The northernmost extension of the Blue Ridge in Pennsylvania is South Mountain, an anticlinorium involving rocks ranging in age from Precambrian through Ordovician. Its east limb is cut out along the Triassic basin in Pennsylvania. Its western limb was deformed by flexural slip, and in the fold core, where deformation was more intense, it was by passive flow. Only one orogenic period of deformation, during the late Paleozoic, (Alleghanian), is apparent. Several stages, including the latest stage of slip-cleavage development, can be recognized within the orogeny.

Folds, cleavage, and faults generally define a simple kinematically congruent model in which regional transport is normal to the regional fold axis. South of the Carbaugh-Marsh Creek right-lateral tear fault near Chambersburg, these structures define a tectonic grain of N. 17° E.; north of the fault the tectonic grain is N. 40° E. This abrupt change in strike of rectilinear tectonic elements accounts for what has been considered as an arcuation of the Blue Ridge.

Analysis of major longitudinal faults by mapping and drilling indicates that there are no shallow thrusts in South Mountain. If a major thrust is present, it is probably deeper than 20,000 feet. South Mountain at its northern terminus passes under the Yellow Breeches thrust sheet which was emplaced by sliding, subsequent to Blue Ridge folding. The rocks of the thrust sheet are multiply-deformed, tectonized carbonate rocks of the Lebanon Valley nappe of probable Taconic age.

The proposed Cornwall-Kelvin right-lateral basement dislocation along 40° N. latitude could account for some of the observed Blue Ridge structure. However, strike-slip faulting is not compatible with paleomagnetic data and certain other structural evidences. It seems more reasonable that an early Paleozoic arcuate structure in the piedmont, east of the Blue Ridge, influenced later structures.

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