A UNIFYING TECTONIC MODEL FOR THE BASEMENT CORE OF THE CENTRAL AND NORTHERN BLUE RIDGE OF VIRGINIA AND MARYLAND

The central and northern portions of the Grenville-age Blue Ridge massif share some common features, including a NE-trending medial Paleozoic fault zone (Rockfish Valley fault zone in the central Blue Ridge and Short Hill fault in the northern Blue Ridge) that separates a comparatively charnockite-rich zone to the west (Pedlar terrane of the central Blue Ridge) from a charnockite-poor (or absent) biotite granitoid-rich zone to the east (Lovingston terrane of the central Blue Ridge) and that may have pre-Paleozoic tectonic origins. These fault zones form a right-stepping en echelon pattern, whereby younger rocks of the Lovingston massif east of the Rockfish Valley fault zone extend northward to the NNE-trending linear belt of the Neoproterozoic Robertson River batholith, northeast of which they are replaced by older Pedlar-equivalent charnockite-bearing rocks lying west of the Short Hill fault.

We propose a model that explains the similarities in lithologic and tectonic framework between the central and northern Blue Ridge basement rocks, their spatially- offset relationship, and the extremely elongate nature of the Robertson River batholith. In this model the Rockfish Valley and Short Hill fault zones were originally part of a single, steeply dipping Grenville-age fault that extended the length of the central and northern Blue Ridge massif and separated the Pedlar and Lovingston terranes. The charnockite-rich central Blue Ridge rocks were deeper-level equivalents of the charnockite-poor northern Blue Ridge rocks. This Grenville-age assemblage was cut in the Neoproterozoic by a normal fault that was east-dipping and had a more northerly strike than the Pedlar-Lovingston boundary. The movement along this fault displaced the northern Blue Ridge rocks downward relative to the central Blue Ridge rocks, and produced the present-day en echelon arrangement of the terranes and the Rockfish Valley and Short Hill fault zones. During and following fault motion this normal fault was the locus of intrusion for the Robertson River batholith, which displays fabrics suggestive of syn-extensional emplacement. The normal faulting and intrusion of the batholith were followed by erosion and deposition of a cover sequence of late Neoproterozoic to Cambrian volcanic and sedimentary rocks, and Paleozoic folding, faulting, and metamorphism.