Constraining fault architecture and fluid flow using crustal noble gases

Deformational features, such as faults, strongly affect the pathways, rates, and length-scales of fluid flow in sedimentary basins. The hydrologic properties of faults vary greatly, allowing them to exhibit hydrologic behaviors spanning the gamut from open and conductive pathways for fluid flow to closed barriers to fluid flow. As a result, determining the role of faults with respect to fluid flow and their impact on geologic fluid migration throughout the life cycle of sedimentary basins remains challenging. Previous studies interrogate fault behavior using structural characteristics of fault cores and damage zones or the geochemistry of fluid inclusions and mineral veins associated with these features. Here, we evaluate the utility of crustal noble gases as tracers of fluid flow in fault systems by examining the composition of a well-constrained fault system in the Northern Appalachian Basin. The Seneca Stone Thrust fault, located near Seneca Falls, NY, USA, displays ~5 m of offset in the Onondaga Limestone and Marcellus Shale. Significant loss of ⁴He and ²¹Ne* near the fault appears to occur prior to basin exhumation, likely resulting from compactive dewatering or hydrocarbon generation and migration. Subsequent cooling shifted fault function from acting as a conduit to acting as a barrier, allowing accumulation of crustal noble gases for extended geologic time (~140 Myr) apparently in a closed-system. We identify significantly lower [⁴He] and ⁴He/²¹Ne* in a discrete zone that includes the fault core and extends ~30 cm into the intensely fractured portion of the fault damage zone as compared to the rest of the fault damage zone (~5 m in width) and other nearby samples from the Marcellus Fm. in the quarry pavement. These results imply that the Seneca Stone Thrust fault has reverted to behaving like a localized conduit in recent geological time. We interpret the discrete zone of extreme ⁴He loss near the fault plane as evidence of recent focused fluid flow within the fault core at temperatures below 94 °C, possibly within the past ~45,000 years. Our data suggest that crustal noble gases can be used to evaluate the timing of crustal isolation and/or hydraulic communication in fault zones locally in the Marcellus Shale and potentially in other geological settings. Future work can further develop this approach to examine the long-term suitability of geological formations for permanent subsurface storage of CO₂, other gases (e.g., H₂), or nuclear waste, as well as to determine if various subsurface intervals represent zones of hydrocarbon accumulation and/or loss over geological time.