THE ROLE OF PRESSURE SOLUTION CREEP AND COMPACTION IN FAULT ZONE RESTRENGTHENING: EXPERIMENTS ON GYPSUM

The repeating stress drops of the seismic cycle require that faults regain strength (heal) during interseismic periods. A variety of mechanisms are likely to contribute to fault restrengthening depending on lithology, temperature, pressure, and chemical environment. Geological observations highlight the importance of fluids in fault zone processes in a variety of settings. With the presence of fluids and the small grain size typical of fault zones, processes involving dissolution/diffusion/precipitation (pressure solution creep) are likely to play a dominant role in the evolution of grain-to-grain contacts when a fault is stationary. To explore the relationship between creep accommodated compaction and fault zone restrengthening, we performed slide-hold-slide friction experiments on simulated gypsum fault gouge at a variety of temperatures ranging from 20 to 60ºC. We present results on the time and temperature dependence of compaction and frictional healing as well as the role of temperature in sliding stability. As one of the main constituents of evaporite rocks, gypsum can play an important role in localizing deformation within sedimentary sequences, particularly in zones of thrust faulting. Laboratory data on the frictional properties of gypsum could provide improved constraints for the tectonic behavior of these settings. Also, due to gypsum’s particularly low resistance to intracrystalline plasticity, the thermal conditions appropriate to induce plastic creep are readily accessible in the laboratory. In this way, gypsum can serve as an analogue material for minerals requiring higher temperature and pressure to activate creep mechanisms. By developing a detailed understanding of the interactions between intracrystalline creep and frictional healing in the laboratory, we can extrapolate to conditions and time scales appropriate to natural fault systems.