Abnormal grain growth in carbonate samples from the North Anatolian Fault: Microstructural evidence of the seismic cycle

Evidence for the nature of fault slip across the seismic cycle is hard to decipher. Fault-related deformation near the fault surface develops over the seismic cycle, characterized by rapid coseismic slip and intense deformation, followed by slower interseismic slip and stress accumulation. While considerable focus has been placed on characterizing deformation through fracturing and mesoscale structures, the analysis of grain-scale plastic processes has been largely neglected. However, transient temperature increases due to frictional heating, combined with the ability of calcite-bearing rocks to deform plastically at relatively low temperatures, suggest that microstructural damage and subsequent recovery processes could leave diagnostic evidence in carbonate fault rocks. Indeed, Pozzi et al. (2019) demonstrated that shearing gouge at seismic rates (~1 m/s) develops a crystallographic preferred orientation (CPO), accompanied by grain growth and sintering. These observations, however, were confined to nanometer-scale grains, localized at the fault surface.

Here, we present a detailed microstructural analysis of carbonate samples from the North Anatolian Fault Zone. We used Electron Backscatter Diffraction to map the calcite grains' orientations and characterize intragrain deformation and grain-boundary morphologies. We identify three distinct layers extending from the fault surface to a distance of ~4 mm. Layer I, with a thickness of tens of µm to 0.5 mm, exhibits predominantly angular grains with grain sizes ranging from unresolved (<1 µm) to tens of µm. Layer II, with a thickness of 0-200 µm, is comprised of small equant grains (1-5 µm) and some larger grains (10-30 µm), characterized by wavy grain boundaries, suggesting active grain boundary migration. No CPO was observed in layers I and II. Layer III, with a thickness of ~2-3 mm, contains large grains (hundreds of µm) that can be divided into two populations of grains. Rounded grains with wavy grain boundaries indicate the progressive consumption of smaller grains. At the core of the layer, grains contain faceted boundaries and are elongated parallel to the fault surface. This layer is the only one to exhibit a distinct CPO with the c-axis oriented normal to oblique to the slip surface. Importantly, the large grains in layer III also comprise small, isolated ‘islands’ of finer grains.

We infer that deformation mechanisms vary systematically with distance from the fault surface. Layer I records cataclastic flow at the fault surface, whereas layer II, characterized by very small grain sizes, exhibits shearing by grain boundary sliding that resulted in grains with low intragrain misorientation and the absence of CPO. The most striking microstructural record is preserved in layer III, which initially shows strong evidence for recovery processes by abnormal grain growth. We propose that this latter process occurred during or immediately after coseismic frictional heating, resulting in the consumption of previously deformed grains, which maintains the CPO record of deformation and provides a microstructural record of the seismic cycle at millimeter-scale distances from the fault surface.

Pozzi, et al., 2019. Coseismic ultramylonites: An investigation of nanoscale viscous flow and fault weakening during seismic slip. Earth and Planetary Science Letters.