The role of clay content during earthquake propagation in carbonate-hosted faults

Carbonate faults often contain small amounts of phyllosilicate in their slip zone. To assess the effect of phyllosilicate content on earthquake propagation in carbonate faults, we performed friction experiments at seismic slip rate (v = 1.3 m/s) on gouges of calcite, phyllosilicate (montmorillonite and illite-smectite) and mixed calcite/phyllosilicate compositions. Experiments were carried out at 9 MPa normal load and under both room-humidity (dry) and water-saturated (wet) conditions. All dry gouges, regardless of clay content, plus the wet calcite, produce a friction evolution curve, comprising an initial slip-hardening phase, during which friction evolves to peak values f = 0.60-0.76, followed by a dramatic slip-weakening phase, during which f decreases to a constant steady-state value of 0.19-0.33 over a distance which ranges from 0.2 m for clay-bearing gouges up to 0.6 m for pure calcite. Conversely, wet gouges with phyllosilicate content ≥ 10 wt.% show negligible slip-hardening, and the attainment of steady-state sliding almost immediately at the onset of slip, with f = 0.05-0.26.

Dry gouges show slip localization and grain size reduction within a narrow (<65 microns) principal slip zone, accompanied by microstructural evidence for thermal decomposition of calcite (although only when clay content is ≤ 50 wt.%). Wet gouges are characterized by distributed deformation and grain size reduction, with no microstructural evidence for thermal decomposition of calcite. We interpret that slip initiates within the wet gouges along interconnected networks of weak phyllosilicates, formed during axial loading compaction prior to shear. This can explain the: 1) measured lack of slip-hardening and peak friction; 2) observed distributed nature of deformation and grain size reduction; 3) lack of evidence for thermally activated processes, due to low frictional heating in accord with small values of friction and lack of slip localization.

Our findings imply that small amounts of phyllosilicate in the slip zone of fluid-saturated carbonate faults can: 1) dramatically change their frictional behaviour, and facilitate rupture propagation to the surface; 2) significantly reduce the amount of frictional heating produced, and mask seismic activity by preventing the development of microscale seismic markers.