Prism 〈c〉 slip in the quartzites of the Oakhurst Mylonite Belt, California

Quartz c-axis fabrics from 21 amphibolite facies quartzites of the Oakhurst Mylonite Belt, California, suggest that prism 〈c〉, prism 〈a〉 and basal 〈a〉 slip systems were activated during deformation. Qualitative two-dimensional kinematic models, consistent with meso- and microscopic shear criteria, are constructed based on a regime of combined pure and simple shear. In this kinematic framework, most of the observed c-axis fabric patterns of the Oakhurst Mylonite Belt can be explained if prism 〈c〉 slip was dominant during the early increments of deformation, until a transition to dominant basal 〈a〉 slip occurred. An apparent relationship between grain size and type of crystallographic fabric is interpreted as resulting from some quartz (syntectonic vein quartz) having recorded only the latest increments of deformation. Comparison with the metamorphic evolution of fault zones of similar tectonic origin in California suggests that prism 〈c〉 slip was enhanced by the circulation of hot fluids which hydrolytically weakened quartz and allowed prism 〈c〉 slip to occur at relatively low temperature.