FLUID HISTORY AND PALEO-OVERBURDEN ALONG A RETRODEFORMED CROSS-SECTION IN THE EASTERN VALLEY AND RIDGE PROVINCE, CENTRAL PENNSYLVANIA

Fracture orientation data were collected in the Middle Devonian to Mississippian clastic sequence along a structural transect from the Appalachian Structural Front to the Great Valley in the eastern part of the Pennsylvania salient. Fractures are commonly planar, and may be bed restricted or cross-cut multiple beds, and are consistently oriented as strike-perpendicular (NNW) or strike-parallel (ENE) sets. The NNW-striking set is commonly mineralized with quartz and less commonly with calcite and/or dolomite.

Fluid inclusion microthermometry provides information on paleo-fluid history, including fluid trapping pressure, temperature and composition. Quartz veins sampled along the structural transect range in thickness from several millimeters up to 2 centimeters. One or more stages of quartz mineralization are present, most commonly occurring as blocky crystals, but can also be fibrous, or prismatic. The quartz contains aqueous brine inclusions that homogenized at 98 to 232 °C. Fluid salinity varies by stratigraphic level with samples in the Mississippian having salinities of 1.9 to 15.1 wt . % NaCl equiv., the Upper Devonian from 9.0 to 24.8 wt. %, and the Middle Devonian from 8.1 to 25.1 wt. %. The quartz also contains CH₄±CO₂ inclusions that homogenize at temperatures as low as -129 °C. The inclusions exhibit a wide range of trapping conditions, which indicate changing deformation conditions during the crystal growth. Calculated trapping pressures are 139 to 287 MPa. Assuming a lithostatic thermobaric gradient, this indicates burial depths during trapping of 5.6 to 11.5 km.

A line-balanced regional cross section was constructed along the sample transect to examine how trapping conditions varied during structural evolution. The resulting structural geometry shows a hinterland-dipping passive-roof duplex of imbricated Cambro-Ordovician carbonate horses dipping slightly to the southeast with each horse having fault bend fold style geometry. Based on preserved stratigraphic thicknesses through the Mississippian, fluid inclusion microthermometry data suggest that the synorogenic (?) Pennsylvanian and Permian thickness was approximately 1.5 km in the north and 6.2 km in the south. This load may have been the driving force for the duplex formation.