THE STRUCTURAL GEOMETRY AND DEVELOPMENT OF THE CENTRAL AND EASTERN PART OF THE PENNSYLVANIA SALIENT

16 regional cross-sections with an along-strike spacing of ~10 km were constructed across the central Pennsylvania Valley & Ridge province, and show that the structural geometry varies significantly along this ~060°-striking segment of the salient. The sections are based on published geologic maps, seismic data, and deep-well logs, and are line-balanced on the C-O carbonate lithotectonic unit. Lateral continuity of structures between sections is maintained.

From the foreland to the hinterland, the major structures include: 1) The Nittany anticline, which has a fault-propagation-style fold geometry and highly-faulted, steep to overturned frontal limb. It is underlain by up to 2 additional thrust horses. 2) The Juniata culmination, which is interpreted to either be a series of imbricate C-O thrust horses or a flat-on-flat structure, where a single >30 km long faulted and folded C-O thrust sheet overlies an imbricated C-O thrust section. Both interpretations accurately fit the available data and have similar retrodefomed lengths. 3) The regionally continuous (>160 km) Jacks Mt. – Berwick anticline C-O thrust structure. 4) The Shade-Blue-Backlog anticlinorium which is a series of detachment folds above a C-O thrust structure. 5) A series of independent C-O thrust anticlines to the east, that merge into an anticlinorium structure in the central part of the salient.

Because the initial cross sections were constructed perpendicular to strike, they converge upon retrodeformation. In order to correct for this, fold segments between the section lines were required to maintain their original length as the sections were pulled back. This resulted in a restoration path that curves 25°-30° to the east. The fault underlying the presently curved Jacks Mt. – Berwick anticline structure, as well as those structures toward the hinterland, restore to a nearly straight fault traces oriented 045°-050°. The Nittany anticline thrust has a general bow shape, whereas the Juniata culmination contains four faults with an en-echelon pattern. Upon restoring the cross sections, there is an increase in the amount of shortening in the C-O section from the eastern part of the salient (~22%) to the central part (~52%). The relatively straight restored faults require a rigid indenter colliding from the southeast to impose the curvature to the salient.