Changing fluid conditions during folding: An example from the central Appalachians

Fluids inclusion microthermometry and stable isotope analyses of vein minerals in the central Appalachian Patterson Creek anticline record dynamic changes in fluid temperature, pressure, and chemistry during folding. These changes resulted from variations in fracture connectivity and fluid communication between stratigraphically restricted fluid reservoirs. Minerals in early-formed bed parallel and ENE-NNW striking cross-fold veins show that prior to folding, fluids in Silurian through lower Upper Devonian rocks were well stratified based temperature, salinity and isotopic composition. Stable isotopes data indicates that the fluid systems were essentially closed, with no communication between layers. As folding commenced, early minerals in NE-NW striking veins in the Siluro-Devonian carbonate rocks record 2. km of burial by syn-tectonic sedimentation, with corresponding increases in fluid temperature and pore-fluid pressure. An additional 2. km of burial along with the influx of low-salinity, methane-saturated brines into the Middle and Upper Devonian clastic rock sequence is recorded by later quartz in the NE-NW striking veins. At some point during folding, Stable isotope data indicate that fluid communication increased along fracture and faults between major stratigraphic fluids reservoirs. The culmination of this process was the breaching of the stratigraphic seal at the base of the Devonian shales, allowing fluid infiltration from the Devonian shale sequence into the upper portion of the Helderberg Group. This resulted in the local precipitation of quartz and possibly the enhancement of solution structures.