An experimental investigation into the role of phyllosilicate content on earthquake propagation during seismic slip in carbonate faults

Carbonate faults commonly contain small amounts of phyllosilicate in their slip zones, due to pressure solution and/or clay smear. To assess the effect of phyllosilicate content on earthquake propagation in carbonate faults, friction experiments were performed at 1.3m/s on end-members and mixtures of calcite, illite-smectite, and smectite gouge. Experiments were performed at 9MPa normal load, under room humidity and water-saturated conditions. All dry gouges show initial friction values ((i)) of 0.51-0.58, followed by slip hardening to peak values of 0.61-0.76. Slip weakening then ensues, with friction decreasing to steady state values ((ss)) of 0.19-0.33 within 0.17-0.58m of slip. Contrastingly, wet gouges containing 10-50wt% phyllosilicate exhibit(i) values between 0.07 and 0.52 followed by negligible or no slip hardening; rather, steady state sliding ((ss)<<0.2) is attained almost immediately. Microstructurally, dry gouges show intense cataclasis and wear within localized principal slip zones, plus evidence for thermal decomposition of calcite. Wet gouges exhibit distributed deformation, less intense cataclasis, and no evidence of thermal decomposition. It is proposed that in wet gouges, slip is distributed across a network of weak phyllosilicate formed during axial loading compaction prior to shear. This explains the (1) subdued cataclasis and associated lack of slip hardening, (2) distributed nature of deformation, and (3) lack of evidence for thermal decomposition, due to low friction and lack of slip localization. These findings imply that just 10% phyllosilicate in the slip zone of fluid-saturated carbonate faults can (1) dramatically change their frictional behavior, facilitating rupture propagation to the surface, and (2) significantly lower frictional heating, preventing development of microscale seismic markers.