LATE PALEOZOIC TRANSCURRENT TECTONIC ASSEMBLY OF THE CENTRAL APPALACHIAN PIEDMONT

Recent investigations in south-eastern Pennsylvania and northern Maryl and have demonstrated a major anastomosing strike-slip shear system. T he Pleasant Grove-Huntingdon Valley shear system emerges from beneath the coastal plain cover at Trenton, New Jersey, and extends to the are a west of Baltimore, Maryland, where it is overlain by the Culpepper M esozoic rift basin. The sense of offset across this system is dextral. In the Susquehanna River region and north of the shear zone, the rock s of the Octoraro Formation contain evidence for two metamorphisms and deformations prior to strike-slip shearing, whereas south of the shea r zone the Peters Creek Formation contains evidence for only one. The discordance in metamorphic and deformational history across the shear zone suggests the now juxtaposed rocks originated in different parts o f the orogen. Although conclusive ages for the strike-slip deformation do not exist at this time, the timing of deformation is loosely const rained where the shear system crosscuts known Taconian structures in t he Piedmont. Comparison of deformation style with other regions in the Appalachian suggests the Pleasant Grove-Huntingdon Valley shear syste m is related to Alleghanian transcurrent tectonics in the Piedmont. Pa linspastic reconstruction of the Pleasant Grove-Huntingdon Valley shea r system reveals fundamental problems in current tectonic models for t he central Appalachian Piedmont. A minimum of 150 km of dextral offset is proposed for the Pleasant Grove-Huntingdon Valley shear system bas ed on reconstruction of the Cambrian-Ordovician shelf edge between nor thern Maryland and southeastern New York. Displacement of this magnitu de can account for the previously proposed tectonic models that portra y a failed Iapetan rift block and microcontinent that contains the Bal timore Grenvillian massifs. Even though a history of early orthogonal collision is preserved within discrete structural blocks, transcurrent shearing has greatly influenced the distribution of those blocks. Mod els not including the strike-slip component of tectonic assembly need serious reconsideration, as evidence grows that the magnitude of oroge n-parallel displacement is equal to or larger than the orthogonal comp onent.