Semibrittle deformation of granite at upper crustal conditions

Abstract

This work addresses semibrittle deformation of rock, characteristic of high temperature and low pressure environments. To enhance the understanding of this multimechanistic deformation regime results are presented on: (1) finite element models which stimulate polyphase aggregate deformation, (2) deformation experiments appropriate to the above environmental conditions, and (3) detailed observations of (2). In the finite element models, volume fractions of "weak" and "strong" elements were varied. 30% (by volume) of a weak member will "control" the aggregate deformation because (1) (GREATERTHEQ) 50% of model strain is accommodated by the weak elements, and (2) model strength is (LESSTHEQ) that of the stronger end member. These results are consistent with recent experimental studies and provide a fundamental basis for understanding them. A series of drained, constant-stress creep tests were conducted on 2 x 4 cm specimens of dry and water-saturated (Pp = 100 MPa) Westerly Granite at 100 MPa effective confining pressure and temperatures of 300(DEGREES)C to in excess of Tm (1000(DEGREES)C). Available equations of flow and time to failure are used as response models to characterize the experimental data, even though the multimechanistic deformation described below and the quasi-steady state flow observed are not compatible with the assumptions underlying the corresponding theories. Data for 600(DEGREES)C and below are well fit by an equation of the form: t(,failure) = t(,o)P('-(alpha)) exp(E/RT-K(sigma)). Using this expression, the apparent activation energies for the summed fatigue processes in the (beta)-quartz field are 7.5 kcal/mole (300-400(DEGREES)C, dry), 16.6 kcal/mole (400-600(DEGREES)C, dry), and 21.6 kcal/mole (400-600(DEGREES)C, wet, Pp of 100 MPa). Quasi-steady state flow was observed in the (beta)-quartz field (700-800(DEGREES)C, P(,p) = 100 MPa) and the flow behavior is described by e = Aexp(-Q/RT)(sigma)('n), where Q = 68 kcal/mole, n = 2.7, and A = 25.2.The deformation is multimechanistic with microfracturing of apparent extensile and shear origin, glide in quartz and biotite, microfracture healing, dissolution, mineral alteration, incipient and partial melt the mechanisms observed and evaluated as functions of temperature and strain. The systematic change in micromechanisms observed with increasing temperature is compatible with, and the origin of, the gradational succession of macroscopic deformation modes from a single narrow fault (T/Tm (LESSTHEQ) .5), to a shear zone (T/Tm (TURNEQ) .6), to multiple shear fractures (T/Tm (TURNEQ) .75), to uniform flow (T/Tm (GREATERTHEQ) .9).