INSIGHTS ON STRUCTURAL CONTROL OF MESOPROTEROZOIC MAGMATIC MINERALIZATION, NEOPROTEROZOIC RIFTING AND HYPOTHETICAL TERRANE BOUNDARIES WITHIN THE VIRGINIA BLUE RIDGE OVERTHRUST PROVINCE NEAR ROANOKE, VIRGINIA

Unpublished geologic maps of the Check and Callaway 7.5-minute quadrangles were reviewed and revised for the 2015 and 2016 USGS/VDGMR I81 Corridor Map Project. New mapping combined with petrographic and geochemical study of of critical exposures helped to update and resolve structural and stratigraphic issues in the complex SW Virginia Blue Ridge imbricate thrust system. Mesoproterozoic Bottom Creek gneiss is thrust over deformed and mineralized Cambrian carbonate and clastic units along the Masons Knob thrust at the eastern edge of the Upper Roanoke Valley. Thrust slices containing metamorphosed mafic to ultramafic rocks within the Bottom Creek Suite form an imbricate stack NW of the Fries Thrust at the Vest Nickel Mine. Careful mapping and structural analysis of the Lick Fork Nickel Mine combined with petrographic and geochemical studies of samples from the mine area shows that coarse gneiss of the Bottom Creek Suite on the flanks of a small dome passes gradationally downward into a tabular body of mafic to ultramafic rock rich in iron, copper and nickel sulfide minerals that may be near the base of a cumulate sequence. Another late anticline that has arched the Copper Hill thrust at Abney Gap on the Callaway quadrangle exposes the Abney Gap pluton which contains similar mafic to ultramafic rocks in the core of a dome.

Neoproterozoic metaclastic and metavolcanic rocks in the southern portion of the study area appear to be separated by SE dipping extensional faults that formed a series of rift basins within Grenville Basement units. These early basin boundary faults were later telescoped to form a complex Mesoproterozoic basement massif during Paleozoic collision. The high strain zone bounded by the Red Valley and the Callaway faults which was proposed to be a major continental suture zone (Jefferson Terrane) does not seem to mark a major structural, stratigraphic or metamorphic break. Similar structure, lithology and metamorphic history preserved in the Eastern Blue Ridge thrust slices may better fit an inverted, late Proterozoic extensional rift basin model proposed by Wehr, 1983. Recent geologic mapping along the Callaway Fault to the SW of Callaway (Mershat, 2011 and Carter and Merchat, 2014) also failed to recognize any major structural variations across the proposed terrane boundary.