Evidence for silica gel and its role in faulting in the Tuscarora Sandstone

Abstract

Silica gel has been proposed as a fault weakening mechanism that may offer a solution to the stress paradox of faulting and be important in promoting seismic slip. Evidence that a gel was present in a fault zone during displacement has proven elusive owing to the unstable nature of a gel and its descendants. Examination of faults in the Middle Silurian Tuscarora Sandstone and other quartz-rich rocks in the region has shown that the microcrystalline quartz in bands and cement in cataclasites was derived from a silica gel. The presence of comminuted grains, silica nanospheres, flow features, amorphous silica, and polygonal crystal domains of low dislocation density within microcrystalline quartz on and adjacent to the fault surfaces is interpreted as evidence of a mobile silica gel precursor. Brecciated microbreccias and mutually crosscutting relationships between microfractures, microcrystalline quartz bands, and microveins along and adjacent to fault surfaces suggest that formation of these features was cyclic. Mutually overprinting textures between brittle and fault creep microstructures (stylolites) indicate alternating episodic brittle deformation and pressure solution. As silica-rich rocks are common, understanding the process of silica gel formation on fault surfaces and its behavior may provide new insights into the mechanics of faulting in the brittle crust.