Co-seismic nanoscale fault gouge deformation mechanisms revealed by automated crystal orientation mapping transmission electron microscopy (ACOM-TEM)

Co-seismic deformation localization in carbonate rocks leads to extensively deformed, fine-grained fault products. With decreasing grain size, the ability to quantify the resulting micro-, and nanostructure as well as the deduction of operating deformation mechanisms becomes challenging. For the first time, we present ultra-high resolution crystal orientation maps on natural carbonate fault mirror surfaces from a seismically active region in Central Greece. We employ ACOM-TEM with a step size of 2 nm on electron translucent TEM samples to quantify the nanostructure and determine the underlying deformation mechanisms leading to the present appearance. The nanostructure comprises a reactivated nano foam-fabric with grain sizes of the recovered grains < 500 nm. The resulting crystallographic preferred orientation is the outcome of high-temperature glide system activation during fault reactivation and subsequent annealing. Prior TEM imaging confirmed the throughout infiltration of an amorphous solid. ACOM-TEM hints to the presence of newly nucleated grains < 50 nm inside the amorphous solid. We propose that our investigation and results on the nanostructural inventory of localized slip in carbonate fault rocks are of major importance for the fault behavior during the seismic cycle.