Semibrittle creep of selected crustal rocks at 1000 MPa

Abstract

Constant stress creep experiments have been conducted on Westerly granite, Quadrant orthoquartzite, Sioux quartzite, and a generic diorite using a solid medium apparatus and talc assemblies. Most of the experiments were conducted at differential stress levels between 200 and 1200 MPa, temperatures between 500 and 800°C, and a constant confining pressure of nominally 1000 MPa; quartz at these conditions remains in the a-stability field. Dry and wet experiments were run on each rock type: the granite and quartzite were vacuum-impregnated to 0.1 wt % and 0.03 wt. % water, respectively, and unjacketed diorite was subject to talc dehydration at temperatures from 800 to 1100°C. Presence of water weakens granite by more than an order of magnitude and reduces the strength of quartzite by 50%. Deformation is semi-brittle variously combining fracture, thermally-activated dislocation motion, and recovery by recrystallization. Transient deformation gives way to steady-state creep at strains less than 1.0% in all experiments except the hot, wet diorite. Strain-time data were fit to the Webster et al. [1969] formulation and parameters for the steady-state function are: [graph]. These values were found to be in reasonable accord with the few published parameters available. Flow laws for wet and dry quartzite are shown to constrain predictions of geologic flow stresses. Quartz controls creep of granite to strains of 5%; feldspar and quartz govern creep of diorite to 10%.