BASIN INVERSION OF NEOPROTEROZOIC VOLCANO-SEDIMENTARY STRATIGRAPHY IN THE MOUNT ROGERS REGION, SW VA

Basin inversion occurs where compression and shortening are superimposed onto formerly extensional basins. Later shortening of syn-rift basins can produce complex structural patterns that include reactivated extensional faults, new compressional structures, and fossil normal faults that were not reactivated. As a result, ordered stratigraphic sequences can become jumbled and age relationships of strata blurred.

An example of such basin inversion occurs in the Blue Ridge of SW Virginia, where Neoproterozoic rift deposits (~760-750 Ma) were overprinted by Paleozoic contraction (~340 Ma). Bimodal volcanic and clastic sedimentary rocks of the Mount Rogers Formation (MRF) formed during early stages of intracontinental rifting of Rodinia. The stratigraphy of the MRF has been described as an “upper” section of rhyolite lavas and pyroclastics, and a “lower” section of bimodal volcanics and sedimentary rocks. Based on field relationships, rocks of the lower MRF have been inferred to be older. However, recent U-Pb zircon ages reported for rhyolites in the upper and lower MRF show that their ages largely overlap, thus the sequence of eruption is unclear. In addition, rhyolite clasts from conglomerates in the lower MRF yield ages ranging from ~780 to ~753 Ma. The youngest ages demonstrate that the conglomerate must be as young or younger than the upper MRF rhyolites.

This apparently jumbled stratigraphy may be explained by a sequence of events in which: 1) pre- to early syn-rift volcanic activity ~780-770 Ma was followed by extrusion of MRF volcanics, ~760-750 Ma; 2) extensional faulting resulted in development of a horst-graben topography, accompanied by erosion of earlier volcanic strata while the down-dropped graben were filled with sediments (shortly after 750 Ma?); and 3) inversion of the rifted terrain by convergent tectonics during early stages of the Alleghanian orogeny, in which northwest-vergent thrust faults and folds uplifted and telescoped the sequence.