--> Relay Ramps and Rhombochasms: Step Overs in the Marcellus and Utica of the Northern Appalachian Basin

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Relay Ramps and Rhombochasms: Step Overs in the Marcellus and Utica of the Northern Appalachian Basin

Abstract

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 successive units. Swings and disruptions in anticlinal traces 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 (for example they have overlap length to width ratios ranging from 1–3.5). 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. To the extent of the 3D seismic surveys, the individual fault segments are straight. How does that observation compare with the oroclinic bend of the Pennsylvania salient where the folds appear to bend around the orocline? In a region where the fold axes have a sharp bend in map view a 3D survey shows the set of orogen-parallel faults that extend to the SW are abutted by a set with a more easterly trend that extend to the NE. The faults do not bend into the different alignments—they are two different sets. This fault set intersection is consistent geometrically with earlier suggestions that the orocline resulted (at least in part) from differently oriented and timed SHmax. In contrast, the nature of stepovers is less clear in the Ordovician Utica of the Mohawk Valley region in eastern NYS near the convergence zone of the Taconics. Fault segments linked by cross-fault stepovers observed in surface mapping (e.g., the Little Falls, Dolgeville, and more eastern NNE-striking faults). However, relay ramps have not been recognized, perhaps because of poor outcrop, or that there is none. One stepover of the Little Falls Fault is a narrow horse with nearly vertical bedding and tight fracture cleavage, suggesting that a relay ramp of consequence does not exist there. Lineaments that extend beyond the stepovers are colinear with proposed WNW-striking faults, some of which reflect local fabrics in pC. In 3D seismic surveys, stepovers on NNE-striking faults appear to be rhombochasms, suggesting right-lateral slip in Trenton/Utica time; surface geology could reflect such rhombochasms.