EVIDENCE FOR POST-ALLEGHANIAN N-S CONTRACTIONAL DEFORMATION IN EASTERN NORTH AMERICA

Evidence of north-south shortening has been found post-dating latest Alleghanian structures in eastern Ohio and affecting early Mesozoic sediments in eastern Pennsylvania. Cleats striking approximately 100˚/280˚ are pervasive in coals exposed along the crest of the Cambridge arch in eastern Ohio. This cleating is commonly associated with small north- and south-verging thrusts that deform structures aligned with the latest phases of Alleghanian orogenesis. Locally, shales and thin-bedded siltstones also exhibit weak spaced cleavage with the same orientation as the coal cleats. Similarly oriented penciling in shales and spaced cleavage in siltstones, all of early Mesozoic age, is observed in the southwestern end of the Newark basin in eastern Pennsylvania. Together, these indicate significant post-Alleghanian N-S shortening affecting portions of eastern North America. No tectonic source of this contractional event is obvious, but it is clear in the Newark basin that this is not related to the previously recognized Neogene NW-SE contractional event.