RELAY RAMPS AND RHOMBOCHASMS: STEP OVERS AND CROSS-STRIKE DISCONTINUITIES 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 at step-overs in the Onondaga/Marcellus and higher units. Swings and disruptions in anticlinal traces mapped at the surface and in shallow coal and oil/gas fields (essentially appearing as local cross-strike discontinuities) correspond to the locations of the relay ramps. These relay ramps have characteristics typical of extensional relay ramps. The extensional faults initiated in Acadian times and are related to slip on, and movement of, the deeper Vernon shales and the Salina salts, partly guided by reactivated Iapetan-opening faults that may have influenced the slope and provided pathways for fluid migration. These extensional faults later developed as generally east-verging reverse faults higher in the section.

In the 3D seismic surveys, the individual fault segments are straight, yet the associated folds appear to bend around the orocline of the Pennsylvania salient. Resolution of this apparent contradiction can be found in a 3D survey that shows the associated faults do not bend. Rather, they are two different sets; one set of faults extends to the SSW and is abutted by a set that extends to the NE. This fault set intersection is consistent geometrically with earlier suggestions that the orocline resulted (at least in part) from differently oriented and timed SH_max.

In contrast, the nature of stepovers is less clear on the NNE-striking, nominally normal faults in the Ordovician Utica of the Mohawk Valley region in eastern NYS near the Taconic convergence zone. Some fault segments are linked by cross-fault stepovers observed in surface mapping. However, relay ramps were not recognized in the past, partly because of poor outcrop or absence. One stepover of the Little Falls Fault is a narrow horse with fracture cleavage, suggesting that a relay ramp of areal consequence does not exist there. Lineaments that extend beyond these stepovers are colinear with proposed WNW-striking faults. In contrast, we identified a partly exposed relay ramp on the Hoffmans Fault. In 3D seismic surveys to the SW in NYS, stepovers on NNE-striking faults suggest rhombochasms with right-lateral slip in Trenton/Utica time; surface geology in the Mohawk Valley region could reflect such rhombochasms.