ASSESSING THE ROLE OF SILICA GEL AS A FAULT WEAKENING MECHANISM IN THE TUSCARORA SANDSTONE

Previous work demonstrated that microcrystalline quartz bands and cement in cataclasites related to faults in quartz-rich rocks may have derived from a silica gel parent fluid. Occurrences of microcrystalline quartz on fault surfaces of all scales in the Tuscarora Sandstone and other lithologies in the central Appalachian foreland raises the possibility that a silica gel may have been present during faulting and significantly reduced the frictional strength. Fractured grains, trapped silica nanospheres, flow features, amorphous silica, uniform grain size distribution and recrystallized grains of low dislocation density within microcrystalline quartz on the fault surface are interpreted as evidence of a silica gel that resulted from comminution and hydration of wallrock asperities along the fault surface. Microcrystalline quartz filling coeval Mode I fractures adjacent to faults suggests that the gel was mobile. Crystallization in these fractures proceeded from opaline phases to coarse-grained quartz in crystal-lined open vugs as water was progressively concentrated in the final phases. Complex mutually crosscutting relations between microfractures and microcrystalline quartz along the faults, as well as presence of brecciated clasts within breccia, indicates that these processes were cyclic. Mutually overprinting textures between brittle and fault creep microstructures (stylolites) suggest alternating brittle and ductile episodes, possibly in response to varying stress levels. As silica-rich rocks are common, understanding the process of silica gel formation on fault surfaces may provide new insights into the mechanics of faulting in these rocks.