Ensemble Shear Strength, Stability, and Permeability of Mixed Mineralogy Fault Gouge Recovered From 3D Granular Models
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Abstract We conduct numerical shear experiments on mixtures of quartz and talc gouge using a three‐dimensional (3D) distinct element model. A modified slip‐weakening constitutive law is applied at contacts. We perform velocity‐stepping experiments on both uniform and layered mixtures of quartz and talc analogs. We separately vary the proportion of talc in the uniform mixtures and talc layer thickness in the layered mixtures. Shear displacements are cycled through velocities of 1 and 10 μm/s. We follow the resulting evolution of ensemble shear strength, slip stability, and permeability of the gouge mixture and explore the mesoscopic mechanisms. Simulation results show that talc has a strong weakening effect on shear strength—a thin shear‐parallel layer of talc (three particles wide) can induce significant weakening. However, the model offsets laboratory‐derived strong weakening effects of talc observed in uniform mixtures, implying the governing mechanisms may be the shear localization effect of talc, which is enhanced by its natural platy shape or preimposed layered structure. Ensemble stability ( a − b ) can be enhanced by increasing talc content in uniform talc‐quartz mixtures. Reactivation‐induced permeability increase is amplified with increased quartz content before the maturation of shear localization. Postmaturation permeability enhances on velocity upsteps and diminishes on velocity downsteps. Talc enhances compaction at velocity downsteps, potentially reducing fault permeability. Evolution trends of stability relating to the composition and structure of the fault gouge are straightforwardly obtained from the 3D simulation. Local friction evolution indicates that talc preferentially organizes and localizes in the shear zone, dominating the shear strength and frictional stability of faults.Keywords:
Talc
Fault gouge
Pseudotachylites are thought to be caused by fault surface melting due to frictional heating during earthquakes. We report on pseudotachylite formation in the laboratory during spontaneous stick-slip on dry, bare-surface granite faults in room temperature triaxial experiments. A continuous melt layer averaging 7 microns in thickness was formed on sawcut surfaces during stick-slip events at 400 MPa confining pressure. At this pressure, dynamic weakening during stick-slip caused total stress drops that ranged from 172 to 414 MPa shear stress (peak normal stress was 249 to 639 MPa) with 1.2 to 4.2 mm slip. In contrast, repeated stick-slip cycles at 50 MPa confining pressure produced fine-grained fault gouge but showed no evidence of melting. Event duration ranged from 0.07 ms for low stress events to 0.32 ms at high stress, and average slip velocity ranged from 0.3 to 20 m/s. Based on thermocouple measurements within 3 mm of the fault, maximum temperatures in some 400 MPa events exceeded 1500°C. By operating at normal stresses 10 to 50 times greater than those used in unconfined rotary machines, triaxial stick-slip experiments are able to develop high transient temperatures and create pseudotachylites, even with limited total slip.
Fault gouge
Overburden pressure
Thermocouple
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Abstract We conduct numerical shear experiments on mixtures of quartz and talc gouge using a three‐dimensional (3D) distinct element model. A modified slip‐weakening constitutive law is applied at contacts. We perform velocity‐stepping experiments on both uniform and layered mixtures of quartz and talc analogs. We separately vary the proportion of talc in the uniform mixtures and talc layer thickness in the layered mixtures. Shear displacements are cycled through velocities of 1 and 10 μm/s. We follow the resulting evolution of ensemble shear strength, slip stability, and permeability of the gouge mixture and explore the mesoscopic mechanisms. Simulation results show that talc has a strong weakening effect on shear strength—a thin shear‐parallel layer of talc (three particles wide) can induce significant weakening. However, the model offsets laboratory‐derived strong weakening effects of talc observed in uniform mixtures, implying the governing mechanisms may be the shear localization effect of talc, which is enhanced by its natural platy shape or preimposed layered structure. Ensemble stability ( a − b ) can be enhanced by increasing talc content in uniform talc‐quartz mixtures. Reactivation‐induced permeability increase is amplified with increased quartz content before the maturation of shear localization. Postmaturation permeability enhances on velocity upsteps and diminishes on velocity downsteps. Talc enhances compaction at velocity downsteps, potentially reducing fault permeability. Evolution trends of stability relating to the composition and structure of the fault gouge are straightforwardly obtained from the 3D simulation. Local friction evolution indicates that talc preferentially organizes and localizes in the shear zone, dominating the shear strength and frictional stability of faults.
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Fault gouge
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Fault gouge
Optically stimulated luminescence
Shearing (physics)
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The experimental sample consists of talc, magnesite and a little quartz, the grade of talc is 80%. By the floatation flowsheet of rougher and two times cleaner, and middling after regrinding return to rougher, the high grade talc concentrate can be obtained. The concentrate contains talc 95.67% (SiO 2 61.68%). Talc recovery rate is 89.43%. By this way, the output of high class talc product is increased and talc resources are exploited very well.
