FRACTURE PATTERNS AND PALEO-OVERBURDEN ALONG A RETRODEFORMED CROSS-SECTION ACROSS THE PENNSYLVANIA SALIENT

Fracture orientation data was collected from multiple formations and stratigraphic levels along a transect across the central part of the Pennsylvania salient. Fractures are typically planar, may be either bed restricted or cut across multiple beds, and are commonly mineralized with one or more stages of calcite and/or quartz. Two pervasive pre- to syn-folding fracture sets were found, a strike-parallel (ENE-striking) set and a strike perpendicular (NNW-striking) set. These fracture sets were found at all stratigraphic levels, and in all lithologies across the entire fold belt. Two definitively pre-folding sets include a WNW-striking (308°±8°) set and a NNE-striking (028°±10°) set. Both sets are found primarily in the Silurian and Devonian clastic rocks.

In order to investigate how fracture orientations vary spatially and temporally during the development of the Valley & Ridge, a line-balanced structural cross section was constructed across the salient using surface geology, depth to basement data, and well control. The structure is a duplex of Cambro-Ordovician carbonate horses that is overlain by a locally faulted passive roof cover rock sequence of Ordovician through Pennsylvanian rocks. The deformed length of the duplex is 97 km, while the retrodeformed length of the carbonate strut is 206 km. The orientations of the ENE and NNW sets are consistent across this restored length, suggesting that the shortening direction must have been the same throughout the development of the duplex. The WNW and NNE pre-folding fracture sets reflect a stress field present before the formation of the salient, and may be due to the 300° directed shortening of the central Appalachians.

Based on the retrodeformed section and preliminary fluid inclusion microthermometry data, syn-Alleghenian sediments accumulated to over 3.6 km near the hinterland side of the belt, tapering to 1.9 km approximately 20 km further toward the foreland, and to <1 km at the Appalachian Structural Front.