Seismic-frequency laboratory measurements of shear mode viscoelasticity incrustal rocks - II: Thermally stressed quartzite and granite

Forced torsional oscillation techniques have been used to explore the seism ic-frequency shear mode viscoelasticity of specimens of two crustal rocks ( Cape Sorell quartzite and Delegate aplite), cycled between room temperature and 700 degrees C under conditions of moderate confining pressure. The ani sotropy and intergranular inhomogeneity of thermal expansivity in these mat erials give rise to large deviatoric stresses, resulting in thermal crackin g at temperatures above a pressure-dependent threshold temperature, associa ted with the onset of very pronounced temperature sensitivity of the shear modulus, in general accord with the predictions of fracture mechanics model s. For Delegate aplite in particular, the shear modulus behaves reproducibl y during multiple thermal cycles at different confining pressures, consiste nt with the notion that the thermal cracks are of low aspect ratio (minimum /maximum dimension), and are therefore readily closed by the prevailing con fining pressure once the thermal stresses are removed. Marked frequency-dep endent dissipation of shear strain energy is observed on heating each rock to temperatures greater than or equal to 500 degrees C, although the attenu ation varies significantly with prior thermal history, probably as a result of progressive dehydration and relaxation of deviatoric stresses. Temperat ure and pressure dependent crack densities For Delegate aplite have been es timated by comparison of the observed shear moduli with those expected for a crack-free aggregate. In parallel with the forced oscillation tests, meas urements have been made of the rate at which (argon) pore pressure equilibr ium is re-established following a perturbation. Combination of these result s, which provide a proxy for permeability, with the inferred crack densitie s indicates that the variation of permeability with crack density is well d escribed by a percolation model with a threshold crack density of similar t o 0.2.