Extensional and contractional deformation in the Blue Ridge Province, Virginia

Abstract

Basement rocks of the Blue Ridge anticlinorium in central and northern Virginia were involved in two temporally distinct deformation episodes. The older event is characterized by a locally pervasive low-strain fabric and high-strain shear zones. A preferred grain shape orientation in recovered quartz, feldspar, and biotite, combined with a poorly developed symmetric quartz c -axis pattern, defines the low-strain fabric. Kinematic indicators are typically symmetric, but where asymmetric, they consistently indicate top down-to-the-southeast extensional motion. Discrete shear zones are within and close to Late Proterozoic plutons; kinematic evidence fron these zones records large noncoaxial strains also with a down-to-the-southeast extensional shear sense. Feldspar microstructures range from cataclastic fractures to complete recovery, suggesting that deformation occurred at greenschist to amphibolite grade. Both low-strain foliations and high-strain shear zones are cut by Paleozoic thrust-related tectonites. On the basis of kinematic evidence and spatial association with Late Proterozoic plutons, cross cutting relations with thrust-related structures, and regional geologic relations, we suggest that extensional deformation is related to Late Proterozoic rifting of the Laurentian margin. The kinematic histories of the Rockfish Valley fault zone and related shear zones appear to be relatively simple and consistent with Paleozoic west- to northwest-directed thrusting. The Rockfish Valley fault zone does not separate distinctly different basement terranes, and thus its displacement is thought to be small. Mylonitic rocks from this fault zone display crystal plastic deformation in quartz and cataclastic deformation in feldspars, under lower to middle greenschist-facies conditions. Solution transfer processes were an important deformation mechanism in these tectonites. Thrust- related mylonitic rocks are overprinted by related cataclasite and semibrittle shear bands, suggestive of a transition toward more brittle deformation as the thrust system cut higher in the crust. The Rockfish Valley fault zone appears to predate the formation of the South Mountain cleavage in the Valley and Ridge province, but it may be part of a lengthy, progressive, thrust-related deformation.