RELAY RAMPS AND RHOMBOCHASMS: EXTENSION, STRIKE SLIP MOTION, AND THE NATURE OF THE PENNSYLVANIA SALIENT/OROCLINE IN THE MARCELLUS AND UTICA OF THE NORTHERN APPALACHIAN BASIN

3D seismic surveys in the Appalachian Basin of western PA reveal relatively short-segmented, orogen-parallel fault systems linked by relay ramps in the Devonian Onondaga/Marcellus and successive units. Swings and disruptions in anticline axes mapped at the surface and in shallow coal and oil/gas fields correspond to the locations of the relay ramps. These relay ramps have characteristics typical of extensional relay ramps elsewhere. The extensional faults initiated in NeoAcadian times related to slip on, and movement of, the deeper Silurian Vernon shales and Salina salts. These structures were partly guided by reactivated Iapetan-opening faults that influenced the fault-block dips and provided pathways for fluid migration.

Individual fault segments are straight within the 3D seismic surveys, yet the fault-related folds at the surface appear to bend around the orocline of the Pennsylvania Salient. A 3D survey in a region where the fold axes have a sharp bend in map view shows the faults do not bend into the different alignments—they are different sets that intersect and abut. The fault set intersection is consistent geometrically with earlier suggestions that the orocline resulted (in part) from differently oriented and timed SH_max associated with “megathrusts” in the hinterland. Strike slip/oblique motion along orogen-parallel faults in the salient is implied by the orientations of the intersecting fault sets.

In 3D seismic surveys in NY south of the Mohawk River, rhombochasms along NNE-striking faults suggest right-lateral slip in Taconic Trenton/Utica time. Outcrop map patterns also reveal possible rhombochasms with Lower Paleozoics in a sea of PC, but these rhombochasms suggest left lateral motion. Kinematic indicators in veins in the Utica along the NNE-striking Hoffmans Fault indicate both left and right lateral motion (in addition to dip slip). Sub-horizontal slickenfibers in Utica occur in core near the NNE-striking Fonda Fault, consistent with strike slip motion. The Taconic rhombochasms imply that thicker Utica can be expected locally and escape tectonics may have played a part in slip along the NNE-striking faults away from the NY Promontory. Complicated patterns of slip out of the core of the PA Salient occurred in Alleghanian times.