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Evolution of Shear Fabric in Granular Fault Gouge from Stable Sliding to Stick Slip and Implications for Fault Slip Mode

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Evolution of Shear Fabric in Granular Fault Gouge from Stable Sliding to Stick Slip and Implications for Fault Slip Mode Evolution of shear fabric in granular fault gouge from stable sliding to stick slip and implications for fault slip mode M.M. Scuderi1*, C. Collettini1, C. Viti2, E. Tinti3, and C. Marone4 1Department of Earth Sciences, Università di Roma La Sapienza, 00185 Rome, Italy 2Department of Physical Sciences, Earth and Environment, Università degli Studi di Siena, 53100 Siena, Italy 3Istituto Nazionale di Geofisica e Vulcanologia (INGV), 00143 Rome, Italy 4Department of Geoscience, The Pennsylvania State University, University Park, Pennsylvania 16801, USA ABSTRACT imposed coseismic velocities, whereas along natural faults, the instability Laboratory and theoretical studies provide insight into the mecha- arises spontaneously and is driven by the elastic interaction between the nisms that control earthquake nucleation, when fault slip velocity is fault zone and the surroundings (e.g., Scholz, 2002). Stick-slip frictional slow (<0.001 cm/s), and dynamic rupture when fault slip rates exceed sliding experiments have provided the foundation for earthquake physics centimeters per second. The application of these results to tectonic (e.g., Brace and Byerlee, 1966; Scholz et al., 1972). Recently, stick-slip faults requires information about fabric evolution with shear and its experiments on bare rock surfaces have documented the melting of asperity impact on the mode of faulting. Here we report on laboratory experi- contacts of the slipping surface during large stress drops (e.g., Hayward ments that illuminate the evolution of shear fabric and its role in con- et al., 2016; Passelègue et al., 2016). However, the mechanisms for strain trolling the transition from stable sliding (v ~0.001 cm/s) to dynamic localization and fault weakening during stick-slip frictional instabilities stick slip (v > 1 cm/s). The full range of fault slip modes was achieved within fault gouge are still poorly understood. In this manuscript we build by controlling the ratio K = k/kc, where k is the elastic loading stiff- on recent works that documented the spectrum of fault slip behavior, from ness and kc is the fault zone critical rheologic stiffness. We show that stable sliding to stick slip on quartz gouge (Leeman et al., 2016; Scuderi K controls the transition from slow-and-silent slip (K > 0.9) to fast- et al., 2016), to characterize the evolution of shear fabric and infer the and-audible (K < 0.7, v = 3 cm/s, slip duration 0.003 s) slip events. controlling mechanism for different fault slip behavior. Microstructural observations show that with accumulated strain, deformation concentrates in shear zones containing sharp shear METHODS planes made of nanoscale grains, which favor the development of Frictional stick-slip instabilities occur when the fault weakening rate frictional instabilities. Once this fabric is established, fault fabric does with slip exceeds the maximum rate of elastic unloading, resulting in a not change signifcantly with slip velocity, and fault slip behavior is force imbalance and acceleration (e.g., Scholz, 2002). In the simplifed mainly controlled by the interplay between the rheological properties case of an experimental fault obeying rate- and state-friction (RSF) laws, of the slipping planes and fault zone stiffness. the occurrence of stick-slip instabilities is controlled by the interaction between the system elastic stiffness, k, and the fault frictional properties, INTRODUCTION which can be written in terms of a critical rheologic stiffness kc (Gu et Understanding the relationship between the evolution of fault fabric and al., 1984). The condition for the nucleation of instability, when sliding fault slip behavior is a long-standing problem in fault mechanics. Tcha- velocity is still slow, is: lenko (1970) used shear box experiments to document fault zone evolution σ ′ ()ba− from Riedel shears to boundary-parallel shears with increasing strain. The kk<=n , (1) c D similarities of the experimental fault zone structure with earthquake fault- c ing were interpreted as indicating similarities in the deformation mecha- where σn′ is the effective normal stress, (b – a) is the friction rate param- nism. Several laboratory and feld studies have expanded on the kinematic eter, and Dc is the critical slip distance. The ratio K = k/kc describes fault descriptions, with the aim of developing an integrated understanding of stability: under quasi-static conditions, where accelerations are negligible, the evolution of fault zone microstructure and friction constitutive prop- sliding is stable for K > 1 and unstable for K < 1. erties (e.g., Sibson, 1977; Logan et al., 1979; Yund et al., 1990; Marone We conducted laboratory experiments using the double-direct shear and Kilgore, 1993; Beeler and Tullis, 1996). For quartzo-feldspathic fault confguration in a biaxial deformation