Structure of Broadtop Synclinorium, Wills Mountain Anticlinorium, and Allegheny Frontal Zone

ABSTRACT The geology of the Broadtop synclinorium, Wills Mountain anticlinorium, and Allegheny frontal zone is interpreted from regional cross sections which use extensive surface, seismic, and subsurface well control in the synclinorium and surface, and scattered well control in the anticlinorium. These major structural features together are about 40 mi wide and extend about 250 mi in a northeast-southwest direction. They make up the western half of the Valley and Ridge province in West Virginia. Structures within the Broadtop synclinorium are similar along its 250-mi length and are shallow rooted above the Whip Cove thrust. The structures within the synclinorium are not related directly to movement on the Broadtop thrust. The gross structures and the differences among the synclinorium, anticlinorium, and plateau are the results of movement on the Broadtop decollement thrust sheet at different stratigraphic positions. The thrust sheet was ramped westward to successively shallower levels as a result of compression from the east. Ramping began wherever the thrust sheets encountered preexisting Cambrian-Ordovician paleotopography or high-angle normal faults. The present elevation of the Wills Mountain anticline in the central segment resulted from piling up and overriding of the leading edge of the Cambrian-Ordovician carbonate rocks (lithotectonic unit 2) in the allochthonous block of the Broadtop thrust. The elevation of the northern and southern segments of the Wills Mountain anticline resulted from breaking and thrusting beneath the Broadtop thrust. This caused the arching of the Broadtop thrust which later was breached by erosion to form two fensters.