DEEP SYNTECTONIC BURIAL OF THE ANTHRACITE BELT, PENNSYLVANIA

The source of heat for anthracitization in the Pennsylvania Anthracite belt has been a point of controversy for many years. Several authors have called on low burial conditions (<5 km) with migrating hot orogenic-derived fluids as the source of heat, while others call on deep basin burial (6-9 km). In this study, fluid inclusions microthermometry of both aqueous and CH₄ ± CO₂ fluid inclusions trapped in syntectonic quartz veins is used to estimate maximum deformation conditions in and near the Anthracite Belt during the Alleghanian orogeny.

CH₄ ± CO₂ inclusions occur in veins in Pennsylvanian rocks, and the Devonian Trimmers Rock and Marcellus Fms. and contain up to 16% CO₂, The lowest density inclusions have homogenization (T_hH) values as low as -137.2 °C in the Pennsylvanian rocks and as low as -152.1 to -155.9 °C in the Trimmers Rock and Marcellus Fms. Using a geothermal gradient of 18 °C km^-1 these low density inclusions correspond to maximum trapping conditions of ~280 °C and 360 MPa (up to 13.4 km depth) in the Pennsylvanian Llewellyn Fm. and up to ~390 °C and 540 MPa (up to 19.0 km depth) in the Trimmers Rock and Marcellus Fms.

Aqueous fluid inclusions occur as two types, low-temperature homogenization (low-T_hA) inclusions (~100 to 155 °C) that are from samples with no evidence for CH₄ ± CO₂ inclusions and need a pressure correction and high-T_hA inclusions (~190 to 245 °C ) that are typically associated with CH₄ ± CO₂ inclusions and do not need a pressure correction. The low-T_hA inclusions are associated with re-equilibrated fluid inclusions that are interpreted to be related to isothermal compression during rapid burial. When a pressure correction is applied, the isochores intersect the maximum trapping conditions determined for the CH₄ ± CO₂ inclusions. The high-T_hA inclusions are interpreted to have been trapped during uplift at lower burial depths.

The anthracite basin is interpreted to be a piggy-back basin that formed forelandward of the deforming Reading Prong massif during the Alleghanian orogeny. Thrusting in the hinterland resulted in the deposition of up to 12 to 16 km of syntectonic sediments in the basin. Continued thrusting then resulted in the folding, uplift and denudation of rocks along the southern margin of the basin.