STRUCTUAL EVOLUTION OF THE NITTANY ANTICLINORIUM THRUST SYSTEM: WEST CENTRAL VALLEY AND RIDGE OF PENNSYLVANIA, MARYLAND, AND WEST VIRGINIA

A series of balanced cross sections and a 3D-reconstruction have been constructed across the Nittany anticlinorium thrust system from northern West Virginia through southern Pennsylvania. Structure controls includes field data, published surface map data, and well data. The anticlinorium was formed during the Late Paleozoic Alleghenian orogeny, is bounded on the west by the Wills Mountain Anticline, and on the east by the Broadtop anticline to the south and the Friend’s Cove anticline to the north.

Toward the south, the anticlinorium is defined by a 17 km thrust duplication of the 2 km thick Cambrian-Ordovician carbonate sequence, with the upper plate thrusted into a duplex of four major horses for a total of 38% shortening. The westernmost horse has a fault-propagation-fold geometry forming the Wills Mountain anticline, whereas the other horses have a fault-bend fold geometry.

To the north, the width of the anticlinorium decreases as the length of duplication of the carbonate sequence decreases. This corresponds to a structural discontinuity near the present location of the Potomac River. In southern Pennsylvania, the anticlinorium consists of two fault-propagation fold-style thrust sheets defining the Wills Mountain anticline on the west and the broad Friend’s Cove anticline on the east. Structure has moderately overturned footwall synclines. The frontal limb of each thrust sheet is interpreted to be deformed into a series of imbricate thrust blocks forming triangle zones.

Just south of the Latitude 40°N tear fault, the structure continues as two thrust sheets with structural duplication with the underlying plate of less than 6 km. Increased displacement of the easternmost thrust results in the leading edge of the sheet being thrusted >3 km beyond its detached and faulted overturned limb. Forlandward of the Wills Mountain anticline, the carbonate sequence is thrusted into a fault-bend style fold as the Schellsburg dome. Total shortening for this section is 43%.