ADVANCES IN FAULT SYSTEM RECOGNITION IN THE APPALACHIAN BASIN: OROGEN-PARALLEL STRIKE-SLIP TRANSPORT IN NEW YORK STATE, PENNSYLVANIA AND WEST VIRGINIA

In 1990 when JF, RDJ and students began identifying and tracing faults in the Appalachian Basin of NYS by integrating 11 traditional and innovative techniques, almost no faults were recognized in NYS west of the Mohawk Valley. After 10 years, we were able to infer the trends of hundreds of faults, using primarily surface geology, soil gas, well logs, and remote sensing. Since the actual faults were commonly hidden, sparse deep wells and no seismic data in the public domain (except for what we shot) meant the actual motion and motion history of most faults were largely unknown; the faults were thought to be primarily dip-slip with multiple stages of deformation. Continued work with 2D seismic, core, fracture mapping, and finally 3D seismic showed that repeatedly-reactivated, orogen-parallel faults in the basin have sustained some degree of strike-slip motion in their tectonic history.

In the Appalachian Basin of PA a 3D seismic survey displays a distinctive left-step restraining bend on the Silurian Rose Hill reflector that indicates right-lateral motion on an orogen-parallel fault system. These faults if imaged in 2D seismic would have been interpreted as dip-slip faults. In the same area other structural elements (e.g., P-shears) are consistent with right-lateral slip. The timing of this motion appears to be Neoacadian or Alleghanian. Right-lateral slip is consistent with African-plate slip past Laurentia in early Alleghanian. That the fault motion occurred ~120 km NW of the Appalachian Front suggests that Iapetan- opening faults associated with the Rome Trough accommodated some of the strain as the African plate collided.

In NYS rhombochasms interpreted in 3D seismic surveys indicate right-lateral motion at Ordovician basal Utica time south of the NY promontory, and fault map patterns suggest left-lateral motion occurred north of the NY promontory. Outcrop and core data suggest both have occurred along some faults in the Utica basin. The Taconic motion could result from escape tectonics or oblique subduction (including aseismic ridge subduction).