Abstract

Late Proterozoic (690-570 Ma) through Ordovician rocks of the Virginia—North Carolina Blue Ridge and Valley and Ridge record the evolution of an eastward-facing passive margin and its subsequent deformation during continent-arc collision (Taconic orogeny). Comparison of Blue Ridge stratigraphy with modern rifts and passive margins suggests that late Proterozoic rifting involved substantial crustal attenuation in the eastern Blue Ridge.

The axial zone of the Blue Ridge from central Virginia southward is occupied by a major fault system (Hayesville-Fries-Rock-fish Valley fault). Contrasts between rocks deposited east and west of this fault suggest that it is a reactivated hinge zone, separating highly attenuated continental crust to the east from continental crust of more normal thickness to the west. During rifting, normal faulting and rapid subsidence of attenuating crust in the eastern Blue Ridge produced rift basins in which deep-water clastic and volcanic rocks accumulated (Lynchburg Group, Ashe Formation). Landward of the hinge zone in the western Blue Ridge, mainly terrestrial to shallow-water clastic and volcanic rocks were deposited in a series of subaerial rift basins (Swift Run/Catoctin, Grandfather Mountain, Mount Rogers Formations). The hinge zone not only controlled depositional patterns throughout the rift and passive-margin history, but it was also reactivated during the Taconic collision, when attenuated crust and its cover were thrust westward over the shelf edge.

Turbidite-dominated rift sequences of the eastern Blue Ridge are unlike “typical” rift deposits, which are characterized by predominantly subaerial and shallow-water sedimentation. Deep-water rifts may be common on attenuated crust (as in the Bay of Biscay); however, they are poorly known from the stratigraphic record because of their susceptibility to destruction during orogeny.

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