article

Jan 1982

The drained residual strength of cohesive soils has been studied extensively. Various correlations between residual friction angle and index properties have been proposed and these are reviewed. Residual strength is measured with least ambiguity in the ring shear apparatus. The mechanisms controlling residual shearing are considered. The results of three series of tests on different soil mixtures, for which the gradings of the soils could be varied artificially, are presented. Three modes of residual shear are demonstrated. These modes are established by studies of brittleness and of the post-shear structure of the soil. The influence of these modes of residual shear on the general behavior of cohesive soils is considered, and the transition from one to another is related to the packing and porosity of the rotund particles present.

The effect of phyllosilicates and matrix on fault strength - Implications for the San Andreas fault zone

Article

Dec 2009

An increasing body of field evidence indicates that mature crustal fault zones are likely to be weak due to the presence of connected strands of foliated, phyllosilicate-rich fault rocks, often showing evidence of fluid-rock interaction. Some recent studies have proposed the formation of talc from serpentinite-reaction in the presence of silica rich fluids as a weakening mechanism for the San Andreas fault zone. Observations from SAFOD (San Andreas Fault Observatory at Depth) however, indicate that only very minor amounts of talc are present in the actively deforming parts of the fault. In this study we aim to quantify the amount of talc required to weaken a serpentinite-rich fault gouge. Furthermore, we will also study this weakening behaviour in case of a quartz matrix. We performed high strain rotary shear experiments on serpentinite/talc mixtures at sliding velocities of 0.03-3.7 mum/s and under conditions comparable to the actively deforming strands of the San Andreas fault zone (temperature: 110 C, effective normal stress: 50 MPa, fluid pressure: 30 MPa). In addition we have determined the weakening effect of talc on fault strength in a system where quartz is the strongest phase instead of serpentinite. Preliminary results indicate that ~10 % of talc is sufficient to cause significant weakening in simulated serpentinite-rich fault gouge at shear strains > 25. Increasing phyllosilicate contents result in further weakening of the serpentinite/talc gouges. Mixtures containing 30 % talc or more are as weak as 100 % talc after ~ 10 mm shear displacement (shear strain ~ 15). Since only a very small amount of talc is present in the actively deforming strands of the San Andreas fault, it is unlikely that its weakness is caused by the presence of talc. A possible alternative might be slip on swelling-clays. Although 30 % talc is sufficient to form a continuous network of weak phyllosilicates, the quartz/talc mixture (70/30) is significantly stronger than the serpentinite/talc mixture (70/30) (friction coefficients are ~ 0.5 and ~ 0.2 respectively), suggesting that not only the weak phyllosilicate phase, but also the matrix plays a crucial role in determining the strength of major crustal fault zones.

Compositional and Fluid Pressure Controls on the State of Stress on the Nankai Subduction Thrust

Article

Nov 2002

We present the initial results of an ongoing experimental study that examines the role that clay minerals play in controlling the properties and state of stress on subduction plate boundary faults. We focus here on the Nankai Trough, SW Japan, because it is representative of a major class of sediment-rich subduction zones (including Alaska, Chile, and Cascadia) that are capable of generating very large earthquakes (M ~8 and greater) and are almost fully coupled in their interseismic periods. The plate boundary fault also seems to be extremely weak because the maximum principal stress currently has a trench parallel orientation in the backstop region. For this to occur the maximum shear stress on the subduction thrust cannot exceed approx. 18 MPa even at the down-dip end of the seismogenic zone (recent numerical simulations by Wang and He). Our preliminary results suggest that both fault mineralogy and regional excess fluid pressure contribute to the low resolved shear stresses on the subduction zone plate boundary. Ring and direct shear tests show that saturated clay phases in the fault possess intrinsically low residual friction coefficients (RFC) at stress levels between 1 and 30 MPa. The RFC values for smectite are 0.14 ñ 0.02, for illite 0.25 ñ 0.01, and for chlorite 0.26 ñ 0.02. We find that the relatively weak illite phase is mechanically dominant in the incoming Muroto section and smectite contributes to even lower RFC values (as low as 0.18) in the incoming strata off the Ashizuri peninsula. Off Muroto, the overall magnitude of the shear stress we predict in the frontal 30 km remains below 2 MPa and only rises towards and above 4 MPa at 50 km from the toe of the wedge because of the overpressuring (lambda* values of between 0.6 to 0.8) and weak clays (RFC values of aprox.0.32 for 60 % Illite + 40 % Qtz mudstone). This is consistent with the low critical wedge taper (4.1degrees) in this region and values are well below the maximum stress estimates derived by Wang and He. Off the Ashizuri region, where the critical taper is higher (7.9 degrees), the basal lambda* value for the decollement could be as low as 0.0 to 0.4 for a decollement lying in a clay-rich portion of the deeper underthrust section, which may have a RFC values as low as 0.24-0.26 even after clay diagenesis has removed the smectite. Even with the lower overpressures we estimate that the basal shear stresses should remain below approx. 12 to 18 MPa, consistent with the Wang and He maximum basal shear stress estimates, further suggesting there is no low stress paradox at this subduction zone. Overall, our data do not, support the proposal that the smectite to illite reaction is directly responsible for the onset of seismogenic behavior throughout the Nankai system because both smectite and illite have low RFC values, tend to velocity strengthen, and there is already a preexisting mechanical dominance of in much of the incoming section (particularly off Muroto).

