Relay ramps and rhombochasms in the northern Appalachian Basin: Extensional and strike-slip tectonics in the Marcellus Formation and Utica Group

Newly identified relay ramps and rhombochasms in the northern Appalachian Basin (NAB) require significant revision to the structural style, structural timing, and hydrocarbon migration components of the basin evolution model for the NAB. Relay ramps demonstrate that Neoacadian (Devonian) extensional tectonics affected the Devonian Marcellus Formation, earlier than the accepted Alleghanian (Carboniferous-Permian) tectonics. Relay ramps also confirm Ordovician Taconic extension affected the Ordovician Utica Group. Rhombochasms and possible Riedel shears in the Utica Group suggest a component of orogenparallel, strike-slip motion during the Taconic Orogeny. In western Pennsylvania, a three-dimensional (3-D) seismic survey reveals that relay ramps terminate straight segments of faults and kink bands along the southeast borders of asymmetric, Salina Group "salt"pillows. The relay ramps are consistent with a local extensional environment associated with evaporite and mud or mudstone withdrawal and transport. Devonian sediments infill the grabens linked with the salt pillows, indicating that the grabens and associated faults initiated during the Devonian Neoacadian orogeny. Since the faults predate hydrocarbon generation, they were potentially migration pathways for hydrocarbons. In New York State, Ordovician (Taconic) rhombochasms in the NAB are inferred from (1) grabens observed in 3-D seismic surveys that occur at right stepovers of northeast-striking faults south of the New York promontory on the Taconic Laurentian margin and (2) grabens mapped at the surface that occur at left stepovers of north-northeast-striking faults north of the New York promontory. The opposite sense of inferred motion north and south of the promontory is compatible with Taconic escape tectonics away from the New York promontory.