FLUID AND FRACTURE HISTORY OF THE TUSCARORA MOUNTAIN ANTICLINE, CENTRAL VALLEY AND RIDGE PROVINCE

The Tuscarora Mountain anticline is located in central Pennsylvania salient and is proximal to the southern margin of the Valley and Ridge province. This anticlinal structure involves the Ordovician Juniata Formation through the Devonian Marcellus Formation. The competent Silurian Tuscarora sandstone provides the structure a high stratigraphic relief. The structural deformation history of this region of the Pennsylvania salient is better understood through the synthesis of multiple methods of analysis, including fluid inclusion microthermometry, quartz vein mineral petrography, and fracture and vein orientations. Dynamically changing fluid conditions (pressure, temperature, chemistry) of mineral vein fluid inclusions are useful for interpreting both the overall deformation trends of the entire salient region, and for highlighting specific local deformation events.

Fractures fall into two bedding perpendicular broad sets after rotating to bedding horizonal: 330°±20° and 550°±20°. Fracture mineralization includes syntaxial veins of euhedral quartz, blocky quartz, elongate blocky quartz, and fibrous quartz, all within lithic sandstone fractures. Fluid inclusion microthermometry of two-phase aqueous inclusions and hydrocarbon (methane) bearing inclusions is used to determine fluid trapping conditions.

Vein samples from the Tuscarora and Bloomsburg Formations have aqueous fluid inclusions that homogenize between 77 to 139 °C, corresponding to a trapping depth of 5.3 to 6.5 km assuming a 20 to 25 °C km^-1. Salinities of 6.1 to 23.3 wt. % NaCl equiv. Quartz vein samples form the Devonian Hampshire Formation have aqueous inclusions that homogenize between 91 to 100 °C, corresponding to a trapping depth of 3.1 to 4.0 km. However, CH₄±CO₂ inclusions homogenize as low as -110 °C, indicating trapping pressure of 142 to 178 MPa (5.5 to 6.8 km depth). A single quartz vein from the Devonian Catskill Formation has CH₄±CO₂ inclusions that homogenize as low as -98.5 °C, indicating trapping pressure of 80 to 96 MPa (3.1 to 3.7 km depth). Based on current stratigraphic thicknesses, an additional 1.0 to 5.5 km of post-Mississippian syntectonic load are indicated by the fluid inclusion data.