Dilatancy toughening of crustal faults

Abstract:

Dilatancy exerts a primary control on fluid pressure evolution during faulting, and has a first order impact on the frictional strength of faults through the principle of effective stress. Dilatancy hardening has long been recognised as a stabilising factor in rock failure and fault slip, and extensive theoretical calculations have been performed to investigate the role of dilatancy in (thermo-) hydro-mechanical fault behaviour. Here we present new experimental results showing direct evidence of pore pressure drops and rupture stabilisation due to dilatancy in initially intact cores of granite. Rupture stabilisation in the experiments is quantitatively explained using a spring-slider model (from Rudnicki and Chen, 1988). Porosity evolution as function of slip and fault zone storage capacity can be directly measured from experimental data. Using these parameter estimates, upscaling to large scale faulting is performed in a dynamic rupture model combining slip-weakening behaviour, slip-dependent dilatancy, and thermal pressurisation of pore fluids. The model predicts that dilatancy increases fracture energy (and overall dissipation) with increasing rupture speed. Dilatancy is also shown to increase the heating rate and produce earlier onset of frictional melting along faults, consistent with field observations of pseudotachylytes in low porosity crystalline rocks.