STRUCTURAL GEOMETRY AND DEFORMATION HISTORY OF THE EASTERN VALLEY AND RIDGE PROVINCE, CENTRAL PENNSYLVANIA

In order to understand the structural geometry of the eastern Valley and Ridge province of Pennsylvania, a line-balanced cross section was constructed from Montoursville in the north to Hershey in the south just east of the Susquehanna River. We used field data published surface data, well data, and depth to basement information. The resulting geometry shows a hinterland-dipping passive-roof duplex of 16 imbricated Cambro-Ordovician carbonate horses dipping slightly to the southeast. Each horse has a fault bend fold style geometry and an average ramp angle of 25°. In the north of the section, the Nittany Anticline is underlain by an antiformal stack of three carbonate horses. To the south in the section, the four more major anticlines (Berwick, Shade Mountain, Broad Mountain, and New Bloomfield) are underlain by either a double or triple imbricate stack of horses. The duplex is overlain by an unbreached cover of Middle-Upper Ordovician to Pennsylvanian rocks. The basal detachment is in the Cambrian clastic sequence and has a gentle dip to the southeast. All other major detachments are in the Middle Ordovician Reedsville Fm. The deformed section length is 109 km and the retrodeformed length is 205 km, resulting in 53% shortening.

Fracture orientation data was collected from sites near the section line. Usually planar, these fractures are either bedding restricted or may cross cut multiple layers, and are rarely mineralized. These fractures are found throughout the region at all stratigraphic levels and are consistently oriented as either strike-perpendicular (NNW) or strike-parallel (ENE) sets. The NNW-striking pre-folding fractures indicate a stress field approximately parallel to the cross-section line.

Fluid inclusion microthermometry data collected from quartz veins in the Middle Devonian clastic sequence provided paleo-overburden, which suggested that the Permian and Pennsylvanian thickness was near 1.5km in the north and 6.2 km in the south. The Permian and Pennsylvanian load may have driven the duplex behavior in the Central Appalachians.