Seven cross-strike structural discontinuities (CSDs), 17 lineaments, and 4 crustal blocks, previously recognized in the central Appalachians, were compiled on side-looking airborne radar (SLAR) image mosaics to develop a structural model. The high-resolution, synoptic view and detailed expression of surficial morphology on x-band SLAR images provide a preliminary means of mapping CSDs and lineaments on the basis of alignment or disruption of structural and geomorphic patterns. Wheeler defined CSDs as structural lineaments or alignments at high angles to regional strike that are recognizable because they disrupt strike-parallel structural, geophysical, geomorphic, sedimentologic, or other patterns in allochthonous fold and thrust belts. CSDs are broad zones that may contain many lineaments of varying size and orientation. Geologic and geophysical data suggest that some Appalachian CSDs and lineaments are the surficial expression of crustal block boundaries.
The previously defined CSDs, lineaments, and crustal blocks were compiled on a 1:1,000,000-scale SLAR image mosaic of eight 1:250,000-scale quadrangles from central Pennsylvania to southern Virginia. Data compiled on the likely origin of these 7 CSDs suggest that three involve basement, three involve only lateral decollement ramps (zones where a decollement transfers to a higher stratigraphic level along regional strike), and one may involve both basement and a lateral decollement ramp. CSD lateral ramps can dip either north or south. Geologic literature suggests that lateral decollement ramps alternate in dip direction along strike of the CSD. Reported evidence for basement involvement in the origin of CSDs includes spatially related centers of earthquake intensity, intrusive igneous rocks, stratigraphic evidence of syndepositional uplift, and basement faults beneath splay faults as seen in seismic reflection data. Highresolution strike-line seismic reflection, gravity, and magnetic profiles are necessary to further understand the origin and formation mechanisms of CSDs.
CSDs are an exploration target for natural gas because they are areas where fracture permeability is enhanced. The spatial relationship of CSDs to gas fields in the Valley and Ridge province suggests structural closure of anticlinal traps due to differential movement along the CSD zone.