Growth of molten zone as a mechanism of slip weakening of simulated faults in gabbro during frictional melting

Article

May 2005
J GEOPHYS RES

1] To understand how frictional melting affects fault instability, we performed a series of high-velocity friction experiments on gabbro at slip rates of 0.85–1.49 m s À1 , at normal stresses of 1.2–2.4 MPa and with displacements up to 124 m. Experiments have revealed two stages of slip weakening; one following the initial slip and the other immediately after the second peak friction. The first weakening is associated with flash heating, and the second weakening is due to the formation and growth of a molten layer along a simulated fault. The two stages of weakening are separated by a marked strengthening regime in which melt patches grow into a thin, continuous molten layer at the second peak friction. The frictional coefficient decays exponentially from 0.8–1.1 to 0.6 during the second weakening. The host rocks are separated completely by a molten layer during this weakening so that the shear resistance is determined by the gross viscosity and shear strain rate of the molten layer. Melt viscosity increases notably soon after a molten layer forms. However, a fault weakens despite the increase in melt viscosity, and the second weakening is caused by the growth of molten layer resulting in the reduction in shear strain rate of the molten layer. Very thin melt cannot be squeezed out easily from a fault zone so that the rate of melting would be the most critical factor in controlling the slip-weakening distance. Effect of frictional melting on fault motion can be predicted by solving a Stefan problem dealing with moving host rock/molten zone boundaries. Citation: Hirose, T., and T. Shimamoto (2005), Growth of molten zone as a mechanism of slip weakening of simulated faults in gabbro during frictional melting, J. Geophys. Res., 110, B05202, doi:10.1029/2004JB003207.

Talc friction in the temperature range 25°-400 °C: Relevance for Fault-Zone Weakening

Article

Jan 2008
TECTONOPHYSICS

Talc is one of the weakest minerals that is associated with fault zones. Triaxial friction experiments conducted on water-saturated talc gouge at room temperature yield values of the coefficient of friction, μ (shear stress, τ/effective normal stress, σ′ N) in the range 0.16–0.23, and μ increases with increasing σ′ N . Talc gouge heated to temperatures of 100°–400 °C is consistently weaker than at room temperature, and μ b 0.1 at slow strain rates in some heated experiments. Talc also is characterized by inherently stable, velocity-strengthening behavior (strength increases with increasing shear rate) at all conditions tested. The low strength of talc is a consequence of its layered crystal structure and, in particular, its very weak interlayer bond. Its hydrophobic character may be responsible for the relatively small increase in μ with increasing σ′ N at room temperature compared to other sheet silicates. Talc has a temperature–pressure range of stability that extends from surficial to eclogite-facies conditions, making it of potential significance in a variety of faulting environments. Talc has been identified in exhumed subduction zone thrusts, in fault gouge collected from oceanic transform and detachment faults associated with rift systems, and recently in serpentinite from the central creeping section of the San Andreas fault. Typically, talc crystallized in the active fault zones as a result of the reaction of ultramafic rocks with silica-saturated hydrothermal fluids. This mode of formation of talc is a prime example of a fault-zone weakening process. Because of its velocity-strengthening behavior, talc may play a role in stabilizing slip at depth in subduction zones and in the creeping faults of central and northern California that are associated with ophiolitic rocks. Published by Elsevier B.V.

Friction of Rocks

Article

Jan 1978

J. D. D. Byerlee

Experimental results in the published literature show that at low normal stress the shear stress required to slide one rock over another varies widely between experiments. This is because at low stress rock friction is strongly dependent on surface roughness. At high normal stress that effect is diminished and the friction is nearly independent of rock type. If the sliding surfaces are separated by gouge composed of Montmorillonite or vermiculite the friction can be very low.

Frictional Behavior of Highly Oriented Pyrolytic Graphite

Article

Jan 1995

Friction of a freshly cleaved highly oriented pyrolytic graphite (HOPG) surface and a roughened graphite surface against a silicon nitride tip is measured using a friction force microscope. The cleaved graphite surface is an atomically smooth surface of (0001) plane with a small fraction of line‐shaped regions. It is observed that the coefficient of friction is extremely small (≪0.006) for a freshly cleaved HOPG surface of (0001) plane. However, the coefficient of friction is more than an order of magnitude larger in some line‐shaped regions on the surface compared with that of smooth regions of (0001) planes. Transmission electron microscopy analysis indicates that the line‐shaped regions consist of graphite planes of different orientations [other than (0001)] as well as amorphous carbon. This result suggests that (0001) graphite planes have the lowest coefficient of friction in comparison with other surface orientations or amorphous carbon. The coefficient of friction of a roughened graphite surface is found to be much larger than that of cleaved graphite. These observations may explain the large differences between the atomic‐scale coefficient of friction of graphite (usually cleaved) (about 0.006 or even less) and its macroscopic counterpart (about 0.1). © 1994 American Institute of Physics.

Graphite as a lubricating agent in fault zones: An insight from low- to high-velocity friction experiments on a mixed graphite-quartz gouge: GRAPHITE AS A LUBRICATING AGENT IN FAULT

Abstract

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