Role of Fluid Composition in the Evolution of Porosity: Examples from Fault Zones in the Central Appalachian Foreland

At low temperatures (<300C), fluid composition is a dominant control on the evolution of porosity in quartz-rich rocks within fault zones. This is well illustrated by two faults in the central Appalachian foreland, the Cove fault, a regional thrust in south central Pennsylvania, and a map-scale backthrust in the Cave Mountain anticline of West Virginia. In the Cove fault, porosity was created by extensional microfracturing and localized dilation at jogs and other irregularities in shear zones. Throughout much of the fault history, quartz cement was precipitated from Ca-rich brines at a rate sufficient to prevent widespread brecciation. The latter portion of the history was marked by a change in fluid composition to Fe-rich, which resulted in widespread dissolution of quartz and the creation of new porosity. In the Cave Mountain backthrust, early aqueous fluids promoted rapid healing of extensional microfractures. These were replaced by methane-rich fluids derived from the underlying shale, which inhibited precipitation of quartz cement resulting in the preservation of open fractures and porous breccia, despite being formed at 6-7 km depth. The presence of Fe-rich fluids late in the fault history locally enhanced porosity through the dissolution of quartz.

The two fault zones illustrate the danger in generalizing fault zones as either fluid conduits or barriers. Depending on fluid composition, fault zones in identical rocks that formed under the same P/T conditions can behave quite differently with respect to fluid flow. Furthermore, changes in fluid composition over time can result in a change in behavior from conduit to barrier or vice versa. Finally, the large dilations recorded by extensive cementation in some fault zones do not necessarily correspond to large fluid-rock ratios. Laboratory experiments have shown that diffusion in response to a concentration gradient is capable of moving large volumes of silica in a static fluid.