Temperature dependence of fracture strength of granite under high confining pressures and its implication in the lithosphere strength

Triaxial testing on a dry granite was conducted at a confining pressure of 1500 MPa and temperatures to 600 degrees C with an averaged strain rate of approximately 2 x 10(-6) s(-1). AU the experiments fall in the high-pressure type fracture regime, which occurs when the compressive strength equals the frictional strength. The strength of the granite decreases with increase of temperature with a trend. The strength values between 200 degrees and 280 degrees C, however, deviate from the trend and very low with a minimum at around 250 degrees C. Micromechanical studies suggest that such behavior is caused by switching the dominating factors from the activation of crack groups to the inhibition of them and the dominance of crystal plasticity with increase of temperature. If we estimate the lithospheric strength using these results and our previous assumption that the high-pressure type fracture occurs in the crust, a low-strength zone between 8 and 12 km depth is suggested. This could provide an alternative explanation for the seismogenic zone in the crust: earthquakes could occur or nucleate more easily where the strength is low.