EXAMINING DEFORMATION CONDITIONS DURING THE FORMATION OF THE SHADE MOUNTAIN ANTICLINE, CENTRAL PENNSYLVANIA SALIENT

The Shade mountain anticline ~60 km long and ~8 km wide and is located in central part of the Pennsylvania Salient. It was formed during the Late Paleozoic Alleghenian orogeny and includes rocks that range from the Ordovician Juniata Formation through the Middle Devonian Mahantango Formation. In order to better understand the fracture and fluid history of this area, joint and vein orientation data was collected in the field. Samples were collected of mineral veins form each fracture set in multiple formations.

Poles to fractures formed tight clusters on the periphery of the stereonet when rotated back to bedding horizontal. All clastic units showed four joint and vein sets: two nearly orthogonal sets are oriented 320° ± 10° and 040° ± 6°, and another two are oriented 341° ± 7° and 070° ± 10°. The carbonate layers within the anticline such as the Devonian Helderberg formation only contained joint and vein sets orientated at 320° ± 10°. The 320° set is 40° anti-clockwise of the normal to the fold axis and likely formed prior to folding, whereas the 341° set is closer to the normal to the fold axis.

Most of the vein samples contain elongate to blocky quartz, although some vein samples were composed of calcite. Fluid inclusion microthermometry was done on several samples to constrain the trapping conditions and fluid chemistry. The Silurian Clinton Formation contains two aqueous inclusion fluid salinity groups, one ranging from 1.4 to 12.7 % wt. % NaCl equiv. and another ranging from 18.9 to 24.8% wt. % NaCl equiv., with homogenization temperatures (Th) in the range of 119.6 to 149.5 °C. Interpreted trapping pressures range from 104 to 171 MPa, indicating a syntectonic load of 2.0 km at maximum. The Devonian Helderberg Formation has with Th values in the range of 200.0 to209.4 °C, and salinities of 13.2% wt. % NaCl equiv. Trapping pressures are interpreted to be 189 to 248 MPa. These values give a syntectonic load of 3.29 to 5.56 km. However, CH₄±CO₂ inclusions have minimum Th values of ~130 °C suggesting trapping pressures as high as 300 MPa (11.5 km depth). The Marcellus and Mahantango Formations have two-phase inclusions with Th values of 82.3 to 223.4 °C and salinities ranging from 6.2 to 23.7% wt. % NaCl equiv. Trapping pressures range from 66 to 274 MPa. Interpreted syntectonic loads range from 1.55 to 7.25 km.