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Talc
Fault gouge
Cataclastic rock
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Previous laboratory studies on rock friction at slow slip rate ( < 1 mm/s) contribute to form the basis of rate- and state-dependent frictional constitutive laws. However, the frictional parameters determined in conventional frictional experiments at the slow slip rates are probably no longer applicable when fault slip approaches to the average slip rate of order of 1 m/s. Recent progress in experimental studies on rock friction reveals that laboratory simulated faults weaken dramatically at fast slip rate conditions ( > 0.1 m/s) with a large slip-weakening distance ( > 1 m). We focus here on the recently reported weakening processes of simulated faults during high-velocity friction from the cases without frictional melting. Several mechanisms has been proposed so far for the cause of the weakening processes of rock friction; thixotropy of a silica gel formed on quartz-rich rocks, formation of a weak material due to thermal decomposition, and drain-off process of adsorbed moisture from the fault due to frictional heating. High-velocity friction of natural fault gouge samples also showed slip weakening with a large weakening distance. Importantly, the laboratory derived weakening distance of the gouge friction decreased with an increase of normal stress. The exact mechanism that leads to the slip-weakening of the gouge samples is unknown, but it may involve a process in which the cause of the weakening is related to the increase of temperature of the gouge material during the high-velocity slip.
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Slip line field
Thixotropy
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Fault zones consist of one or more fault cores sandwiched by a damage zone surrounded by less deformed wall rocks. Most of the deformation is accommodated in the fault core through slip along one or more principal slipping zones. The thickness of fault cores (mm to m) and individual slipping zones (µm to dm) increases with fault slip displacement. In particular, small-displacement or immature faults have such thin slip zones that resemble bare rock surfaces. When exhumed from <5-6 km depth, slip zones are made by poorly cohesive fault gouges.Several laboratory experimental configurations aim to reproduce the deformation processes activated during seismic slip episodes. In the laboratory, the slip zone is represented as the interaction volume of two bare rock surfaces (i.e., immature faults) or as a mm-thick gouge layer (i.e., more mature faults). Most studies have focused on the frictional behavior of gouge layers or bare rocks during single seismic events, and only a few on the mechanical and microstructural evolution of a gouge layer subjected to multiple events of seismic slip (e.g., Smith et al., 2015). Here, we present rotary-shear friction experiments that reproduce seismic slip on both gouge layers and bare rocks derived from calcite-rich marble. The aim of this study is to analyze the frictional evolution of a gouge layer undergoing multiple seismic slip pulses: four trapezoidal slip pulses at 1 m/s for 1 m of slip, with hold time of 120 s between each pulse. Moreover, we compare this evolution with one of bare rocks of the same material but slid only once at 1 m/s for a total slip higher than 1 m. Experiments were performed at normal stress of 10, 20, and 30 MPa under room humidity conditions.Our experimental results show that despite the static and dynamic friction coefficients are higher in the gouge layer than in the bare rock experiments, the frictional work to achieve the dynamic friction decreases at each seismic slip pulse in the gouge experiments and is comparable with the bare rock one after the second pulse. High-resolution scanning electron microscope investigations of the sheared gouge layers show that in the first two slip pulses most of the frictional work is spent on (1) strain localization into newly-formed slip zones bounded by continuous ultra-smooth surfaces and, (2) grain size reduction, sintering and compaction (i.e., porosity reduction) within the bulk gouge layer. However, after the second pulse, the slip is localized in one or more well-developed slip zones bounded by ultra-smooth surfaces, that cut through the compacted gouge layer, and the mechanical behavior is similar to that of bare rocks.Carbonate-bearing fault zones are common seismogenic sources in the Mediterranean area (e.g. 2009 L'Aquila Mw6.3 and 1981 Corinth M6.6 earthquakes). In a series of subsequent seismic slip events, it is shown that the evolution of a gouge layer in carbonate-bearing fault rocks tends to produce a similar mechanical behaviour of bare rocks although the volumetric distribution of strain is significantly different. Importantly, the energy spent by apparently different mechanical processes is eventually similar.
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Slipping
Slip line field
Echelon formation
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Abstract In addition to the velocity dependence of friction, slip dependence may play a major role before and during earthquake slip in fault zones. We performed laboratory friction experiments on simulated fault gouges, measuring both the velocity and slip dependence of friction in velocity step tests. The pure velocity‐dependent component of friction measured over short displacements shows both velocity strengthening and velocity weakening friction, depending on the amount of slip considered. However, we observe that increases in sliding velocity can induce slip weakening behavior which overwhelms the velocity dependence resulting in large overall weakening, especially at rates > 1 µm/s. On natural tectonic faults, this suggests that a velocity perturbation, such as coseismic rupture propagating onto a fault patch, could induce instability via large slip weakening. Therefore, a fault which is experiencing a transient slip or slow earthquakes may be more easily induced to slip coseismically if a dynamic rupture from large earthquake propagates onto the fault.
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