PALEO-FLUID AND DEFORMATION CONDITIONS DURING THE ALLEGHENIAN OROGENY: SOUTHERN PENNSYLVANIA VALLEY AND RIDGE PROVINCE

Fluid inclusion microthermometry of syn-tectonic vein minerals provides information on the fluid history, including trapping conditions (PT), composition, and changes over time. This data in turn can provide insight into the paleo-hydrologic structure and conditions present during deformation. Quartz and calcite veins were sampled from the multiple stratigraphic levels along a transect in the Pennsylvania Valley and Ridge province, from the Appalachian Structural Front to the Great Valley. The area was deformed during the Late Paleozoic Alleghenian orogeny.

In both Ordovician carbonate and clastic rocks, blocky calcite contains aqueous brine inclusions that were trapped at 104 -139 °C and have salinities of 23.7 to 28.0 wt. %, along with uncommon CH₄ inclusions. Fluid trapping pressures range from 84 to 97 MPa. In the Silurian - Devonian carbonate rocks, blocky calcite contains aqueous brine inclusions that were trapped at 79 -138 °C and have salinities of 24.0 to 28.0 wt. %. Fluid trapping pressures are interpreted to be 130 to 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 to 18.6 wt. %. The quartz also contains abundant CH₄±CO₂ inclusions that homogenize as low as -122 °C and have up to 6% CO₂. Preliminary SEM-CL analysis of individual quartz crystals show a common pattern of an internal dark-gray core with a bright white rim. The cores contain primary CH₄±CO₂ inclusions while the rims contain both primary CH₄±CO₂and aqueous brine inclusions. Trapping pressures increase from core to rim, with maximum trapping pressures of 230 MPa. In contrast, Middle Devonian rocks at the Appalachian Structural front on the western margin of the Valley and Ridge contain only liquid hydrocarbon and condensate-like fluid inclusions that reflect low trapping pressures. Interpreted syn-Alleghenian sediment and/or thrust loads range from 4 to 5 km within the Valley and Ridge to less than 0.5 km at the Structural Front.