Clay–clast aggregates in fault gouge: An unequivocal indicator of seismic faulting at shallow depths?

A common problem encountered in studies of gouge-bearing natural faults is the difficulty of ascertaining whether the observed gouge was sheared seismically or aseismically; this problem arises because of the scarcity of indicators of fault slip rates for gouge. Recently, clay–clast aggregates (CCAs; a CCA comprises a clastic core mantled by a rim of ultrafine particles) were proposed as a possible indicator of seismic slip in gouge, on the basis of shear experiments on gouge at seismic slip rates. To examine the processes and conditions of CCA formation, we conducted rotary shear experiments on quartz and quartz–bentonite gouges under normal stresses (0.3–3.0 MPa) and slip rates (0.0005–1.3 m s−1), and in both room-humidity (room-dry) and water-saturated (wet) conditions. We found that CCAs could be produced in room-dry gouges even at the lowest slip rates, which are considerably slower than actual seismic slip rates. This finding demonstrates that thermal pressurization and fluidization at elevated temperature during seismic slip are not necessarily needed for the formation of CCAs, contrary to previous views. Given the occurrence of CCAs over a wide range of slip rates, we suggest that the presence of CCAs is not an unequivocal indicator of fault slip at seismic slip rates.