Tectonic evolution of the northern Blue Ridge massif, Virginia and Maryland
Published:January 01, 2004
William C. Burton, Scott Southworth, 2004. "Tectonic evolution of the northern Blue Ridge massif, Virginia and Maryland", Proterozoic Tectonic Evolution of the Grenville Orogen in North America, Richard P. Tollo, James McLelland, Louise Corriveau, Mervin J. Bartholomew
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Detailed mapping in the Mesoproterozoic northern Blue Ridge massif has delineated ten high-grade metamorphic map units of mostly granitic composition occurring in association with charnockite and paragneiss. U-Pb isotopic dating of zircons from these rocks defines three episodes of protolith intrusion during the interval from 1150 to 1055 Ma. Crosscutting relationships, structural analysis of foliations and lineations, and comparison of deformational fabrics of gneisses with different protolith ages indicate three episodes of Grenvillian deformation under differing stress regimes: D1, post–1140 Ma and pre–1120 Ma, involving regional coaxial compression; D2, post–1055 Ma and pre–1035 (?) Ma, noncoaxial ductile shear; and D3, post–1035 (?) Ma and pre–1030 (?) Ma, late-stage compression. The Short Hill fault, a Paleozoic structure of possibly Grenvilleage origin, separates predominantly older gneisses with D1 foliation to the west from predominantly younger gneisses with D2 and D3 structures to the east. Geologic features in the central Blue Ridge massif that are similar to those in the northern Blue Ridge include a central fault zone, the Rockfish Valley fault zone, separating a more charnockitic suite of rocks to the west from a more leucogranitic suite to the east. We propose a model in which the Short Hill fault and the Rockfish Valley fault zone were formerly part of a single discontinuity that was offset by a younger Neoproterozoic normal fault during early rifting of the Laurentian margin. This rift-related normal fault became the locus of intrusion of the 735–702 Ma Robertson River batholith.