CONTRASTING TECTONO-THERMAL HISTORIES ACROSS THE ROSEMONT SHEAR ZONE IN THE CENTRAL APPALACHIAN PIEDMONT OF SE PENNSYLVANIA AND NORTHERN DELAWARE

The Rosemont Shear Zone (RSZ) is a NNE-SSW striking zone of vertical foliation extending from the Pleasant Grove – Huntingdon Valley Shear Zone (PGHV) north of Philadelphia south to Newark De, where it is covered by Cretaceous sediments of the Coastal Plain. The RSZ separates rock with distinct origins and tectono-thermal histories and may be the southern continuation of Cameron’s Line. Rifted Laurentian margin rocks, comprising Mesoproterozoic gneiss and metamorphosed early Paleozoic-aged cover rocks, the West Grove Metamorphic Suite (WGMS) and Glenarm Group, lie to the west of the RSZ; an Ordovician-aged magmatic arc, the Wilmington Complex, and related rock of the Wissahickon Fm. and Chester Park Gneiss, which may be a fragment of the Moretown Terrane, lie to the east. Two discreet periods of metamorphism have been documented in the Wissahickon: high temperature, low to moderate pressure metamorphic assemblages, which occur near Silurian (~430) intrusions, were overprinted by regional-scale kyanite-bearing assemblages in the middle Devonian (~380 Ma). West of the RSZ, Mesoproterozoic gneiss and the West Grove Metamorphic Suite occur in a series of thrust sheets. Monazite ages (Bosbyshell et al., 2016) indicate successive stacking from SE to NW, beginning in the Silurian (~425 Ma), with the highest-grade rocks occupying the highest structural levels. This convergence continued into the Middle Devonian (~380 Ma), when emplacement of the amphibolite facies West Chester nappe produced an inverted metamorphic gradient in greenschist grade Peters Creek Schist below. Thrust sheets are bounded by the steeply dipping Rosemont and PGHV shear zones; a geometric configuration which suggests that shortening occurred in a sinistral transpressive regime. However, outcrop-scale evidence for the sinistral component is rare, due to overprinting by younger deformation. New EPMA U-Th-total-Pb monazite ages from pelitic mylonite document early Devonian sinistral deformation in the RSZ. Analyses from asymmetric tails on foliation-parallel monazite, which indicate a left-lateral sense of shear, yield ages from 420 ± 10 Ma to 404 ± 9 Ma. This is contemporaneous with shortening in rocks to the west and supports the sinistral transpressive model.