CONTROLS ON THE STYLE OF LOW TEMPERATURE, FAULT-RELATED DEFORMATION IN QUARTZ-RICH ROCKS: A TALE OF TWO FLUIDS

Fluids play an important role in the deformation of quartz-rich rocks having both a mechanical and chemical effect. In addition to the presence or absence of fluid, the fluid composition can also affect deformation. The purpose of this presentation is to describe the differences and changes in deformation style in two fault zones as a result of differing fluid histories. The Alleghanian structural style and history of quartz arenite deformed in the Cove fault zone in Pennsylvania is compared to that of a map-scale backthrust in the core of the Cave Mountain anticline in West Virginia. In the Cove fault zone, early distributed cataclasis was followed by localized cataclasis and dilational microfracturing, all of which were accompanied by locally significant crystal-plastic deformation. Throughout these events, quartz cementation kept pace with microfracturing. In the Cave Mtn. backthrust, early microfracturing was accompanied by quartz cementation, but the dominant deformation, extensive brecciation and dilational microfracturing, occurred in the near absence of quartz precipitation. Crystal-plastic deformation is unimportant in this fault zone. Whereas fluid inclusions in the Cove fault zone are two-phase aqueous (CaCl2-rich brine) inclusions, those in the Cave Mtn. backthrust are all single-phase methane (+/- CO2) inclusions.

Because deformation in both fault zones took place at similar depths (~6 km) and temperatures (~220 C), the difference in structural style is believed to result from the difference in fluid composition. In the Cove fault zone, aqueous fluids promoted rapid cementation of breccias and microfractures resulting in a well cemented cataclasite. In the backthrust, the fluid was methane-dominated; hence, the growth of cement was inhibited resulting in open microfractures and a porous fault breccia. Furthermore, the relatively greater importance of crystal-plastic deformation in the Cove fault zone is believed to be due to hydrolytic weakening from the greater availability of water. In addition to affecting the fault zone behavior and deformation style in terms of the role of fracture healing, this study also suggests that methane may act like to water to promote brittle deformation through the reduction of effective normal stress.