Development of a dilatant damage zone along a thrust relay in a low-porosity quartz arenite

by 1978- Cook, Jennie E.

A damage zone developed along a backthrust fault system in well-cemented quartz arenite of the Tuscarora Sandstone in the Alleghanian foreland thrust system consists of a network of NW-dipping thrusts that are linked by multiple higher-order faults and bound a zone of intense extensional fractures and breccias. The damage zone is unusual in that it preserves porous brittle fabrics despite formation at > 5km depth. The damage zone developed at an extensional step-over between two independent, laterally propagating backthrusts. Continued displacement resulted in breaching of the relay and formation of faultbounded horses, and favored the formation of extensional fractures. The presence of pervasive, late-stage fault-normal joints in a fault-bounded horse in the northwestern damage zone indicates formation between two near-frictionless faults. This decrease in frictional resistance was likely a result of increased fluid pressure. In addition to physical effects of fluid, chemical effects of fluid also influenced damage zone development. Quartz cements, fluid inclusion data, and FTIR analysis indicate that both aqueous and methane-dominated fluids were present within the damage zone. Although aqueous fluids are commonly present in the Tuscarora Sandstone, the presence of methane-saturated fluids is atypical. The backthrust network likely acted as a fluid conduit system, bringing methane-rich fluids up from the underlying Martinsburg Formation and displacing the resident aqueous fluids. The presence of methane was important for damage zone development in two ways. First, methane enhanced the effects of pore fluid pressure and facilitated brittle fracturing; and second, methane inhibited nucleation and precipitation of later-stage quartz cements, and thus the healing of later-stage fractures and breccias. The cumulative effect of damage zone structures produced a zone of localized high porosity and permeability within a well-cemented quartz arenite at depth. The development and preservation of the dilatant damage zone results from (1) its formation at an extensional step-over, (2) lack of later-stage cementation, (3) and presence of locally elevated fluid pressures. iii