STRUCTURES AND DEFORMATION INSIDE THE GRANDFATHER MOUNTAIN WINDOW: THE NORTHERN GOLDMINE BRANCH FAULT

Mesoproterozoic Laurentian basement gneisses and overlying Neoproterozoic rift sequences of the Grandfather Mountain Formation (GFMF) are exposed in the Grandfather Mountain Window (GFMW). Throughout the Southern Appalachian Orogen, the contact between similar lithotectonic units is recognized as a nonconformity between a younger cover sequence deposited as proximal rift basin sediments on an older Grenvillian basement massif. A nonconformable relationship is not observed, however, within the northern GFMW, where recent detailed geologic mapping in the Boone 7.5-minute quadrangle confirms that the 1.2 Ga Blowing Rock Gneiss (BRG) is structurally juxtaposed atop the GFMF along the Goldmine Branch Fault Zone (GBFZ).

Foliations striking ~N30°E and dipping gently to moderately (~24°–50°) SE within the BRG give way to variable N70°W–N30°E striking, NE to SE dipping protomylonitic-to-ultramylonitic fabrics within the fault zone. The GBFZ dips moderately (~35°) to the S–SE and trends variably – approximately parallel to the NE striking BRG foliations west of NC321 to nearly orthogonal to those foliations in the east at the headwaters of Goldmine Branch. Gently to moderately plunging (26°–53° N10–15E) asymmetric folds and tailed quartz porphyroclasts indicate a topped-to-the-NW sense of motion along the fault.

The geometry of the GBFZ varies, with footwall GFMF rocks appearing locally atop the BRG. Our fault projections require that either (1) folding, or (2) additional complex faulting, occurred subsequent to initial development of the GBFZ. Current data favor the former.