ON THE INFLUENCE OF POROSITY ON THE LOW-TEMPERATURE BRITTLE DUCTILE TRANSITION IN SILICICLASTIC ROCKS

A compilation of available experimental data, coupled with new results on Tennessee sandstone, shows that the low-temperature brittle faulting to cataclastic flow transition in siliciclastic rocks takes place at progressively higher confining pressures as porosity is reduced. The collapse of initial porosity compensates for the tendency for brittle deformation to be dilatant. According to a stability criterion, this in turn favours spreading of the cataclastic deformation throughout the rock volume instead of fault localization. At sufficiently high strains, dilatation, and hence fault localization, supervenes. From microstructural observations on Oughtibridge Ganister (7% porosity), deformed under conditions of the faulting to flow transition, the mechanism of pore collapse involves crushing and sliding on shear-oriented grain boundaries, with accumulation of wear products in the pore spaces. This explains the enhancement of pore collapse by differential stress relative to purely hydrostatic compression.