Clay clast aggregates in gouges: New textural evidence for seismic faulting

Spherical aggregates named clay‐clast aggregates (CCAs) have been reported from recent investigations on retrieved clay‐bearing fault gouges from shallow depth seismogenic faults and rotary shear experiments conducted on clay‐bearing gouge at seismic slip rates. The formation of CCAs appears to be related to the shearing of a smectite‐rich granular material that expands and becomes fluidized. We have conducted additional high‐velocity rotary shear experiments and low‐velocity double‐shear experiments. We demonstrate that a critical temperature depending on dynamic pressure‐temperature conditions is needed for the formation of CCAs. This temperature corresponds to the phase transition of pore water from liquid to vapor or to critical, which induced gouge pore fluid expansion and therefore a thermal pressurization of the fault. A detailed examination by energy dispersive X‐ray spectrometry (EDX‐SEM) element mapping, SEM, and transmission electron microscopy (TEM) shows strong similar characteristics of experimental and natural CCAs with a concentric well‐organized fabric of the cortex and reveals that their development may result from the combination of electrostatic and capillary forces in a critical reactive medium during the dynamic slip weakening. Accordingly, the occurrence of CCAs in natural clay‐rich fault gouges constitutes new unequivocal textural evidence for shallow depth thermal pressurization and consequently for past seismic faulting.