THE ROLE OF FLUIDS IN THE MICROSTRUCTURAL EVOLUTION OF QUARTZ-RICH ROCKS DURING LOW TEMPERATURE, FAULT-RELATED DEFORMATION

The Big Cove anticline in the Valley and Ridge province of south-central Pennsylvania is cut by a steeply-dipping oblique slip fault that was a fluid pathway during the Alleghanian orogeny. Horses of quartz arenite from the Lower Silurian Tuscarora Sandstone along this fault contain numerous microstructures that were used to detail the fluid history and infer the role of fluids in the selection and operation of grain-scale deformation mechanisms. In most samples, brittle microstructures (fluid inclusion planes, microveins, cataclastic bands, and distributed cataclasis) predominate, although pressure solution microstructures (transgranular stylolites, sutured and truncated grains) are better developed in some samples. Crystal-plastic microstructures (undulose extinction, deformation lamellae and bands) are subordinate to other microstructures and are typically developed near fractures and stylolites. Crosscutting relations show two phases of brittle deformation: cataclasis then systematic extensional microfracturing. Fluid inclusion homogenization temperatures for the two phases of brittle deformation are 160º and 120º C, respectively. Pressure solution preceded and followed both phases of brittle deformation. The spatial relationship between crystal-plastic microstructures and fractures or stylolites implies that they were coeval. The evolution of fluid pathways and the fluids contained therein largely controlled the timing and nature of grain-scale deformation, and hence, the rock behavior in the fault zone. Early cataclasis, locally facilitated by primary porosity, created abundant secondary porosity. Fluids promoted extensive redistribution of silica through pressure solution and cementation of fractures. Grains adjacent to fractures and stylolites were able to deform plastically due to the localized availability of water. During deformation, fluid pathways evolved in location, from exterior to interior of blocks, and in scale, from fault to fracture and stylolite to lattice. Compared to weakly cemented porous sandstones, which can utilize existing fluid pathways for long periods of time, rapid cementation during deformation in the Tuscarora required the continual formation of new pathways to maintain fluid flow.