Temperature and pressure sensitive processes modify rock strength and fault properties with depth. A series of triaxial shear fracture experiments have been conducted on granite under fixed confining pressures (70 to 480MPa), pore fluid pressures (10 to 300MPa) and temperatures (room temperature to 4800C) to investigate relationships between ambient crustal conditions, rock strength and fault displacement. We report here the first investigations of rock fracture experiments under elevated temperatures in the presence of high pore pressures. Our results improve existing models of fault slip displacement and rock strength with depth, based on dry experiments, by incorporating a more realistic water saturated crust, and considering the sealing effect on faults caused by high temperature welding.

The effect of ambient conditions on rock shear fracture parameters may be summarised as follows: rock strength with depth of burial is relatively constant up to temperatures of 3000C, after which strength decreases rapidly. Increasing pore fluid pressure mechanically reduces the strength of rock at all temperatures, and also reduces the amount of fault displacement. Temperature dependent processes at depth may allow a fault to seal (due to welding and mixed deformation processes), but simultaneously leak' nearer the surface (due to brittle fracture induced porosity).

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