MICROMECHANICS OF FAULTING IN WESTERLY GRANITE

Scanning electron microscope (SEM) observations were made on two suites of ion-thinned samples deformed at 400 MPa, 350°C and 250 MPa, 150°C. The latter suite includes 5 samples retrieved at different stages in the post-failure region. The pre-failure samples in both suites show numerous transgranular cracks at low angles (<15°) to maximum compression direction. We did, however, also observe many high-angle(/lt;15°) transgranular cracks. The deformation in post-failure samples is localized. Typically a localized zone is comprised of a number of almost coplanar cracks inclined at angles of 15°–45° to the maximum compression direction. Elsewhere, axial crack arrays extend over entire grains forming slender columns. In plagioclase, complex crack networks link up with the pores. The crack arrays and networks act as ‘barriers’ to the joining up of the inclined cracks to form a shear zone. A through-going fault is formed by coalescence of arrays and networks with the inclined cracks, accompanied by extensive crushing into fine-grained gouge. Elastic anisotropy and pore, as well as grain scale inhomogeneity, can influence the development of stress-induced cracks. Consequently, the four minerals in Westerly granite behave differently during faulting, the failure mechanisms being dependent on both mineralogy and grain orientation.

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