The role of microcracking in shear-fracture propagation in granite

Abstract

Microcracking related to the formation of a laboratory shear fracture in a cylinder of Westerly granite has been investigated using image-analysis computer techniques. Well away from the fracture (farfield), the deformed granite has about twice the crack density (crack length per unit area) of undeformed granite. The microcrack density increases dramatically in a process zone that surrounds the fracture tip, and the fracture tip itself has more than an order of magnitude increase in crack density over the undeformed rock. Microcrack densities are consistently higher on the dilational side of the shear than on the compressional side. Microcracks in the undeformed rock and in the far-field areas of the laboratory sample are concentrated within and along the margins of quartz crystals, but near the shear fracture they are somewhat more abundant within K-feldspar crystals. The energy release rate, g II , for mode II fracture progagation is estimated from the microcrack density data to be ≥ 1.7–8.6 kJ m −2. The microcracks that formed during the experiment are principally tensile cracks whose orientations reflect the local stress field: those formed prior to the nucleation of the fault are roughly parallel to the cylinder axis (loading direction), whereas those generated in the process zone make angles averaging 30 ° to the overall fault strike (and 20 ° to the cylinder axis). The preferred orientation and uneven distribution of microcracks in the process zone tends to pull the propagating fracture tip towards the dilational side, even though the trend is away from the overall fault strike. As a result, the propagating shear follows the microcrack trend for some distance and then changes direction in order to maintain an overall in-plane propagation path. This recurring process produces a zig-zag or sawtooth segmentation pattern similar to the sawtooth geometries of faults such as the San Andreas fault.