THE MECHUM RIVER FORMATION, VIRGINIA BLUE RIDGE: A RECORD OF NEOPROTEROZOIC AND PALEOZOIC TECTONICS IN SOUTHEASTERN LAURENTIA

摘要

Neoproterozoic metasedimentary rocks of the Medium River Formation crop out in an elongate northeast-southwest trending belt in the central Virginia Blue Ridge province. The northwestern contact of the Medium River Formation is nonconformable above Mesoproterozoic basement, whereas the southeastern contact is a set of steeply dipping, brittle reverse faults that juxtapose basement against Medium River rocks. The internal structure of the belt consists of northwest-verging, open to tight, moderately to steeply plunging asymmetric folds. Although previous workers have interpreted the outcrop belt of the Medium River Formation as a graben formed during Laurentian rifting, structural relations require that its present geometry is a structural inlier related to Paleozoic contractional deformation. Strain analysis reveals that grain-scale deformation processes produced up to 35 percent shortening during foliation development whereas map-scale folds account for an additional 30 to 50 percent shortening. Restoration of the Medium River Formation to its pre-contractional geometry reveals little about the geometry of the original depositional basin. The bounding reverse faults on the east side of the Medium River Formation are interpreted as out-of-sequence structures that developed after regional folding, metamorphism and foliation development that may be related to the emplacement of the Blue Ridge thrust sheet over a tectonic ramp. Sediment transport indicators are consistent with a source area to the east, suggesting that the Medium River Formation was separated from similar units in the eastern Blue Ridge by an asymmetric basement high that may have been produced by block rotation above listric normal faults.