Formation of clast-cortex aggregates in experimental fault gouges

Clast–cortex aggregates (CCAs) have been commonly reported in experimental and natural fault gouges. Here we discuss the formation process of CCAs and its mechanical implications, based on experimentally simulated fault gouges. We conducted low- to high-velocity rotary shear experiments on Ca-bentonite gouges at a normal stress of 1 MPa, slip rates of 0.01–1.31 m s⁻¹, and ambient temperature and humidity conditions. The CCAs consist of a central clast (mostly quartz fragments with subordinate clay aggregates) and a concentric rim of nano-clay particles (i.e., cortex). The CCAs range in size from 1 to 100 μm and developed adjacent to a foliated high-strain layer at all slip rates. The microstructures suggest that the CCA-rich layer was mainly deformed by granular flow, whereas the foliated high-strain layer was deformed by cataclastic flow. The increase in average size of the CCAs with displacement and the formation of their concentric rims suggest that the CCAs grew like a rolling snowball. The estimated strain rates obtained from the concentric layers of the CCAs and strain markers indicate the existence of a range of shear strain rates that favors granular flow and the formation and growth of CCAs.