Abstract
Highly reflective, continuous smooth surfaces, known as mirror-like (MSs), have been observed in experimental carbonate-hosted faults, which were sheared at both seismic and aseismic velocities. MSs produced during high-velocity friction experiments (>0.1 m s–1) are typically interpreted to be frictional principal slip surfaces, where weakening mechanisms are activated by shear heating. We re-examined this model by performing friction experiments in a rotary shear apparatus on calcite gouge, at seismic velocities up to v = 1.4 m s–1 and normal stress σn = 25 MPa, to analyze the evolution of microstructures as displacement increases. After the onset of dynamic weakening, when the friction coefficients are low (μ << 0.6), sheared gouges consistently develop a well-defined, porosity-free principal slip zone (PSZ) of constant finite thickness (a few tens of micrometers) composed of nanometric material, which displays polygonal grain shapes. MSs occur at both boundaries of the PSZ, where they mark a sharp contrast in grain size with the sintered, much coarser material on either side of the PSZ. Our observations suggest that, with the onset of dynamic weakening, MSs partition the deformation by separating strong, sintered wall rocks from a central weak, actively deforming viscous PSZ. Therefore, the MSs do not correspond to frictional slip surfaces in the classical sense, but constitute sharp rheological boundaries, while, in the PSZ, shear is enhanced by thermal and grain-size–dependent mechanisms.
Highlights
Reflective smooth surfaces, known as “mirror-like surfaces” (MSs), have long been found and described along natural exhumed faults
We propose a conceptual model where WPSZ is compared to a theoretical thickness WT (Fig. 4B)
We propose that weakening is driven by thermally induced grainsize–sensitive mechanisms active within a principal slip zone (PSZ), whose finite thickness is controlled by grain size, temperature, and shear stress
Summary
Known as “mirror-like surfaces” (MSs), have long been found and described along natural exhumed faults. We integrate mechanical data and microstructural observations to develop a new conceptual model concerning the role and significance of MSs throughout the weakening history of calcite gouges sheared at seismic velocities. The internal structure of the PSZ shows fairly homogeneous grain size, triple junctions, and oblique foliation, marked by a shape-preferred orientation of the crystals, consistent with the overall (sinistral) sense of shear (Fig. 3B; Section DR5, Fig. DR17). This texture is similar to that observed in some natural (Bestmann et al, 2000; Herwegh and Kunze, 2002) and experimental (Barnhoorn et al, 2004) calcite ultramylonites. This confirms that decarbonation reactions were not quantitatively significant during our experiments
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
