Abstract

The Piedmont Province of SE Pennsylvania consists of gneisses overlain by, or in fault contact with, metasedimentary phyllites and schists. Correlation of these crystalline rocks with similar units in Maryland and isotopic ages of the metamorphic events suggest ages from pre-Grenville (1000 Ma) to Ordovician; Cambrian and Ordovician fossil-bearing units occupy a narrow band across the central part of the region. A 1000 Ma regional granulite facies episode is documented in the basement gneiss units. A second younger and lower pressure granulite facies metamorphism centres around an intrusive anorthosite-norite complex. A third metamorphism ranging from greenschist to upper amphibolite facies affects the entire terrain; this is probably close to 440 Ma old. Major folds and faults formed during or after the last metamorphic episode; earlier structural features are either obliterated or not exposed. The proposed tectonic model includes a rifting of continental basement forming a basin with a lower Palaeozoic carbonate bank on the NW margin and an island arc to the SE. Compression produced major nappes and possible thrusts during the last regional metamorphism. Vertical movements associated with major block faulting marked the end of the orogenic activity

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