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Superplastic Nanofibrous Slip Zones Control Seismogenic Fault Friction

Abstract

Displacement in seismically-active fault zones is frequently localized within narrow (<0.1 m) principal slip zones composed of nanogranular fault rock. We report on sheared simulated calcite fault gouges recovered from direct shear experiments performed (nominally) dry, using an (effective) normal stress of 50 MPa, temperatures of 18–150°C, and sliding velocities of 0.1–10 μm/s. The mechanical data show a transition from stable velocity strengthening slip below ~80°C, to potentially unstable, velocity weakening slip at higher temperatures. After each experiment, all sheared samples split along a shear band fabric defined by mainly R1- and boundary shears. Thin sections prepared normal to the shear plane and parallel to the shear direction, investigated using a light microscope, show that the shear bands are 10–100 μm wide and characterized by an ultra-fine (nanoscale) grain size and a strong optical anisotropy indicative of a crystallographic preferred orientation (CPO). Loose sample fragments recovered from the direct-shear piston interface, viewed normal to the shear plane, show elongate, shiny, striated patches aligned parallel to the shear direction, corresponding with the ultra-fine grained boundary-parallel bands seen in thin section. Focused ion beam – scanning electron microscopy (FIB-SEM) revealed that these “mirror-like” slip surfaces consist of ultra-fine grained films composed of ~100 nm wide fibers aligned sub-parallel to the shear direction, overlain or juxtaposed by granular zones of ~100 nm spherules constituting the shear band. The nanofibers are demonstrably ductile at room conditions, and show extreme extensibility and fracturing without localized necking, characteristic for superplasticity. Transmission electron microscopy (TEM) applied to isolated nanofibers revealed the presence of similarly oriented, 5–20 nm calcite nanocrystallites. Our results point to a fault slip deformation process involving competition between granular flow and diffusive mass transport operating at the nanoscale, suggesting that (potentially unstable) velocity weakening slip is produced by enhanced diffusion-driven compaction rates at elevated temperatures. We argue that the nanofibers and CPO are formed by a process of orientation-dependent nanoparticle sintering. Given the abundant recent observations of nanogranular fault surfaces in tectonically-active terrains, the proposed mechanism may be relevant to crustal seismogenesis in general.