Siluro-Devonian Assembly of the High-grade Core of the Central Appalachian Piedmont

Howell Bosbyshell, West Chester University of Pennsylvania
Gale Blackmer, Pennsylvania Geological Survey
LeeAnn Srogi, West Chester University of Pennsylvania
Sandy Schenck, Delaware Geological Survey

Amphibolite facies metamorphic rock of the West Grove Metamorphic Suite in the central Appalachian Piedmont of SE Pennsylvania and northern Delaware occurs in a stack of basement gneiss-cored nappes or thrust sheets bounded to the north by the Pleasant Grove-Huntingdon Valley shear zone (PGHV) and to the southeast by the Rosemont shear zone (RMZ); both are steeply dipping transcurrent shear zones. The granulite facies Wilmington Complex (WC) and amphibolite facies Wissahickon Fm. (WF) occupy the block east of the RMZ. From NW to SE and structurally lowest to highest, the nappes include the West Chester massif, the Woodville nappe, the Avondale massif and the Mill Creek Anticline. U-Th-total Pb monazite ages indicate that maximum temperatures in the Mill Creek were attained in the late Silurian prior to the highest temperatures in the structurally lower Avondale nappe. In turn, peak metamorphism in the structurally lowest unit, the Doe Run schist in the West Chester nappe, is even younger – maximum temperatures were not reached until 410 Ma (early Devonian). We interpret this sequence to represent successive stacking of thrust sheets from southeast to northwest with the warmer overriding sheets contributing to the thermal budget of lower sheets. This deformation and metamorphism is interpreted to be the result of the Silurian collision of Ganderia, in a sinistral transpressive tectonic regime (Hibbard et al., 2007; 2010). The geometry of thrust sheets relative to the steeply dipping PGHV shear zone is consistent with the sinistral restraining bend at the New York promontory in the Hibbard model.

The most recent ductile deformation in the PGHV is thought to reflect dextral motion; such motion could have transported the assembled nappes from a more northerly location. The middle Devonian age of monazite in the WF suggests that Barrovian metamorphism there is the result of crustal thickening during the Acadian orogeny, the accretion of Avalon in the northern Appalachians. Given the evidence for late Devonian and younger dextral transcurrent motion regionally on the PGHV and RMZ, and throughout the Appalachians, it is likely that the crustal block east of the RMZ which contains the Wissahickon Fm. and Wilmington Complex was originally located some distance to the north.