DEVELOPMENT OF A DILATANT DAMAGE ZONE ALONG A THRUST RELAY IN A LOW-POROSITY QUARTZ ARENITE

A damage zone that developed along a backthrust fault system in well-cemented quartz arenite of the Silurian Tuscarora Sandstone in the Appalachian Valley and Ridge consists of a network of NW-dipping thrusts that are linked by multiple higher-order faults, bounding 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 fault-bounded horses, and favored the formation of extensional fractures. The presence of pervasive, late-stage, near fault-normal joints in a horse in the northwestern part of the damage zone also indicates propagation between two near-frictionless faults. This decrease in frictional resistance during late damage zone development was likely a result of increased fluid pressure.

In addition to physical effects of fluid, chemical effects also influenced damage zone development. Quartz cements, fluid inclusion data, and FTIR analyses indicate that both aqueous and methane-dominated fluids were present during damage zone formation. While aqueous fluids are commonly present in the Tuscarora Sandstone, the abundance of methane and near absence of aqueous 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 likely enhanced the effects of pore fluid pressure, facilitating brittle fracturing; and second, methane inhibited nucleation and precipitation of later-stage quartz cements, and thus the healing of open fractures and breccias.

Cumulatively, deformation 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, and (3) presence of locally elevated fluid pressures.