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Fault weakening and slow earthquakes as a consequence of fault gouge strengthening in hydrothermal regimes
- insights from laboratory experiments

Silvio B. Giger 1, Stephen F. Cox 2, Eric Tenthorey3

1 Research School of Earth Sciences, Australian National University, Canberra, Australia
2 Department of Earth and Marine Sciences, Australian National University, Canberra, Australia
3 Research School of Earth Sciences, Australian National University, Canberra, Australia

A laboratory study was conducted to investigate how solution transfer processes influence the mechanical behaviour of simulated quartz fault wear products at high temperature, hydrothermal conditions. Experiments were performed under nominally dry conditions, as well as in the presence of an aqueous pore fluid, at elevated temperatures (500 to 927°C), and at 100MPa effective confining pressure. The mechanical data and detailed microstructural analysis indicate that the kinetics of solution transfer processes exert a fundamental control on the time-dependence of the mechanical behaviour of fault wear products. At nominally dry conditions, the gouges deform by cataclastic creep with distributed shear. The strength and microstructural evolution are relatively temperature insensitive. At moderately chemically reactive, hydrothermal conditions (T=500C°, coarse grain size, fast deformation rate), the gouge sliding resistance is slightly lower than at dry conditions, most likely due to the operation of chemically-enhanced fracture growth and dissolution of ultra-fine particles and surfaces of coarser particles, assisting cataclastic flow. At highly chemically reactive, hydrothermal conditions (T≥600°C, small grain size, slow deformation rate), substantial gouge compaction and low, distributed shear strains, are accommodated largely by dissolution-precipitation creep processes. Increase in average grain contact areas during compaction by dissolution-precipitation creep is associated with strain hardening.  However, at axial displacements of 0.5 to 1.0mm, a peak strength is reached (Figure 1), followed by up to 50% stress drop over several minutes. Slow stress drop is associated with slip localisation at the gouge-hostrock interface. Subsequent frictional sliding on this interface occurs at friction coefficients as low as 0.4.

The experimental results indicate that the presence of reactive pore fluids can lead to rapid strengthening of wear products during interseismic intervals.  Earthquake recurrence in faults containing reactive pore fluids will thus be influenced by the relative rate of recovery of shear strength and the rate of tectonic loading. The low resistance to frictional sliding after slip localisation is interpreted to reflect a role of dissolution processes play in overcoming frictional barriers during slow slip. It is speculated that fluid-assisted shear strength recovery, and the potential effects of fluids in controlling slow stress drop during slip localisation, may be pertinent to understanding the role of fluids in generating slow earthquakes in parts of subduction systems and in the continental seismogenic regime.

Figure 1. Shear stress versus axial displacement for quartz gouges deformed at high pressure hydrothermal conditions and at temperatures in the range 500°C to 927°C.  The nominal displacement rate was 0.18mms-1.  Experiment 4372 was conducted without pore water, but at the same effective confining pressure conditions as the other experiments.