PRELIMINARY GEOLOGIC MAP OF THE MOUNT ROGERS AREA, VA–NC–TN: FIELD RELATIONSHIPS OF NEOPROTEROZOIC VOLCANISM AND GLACIATION IN THE BLUE RIDGE

New and legacy 1:24,000-scale geologic mapping are combined to create a detailed geologic map of eight 7.5-minute quadrangles that encompass the southern part of the Mount Rogers National Recreational Area in southwestern VA and parts of NC and TN. The map spans from folded and faulted Paleozoic strata of the Valley and Ridge to metamorphic and igneous rocks of the Blue Ridge. The map presents new interpretations of (1) the northeastern end of the Mountain City window; (2) field relations between the Neoproterozoic Mount Rogers and Konnarock formations; (3) separation of Proterozoic rocks of the Blue Ridge into three age groups; and (4) delineation of Paleozoic high-strain zones including the Fries and Gossan Lead faults.

In the Valley and Ridge, Cambrian to Middle Ordovician carbonate and clastic rocks are exposed in a syncline on the Pulaski thrust sheet; these rocks are overridden by the Blue Ridge thrust sheet. The northeast end of the Mountain City window is interpreted as a simple window; the Stone Mountain fault is folded and continues as the Iron Mountain fault on the NW-side of the window. The Stone Mountain fault does not exist to the NE near the Razor Ridge volcanic center; instead a continuous section of Proterozoic gneisses, Mount Rogers Formation, Konnarock Formation and Chilhowee Group is now recognized. Rhyolites of the Mount Rogers Formation range from 750–760 Ma, with detrital zircon age populations from associated volcaniclastic rocks indicating magmatism began by ~780 Ma. Rhyolite blocks in the Konnarock Formation and a change from rift-related clastic rocks of the Mount Rogers Formation transitioning to maroon laminites and laminites with dropstones, suggest that the Konnarock Formation may be as old as ~750 Ma. Proterozoic crystalline rocks of the Blue Ridge, previously referred to as the Cranberry Gneiss, are separated based on field relationships and SHRIMP U–Pb geochronology: (1) pre-Grenvillian crust,1.3–1.2 Ga; (2) 1190–1140 Ma granitoids; and (3) 1075–1030 Ma granitoids. Multiple greenschist-facies high-strain zones, including the 2–11 km wide Fries high-strain zone, crosscut the Blue Ridge thrust sheet. Fabrics across the Fries and Gossan Lead faults have similar orientations and top-to-NW contractional deformation associated with the Alleghanian orogeny.