PRELIMINARY GEOLOGIC MAP OF THE MOUNT ROGERS AREA, VA–NC–TN

New and existing 1:24,000-scale geologic maps and geochronologic data have been compiled into a geologic map database 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 area covers folded and faulted Paleozoic strata of the Valley and Ridge and 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 (MR) and Konnarock (KF) formations; (3) separation of Mesoproterozoic rocks of the Blue Ridge into three age groups; and (4) delineation of Devonian-Carboniferous high-strain zones, some of which transect the Fries and Gossan Lead faults.

In the Valley and Ridge, Cambrian to Middle Ordovician carbonate and clastic rocks are exposed in a syncline in the Pulaski thrust sheet, and are overridden by the Blue Ridge thrust sheet. The northeast end of the Mountain City window is interpreted as a simple window through the Stone Mountain and Iron Mountain faults. A continuous section of Mesoproterozoic gneisses, MR, KF and Chilhowee Group is recognized locally and requires no faults as previously shown. Rhyolites of the MR range in age from 760–750 Ma; underlying volcaniclastic rocks contain detrital zircon age populations of rhyolites as old as ~780 Ma. Rhyolite domes and blocks within the glaciogenic KF and a transitional facies suggest that the KF was associated with volcanism ca. 750 Ma. The Early Cambrian Unicoi Formation disconformably overlies the KF. K-feldspar in Unicoi conglomerate yield complicated ⁴⁰Ar/³⁹Ar age spectra and suggest basalt flows ~545 Ma. K-feldspar amygdules in the basalt flow yield an ⁴⁰Ar/³⁹Ar age of ~300 Ma, related to the Alleghanian. Mesoproterozoic rocks of the Blue Ridge are subdivided based on field relationships and SHRIMP U-Pb geochronology: (1) pre-Grenvillian crust,1.3–1.2 Ga; (2) 1.19–1.14 Ga; and (3) 1.085–1.03 Ga. A 2 to 11 km wide zone of anastomosing greenschist facies high strain zones in the basement rocks transects the former Fries fault. Mylonitic foliations across the Stone Mountain, Fries, and Gossan Lead faults have similar orientations and record top-to-NW thrust deformation associated with the Alleghanian orogeny at ~340 Ma.