apparatus (Fig. 1A). Fault zones ® gouge, increasing strain causes an evolution from distributed to localized were composed of quartz gouge (MIN-U-SIL 99.5% SiO2), with a grain deformation along fault-parallel shear planes. This microstructural evo- size in the range of 2–50 μm and mean size of 10–15 μm. Gouge layers lution is accompanied by a transition from a velocity-strengthening (i.e., were sheared at constant velocity of 10 µm/s and normal stresses ranging aseismic creep) to velocity-weakening behavior, which is a necessary con- from 13 to 35 MPa (Table DR1 in GSA Data Repository1). We performed dition for frictional instability, neglecting cohesion (e.g., Marone, 1998). velocity step experiments to retrieve the RSF parameters and characterize In the last decade, high-velocity friction experiments have shown that at the fault critical rheologic stiffness, kc (Fig. DR2 in the Data Repository). earthquake slip velocities (≥10 cm/s), signifcant grain-size reduction and To explore a range of stability conditions, we artifcially reduced the shear localization occurs in granular materials (e.g., Goldsby and Tullis, 2002; loading stiffness by inserting an elastic element in the loading column Di Toro et al., 2011; De Paola et al., 2015; Smith et al., 2015). These stud- ies have been fundamental to characterizing fault rocks produced during 1 GSA Data Repository item 2017242, summary of experiments and boundary the earthquake slip phase; however, these experiments are conducted with conditions, methods and calculations, and supplemental information, is avail- able online at http://www.geosociety.org/datarepository/2017/ or on request from *E-mail: [email protected] [email protected]. GEOLOGY, August 2017; v. 45; no. 8; p. 731–734 | Data Repository item 2017242 | doi:10.1130/G39033.1 | Published online 9 June 2017 ©GEOLOGY 2017 Geological | Volume Society 45 | ofNumber America. 8 For| www.gsapubs.org permission to copy, contact [email protected]. 731 Shear Stress ACσn = 35 MPa K = 0.5 a Spring Figure 1. A: Double direct 1 MP On Board LVDT cm/s shear configuration with (Vertical Slip) y, spring used to reduce stiff- Gouge Layers ness of loading system. elocit Displacements are mea- On Board LVDT V sured via a Linear Variable (Dilation - Slip Differential Transformer Stress Compaction) (LVDT). B: Mechani- Normal a Side Shields σn = 25 MPa K = 0.6 cal data for six runs at Rubber normal stress 13 < < ), MP σn Membrane τ B ( 35 MPa. Experiments at mm/s σn = 25 MPa run at higher σ = 35 MPa y, n stiffness (red and green 20 lines) produce stable slid- σn = 30 MPa elocit a V ing. White dots represent Shear Stress microstructural samples. σn = 25 MPa Slip ), MP 15 t C: Details of stick-slip ( cycles. With increasing σn = 20 MPa σn = 15 MPa K = 0.9 normal stress, we observe 10 σn = 15 MPa an increase in stress drop, peak slip velocity, and µm/s Shear Stress = 13 MPa σn y, recurrence intervals. K = 5 k/kc, where k is the elastic loading stiffness and k is elocit c V the fault zone critical rheo- 0 logic stiffness. 0 5101520 Slip Shear Strain (γ) Time, seconds (Fig. 1A). For these conditions, the transition from stable to unstable slip, we do not observe signifcant differences (Fig. DR4). In both cases, sliding occurs at a normal stress of 14 MPa (Fig. 1B), and we explored a most of the deformation occurs along B-shear zones and is localized range of normal stresses up to 35 MPa to study the transition from stable along Y-shears (Fig. 2E). The Y-shears consist of tightly packed zones of to unstable sliding (Figs. 1B and 1C). At the end of the experiments, we randomly oriented quartz nanograins, with grain dimensions of 50–500 collected samples for scanning electron microscopy and transmission nm, characterized by angular to sub-spheroidal shape and intense disloca- electron microscopy investigations. We also analyzed microstructures for tions (Fig. 2E, green inset). In some instances, grains with dimensions of experiments at low and high shear strain that were performed at higher 0.5–1 μm are fractured and show sharp grain surfaces. Some amorphous stiffness, resulting in stable sliding at normal stress of 25 MPa (Fig. 1B). material is present at the boundaries of the smaller grains, however the development of amorphous material in high-strain zones is not a promi- COUPLING BETWEEN MECHANICAL AND nent feature of the microstructure (De Paola et al., 2015). Nanograins, MICROSTRUCTURAL EVOLUTION <200 nm, likely form by dislocation pileup within larger grains. Outside Our experiments document a spectrum of slip behaviors from stable to the B-shear zones, the quartz grains are larger and contain pervasive quasi-dynamic (i.e., the acceleration is slow with negligible inertia effects) fractures (Fig. 2E, red inset). and fully dynamic sliding (i.e., abrupt acceleration) (Figs. 1B and 1C). With decreasing K the recurrence time of stick-slip cycles becomes longer, DISCUSSION the stress drop increases from ~0.4 to ~2.4 MPa, and the slip velocity Our experiments show that fault slip behavior is controlled by the increases from ~150 μm/s for K = 0.9 to ~3 cm/s for K = 0.5 (Fig.
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