Bridging the Gap Between Seismic and Sub-seismic Mirror-Slip Surfaces in Carbonate Fault Gouge

Specularly light reflective fault plane interfaces known as Mirror-Slip Surfaces (MSS’s) are common in seismically active fault zones around the world and thus their role in controlling fault strength and stability is of great interest. MSS’s have been experimentally produced in simulated carbonate faults at relatively high (10^-1-10⁰ m/s) and low (10^-7-10^-5 m/s) sliding velocities (resp. HV and LV). However, their role in controlling fault mechanical properties at sub-seismic vs seismic fault-slip velocities remains enigmatic. With the aim to unravel the structural development of MSS’s with increasing shear displacement (rate) and effective normal stress, we conducted HV and LV shear deformation experiments on simulated faults composed of granular calcite. We employed a ring shear set-up in a HV rotary shear apparatus as well as a saw-cut assembly mounted in a triaxial cell, which enabled fault-slip tests under a wide range of slip velocities (v = 10^-7 - 10^-1 m/s) and effective normal stresses (σ_n ≈ 10 – 170 MPa). All experiments were carried out under room-dry conditions, at room temperature. Post-mortem microstructure analysis of recovered fragments was carried out through visual inspection, incident light and scanning electron microscopy, as well as using Raman spectroscopy.

MSSs develop at sub-seismic slip velocities (v = 10^-7 m/s) initially as visibly striated patches after 0.0062 m (σ_n ≈ 10 MPa), 0.004 m (σ_n ≈ 50 MPa) and 0.0026 m (σ_n ≈ 170 MPa) of shear displacement. The area covered by MSSs systematically increases with displacement to form continuous coatings after 0.042 (σ_n ≈ 10 MPa), 0.0062 m (σ_n ≈ 50 MPa) and 0.0036 m (σ_n ≈ 10 MPa).  As displacement rate is increased (10^-5 – 10^-4 m/s) MSSs are no longer observed however continuous MSSs are visible again at seismic slip velocities (>10^-1 m/s). Our microstructural analysis revealed that MSSs are layers of (nano)crystalline calcite some of which contain elongated nanofibrous structures. In addition, discrete, 3 - 20 micron-sized patches of amorphous carbon are produced at seismic slip velocities, and at sub-seismic velocities under high normal stresses (σ_n > 160 MPa). We could not however identify any microstructural characteristics that are diagnostics of MSSs produced at certain slip rates or normal stress.

Our interpretation is that MSSs form by sintering of nm-sized particles within ultrafine-grained shear bands. With increasing shear displacement, MSS patches connect into continuous veneers. The formation of (continuous) MSSs at low as well as high sliding velocities in our experiments implies that natural MSSs are unreliable indicators for palaeoseismicity.