FLUID HISTORY AND PALEO-OVERBURDEN DURING THE ALLEGHENIAN OROGENY: CENTRAL PENNSYLVANIA VALLEY AND RIDGE PROVINCE

Fluid inclusion microthermometry of syn-tectonic vein minerals provides information on fluid history, including trapping conditions (PT), composition, hydrologic structure, and changes during deformation. Quartz and calcite veins were sampled from multiple stratigraphic levels along a transect in the Pennsylvania Valley and Ridge province, from the Appalachian Structural Front to the Great Valley. A line-balanced structural cross-section was constructed along the same transect to provide insight on the structural geometry of the area. The structural style across the Valley and Ridge changes from a series of closely-spaced C-O carbonate imbricate horses toward the hinterland to a long flat-on-flat toward the foreland, culminating in an antiformal stack of three carbonate layers.

In both Ordovician carbonate and clastic rocks, blocky calcite contains aqueous brine inclusions trapped at 104 -139 °C with salinities of 23.7-28.0 wt. %, and uncommon CH₄ inclusions. Fluid trapping pressures range from 84-97 MPa. In the Silurian - Devonian carbonate rocks, blocky calcite contains aqueous brine inclusions trapped at 79-138 °C and have salinities of 24.0-28.0 wt. %. Fluid trapping pressures are interpreted to be 130-155 MPa. Middle Devonian clastic rocks contain early blocky calcite and dolomite with light-brown tinted hydrocarbon inclusions. Later quartz is blocky, but also occurs as prismatic and euhedral crystals up to 3 cm across. These contain aqueous brine inclusions that homogenize at 148-204 °C and have salinities of 7.9-18.6 wt. %. The quartz also contains abundant CH₄±CO₂ inclusions that homogenize as low as -122 °C with up to 6% CO₂.There are at least two major growth phases within the quartz crystals as determined by SEM-CL analysis as well as numerous smaller incremental growth zones. There is a wide range of trapping conditions, with increasing trapping pressures from center to rim, indicating changing deformation conditions during crystal growth. In contrast, Middle Devonian rocks at the structural front contain only liquid hydrocarbon and condensate-like fluid inclusions that reflect low trapping temperatures. There is an apparent significant drop in paleo-overburden at this structural transition ranging from 4-5 km within the Valley and Ridge to less than 0.5 km at the Structural Front.