Slip localization and fault weakening as a consequence of fault gouge strengthening — Insights from laboratory experiments

Abstract

A laboratory study of simulated quartz gouges was conducted to investigate how solution transfer processes influence the mechanical behaviour of 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 effective confining pressure conditions ( σ 2′ = σ 3′ = 100 MPa) to simulate, on a laboratory timescale, processes that may be important in fluid-active fault zones at depth in the continental crust. The mechanical data and microstructural analysis indicate that the kinetics of solution transfer processes can exert a fundamental control on the mechanical behaviour of fault wear products. It is found that, at nominally dry conditions, gouges deform by cataclastic creep and distributed shear, with strength and microstructures being relatively unaffected by temperature. At moderately chemically reactive, hydrothermal conditions (500–600 °C, coarse grain size, or fast deformation rate), the presence of a reactive pore fluid slightly reduces the shear strength with respect to dry conditions. However, at highly chemically reactive, hydrothermal conditions (600–927 °C, small grain size, and slow deformation rate), rapid porosity reduction is accommodated by dissolution–precipitation processes. Deformation under such conditions results in a fast increase of grain contact area and the development of cohesive bonds between adjacent particles, which in turn inhibits cataclastic granular flow. With increasing displacement and compaction of the quartz gouge, there is a sudden transition from distributed cataclastic flow, to slip localization at the interface between the gouge and one of the forcing blocks. This deformation mode switch is associated with dramatic weakening (up to 50% drop in shear resistance, and changes in the apparent coefficient of friction from > 0.7 to ≈ 0.4). Stress drop occurs over many minutes in the laboratory. It is speculated that solution-assisted gouge compaction, and consequent slip localization with associated slow, yet dramatic stress drop, could provide a mechanism for the occurrence of slow earthquakes.