The role of dilation and cementation in the formation of cataclasite in low temperature deformation of well cemented quartz-rich rocks

An important textural component of cataclasites in quartz-rich rocks deformed at low temperatures is a fine-grained (<20 μm) matrix composed of anhedral quartz. As cataclasites form during shearing, this component would typically be interpreted to result from extreme grain size reduction through comminution. Tabular bands of fine-grained quartz that texturally resemble cataclasite found in faulted quartz arenites and other quartz-rich, well cemented sandstones have characteristics that are incompatible with a shearing origin. These characteristics include a lack of shear displacement and wall rock fragments, gradational contacts with the wall rock, and a fine-grained quartz fill that has a different cathodoluminescence, water content, oxygen isotope chemistry, and in some cases, mineralogy, than the wall rocks. Rather than being the result of cataclasis, the fine-grained quartz in these tabular bands was precipitated in a dilating fracture. In essence, these structures are veins. lt zones, where cataclasite is well developed and shearing is unequivocal, the fine-grained quartz matrix representing up to 50% of the fault rock volume has characteristics similar to those in the tabular bands. We interpret these volumes to be in many cases a result of cement precipitation rather than as a product of comminution. If correct, then the brittle deformation of well cemented, quartz-rich rocks deformed at low temperatures involves much more dilation and cementation than previously recognized.