STRATIGRAPHY AND TECTONIC ASSEMBLAGE OF ROCKS OF THE IAPETAN RIFTED MARGIN OF THE PENNSYLVANIA EMBAYMENT: FRAMEWORK OF THE WESTERN PIEDMONT WESTMINSTER TERRANE, FREDERICK VALLEY SYNCLINORIUM, AND BLUE RIDGE-SOUTH MOUNTAIN ANTICLINORIUM, MARYLAND AND PENNSYLVANIA

Geologic mapping, ⁴⁰Ar/³⁹Ar geochronology, and fission track analyses of lower greenschist-facies phyllite, arkose, and marble support a new interpretation for the Paleozoic tectonic history of the Westminster terrane (WT), Frederick Valley synclinorium (FVS), and Blue Ridge-South Mountain anticlinorium (BR-SMA). The WT and FVS are preserved within the Pennsylvania embayment at the northern terminus of the BR-SMA. Although the WT is truncated by the Potomac terrane (PT) along the Pleasant Grove fault (PGF) and overlain by the Mesozoic Culpeper basin from Pennsylvania south to Virginia, the distribution of the rocks may reflect the Neoproterozoic rift architecture. The WT is comprised of slope and rise prism sedimentary and volcanic rocks interpreted to be Iapetan rift deposits of Neoproterozoic and Cambrian age. The western WT consists mostly of phyllite containing map-scale bodies of quartzite, metabasalt, marble, and metalimestone, whereas the eastern WT consists of schist with thin bodies of metagraywacke and minor greenstone. Northward in Pennsylvania phyllite and schist dominate with few layers of metabasalt in the southwestern WT. This accretionary wedge and their internal premetamorphic faults were emplaced onto continental margin (Sugarloaf Mountain anticlinorium, SMA) and shelf (FVS) rocks along the Martic fault (MF), ca. 445-441 Ma during late Taconian deformation. WT rocks were deformed at ca. 430 Ma to produce the SMA; continental margin rocks are exposed in a tectonic window beneath the slope and rise rocks above the MF. Deformation progressed westward into Blue Ridge rocks at ca. 404-390 Ma. Out of sequence faulting along the PGF (370-360 Ma) produced shear band cleavage during dextral transpressive motion and second-generation cleavage developed in the eastern WT. However, the distribution and correlation of rocks across strike do not support major strike slip displacement along the MF. Deformation and metamorphism occurred episodically before 370 and after 311 Ma in the western PT, eastern WT, and the BR-SMA, with final juxtaposition and thermal convergence in the Alleghanian orogeny (ca. 300 Ma) associated with emplacement of the North Mountain thrust sheet. Motion along the Parrs Ridge fault (eastern WT) at this time produced the regional Marston and Tucquan folds in the western WT.