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Maps and summary tables are presented of microcracks generated during a brittle faulting experiment on Westerly granite. Granite located well away from the shear fracture in the stressed sample contains twice the crack density of the undeformed granite, and the stressinduced microcracks of the far-field area have a roughly axial orientation consistent with tensile cracks formed in response to loading. Microcrack densities increase markedly in the process zone in front of the fracture tip, and at the tip the densities are over an order of magnitude higher than in the undeformed rock. Microcracks generated within the process zone are predominantly tensile cracks that reflect the stress fields associated with the shear fracture, but some cracks form in response to the physical requirements of extending the fracture. The orientations found for the total crack populations also hold for the grain-boundary and intragranular cracks considered separately. The largest number of cracks and the smallest average crack length were found in the process zone immediately in front of the fracture tip. Grain-boundary cracks comprise about half the microcrack population of the undeformed granite but only a small proportion of the highly cracked areas in the process zone and adjacent to the shear fracture. The grain-boundary cracks are nevertheless significant in the stressed sample in providing a connected network of cracks. Microcracking in the undeformed granite sample is highly concentrated within and along the boundaries of quartz. Near the shear fracture, intragranular cracks are slightly concentrated in K-feldspar but grain-boundary cracks remain most closely associated with quartz. Comparison of the crack and mineral maps reveals marked crystal-to-crystal variations in crack density that are caused by varying crystallographic orientations relative to the imposed stress fields.

Microcrack populations associated with a propagating shear fracture in granite