STRUCTURE OF THE NORTHEAST END OF THE MOUNTAIN CITY WINDOW, VIRGINIA–NORTH CAROLINA–TENNESSEE

Recent field studies in the Grayson, Konnarock, Troutdale, and Middle Fox Creek 7.5–minute quadrangles in VA, NC, and TN provide important constraints on the structural and stratigraphic relationships at the northeast end of the Mountain City window, and the overriding Blue Ridge thrust sheet (BRTS). The northeast end of the Mountain City window is framed on the southeast by the Catface fault, part of the Stone Mountain fault system. Framing the window on the northwest, the Iron Mountain fault is a discrete, brittle, bedding–parallel fault. Collectively, these faults form the base of the BRTS and become the Holston Mountain fault on the northwest flank of the Shady Valley thrust sheet (SVTS). Rocks in the northeast end of the Mountain City window are weakly to moderately cleaved and constitute an antiformal syncline that plunges 20°, N 65° E. Overturned Konnarock and Unicoi formations in the window require a ramp–flat geometry in the hanging wall of the SVTS. The Catface fault is interpreted as equivalent to the Stone Mountain fault proper as they both occupy identical structural positions at their type localities. The intensity of deformation within the Pond Mountain volcanic center is atypical of the Mountain City window and the segment of the Stone Mountain fault on the southeastern flank of the volcanic center is instead correlated with the multiple greenschist–facies, high–strain zones that crosscut the BRTS. Undulose quartz, fractured feldspars, and mylonitic foliations from the Catface fault indicate top–to–the–NW motion, and ductile deformation near ~300° C along the base of the BRTS on the southeast side of the window. The Stone Mountain fault was not recognized northeast of Troutdale, VA. Instead, a continuous NW–dipping section of Mesoproterozoic basement, Mount Rogers and Konnarock formations, and Chilhowee Group was recognized. The SVTS, which includes the Stony Creek syncline cored by Ordovician rocks, is connected to the BRTS. The southeast flank of the Mountain City window may have acted as a footwall ramp that caused deformation in the BRTS to change from ductile to the southeast to brittle deformation to the northwest.