Multiple Seismic Slip-Rate Pulses and Mechanical and Textural Evolution of Calcite-Bearing Fault Gouges

Natural fault zones are complex, spatially heterogeneous systems. Rock deformation experimental studies simplify the complexity of natural fault zones either as a surface discontinuity between intact rocks (bare-rock surfaces) or as a few mm-thick gouge layer. However, depending on the simplified fault type and its slip history, the response to applied deformation can vary. In this work, we conduct laboratory experiments for investigating the evolution of mechanical parameters of simulated faults made of calcite gouge subjected to multiple (four) identical seismic slip-rate pulses. We observed that, as the number of applied slip-rate pulses increased, (a) initial friction and steady-state friction remained approximatively constant, (b) peak friction and normalized strength excess increased and, (c) the slip distances to achieve peak and steady-state friction, Da and Dc, decreased. The greatest changes occurred between the first and the second slip-rate pulse. From this pulse onward, the dissipated energy of the calcite gouge fault was similar to those obtained in bare-rock surfaces experiments. Microstructural analysis showed that, strain is localized in up to two (recrystallized) principal slip zones (PSZ) with sub-micrometric grain size, surrounded by low porosity sintered and non-sintered comminuted gouge domains. We conclude that previous seismic slip episodes impact on both the structure and the strain localization processes within a fault, contributing to its shear fabric evolution. We highlight that the strain localization process identifies the PSZ, dissipating the least amount of energy within the entire experimental fault zone.