DETAILED GEOLOGIC MAPPING OF THE BRUSHY MOUNTAINS, INNER PIEDMONT, NEAR LENOIR, NORTH CAROLINA

The Inner Piedmont (IP) is the Acadian metamorphic core of the southern Appalachians and consists of a series of distal slope-and-rise sediments shed from the Laurentian margin, and igneous rocks of various temporal and geochemical affinities. These rocks were then subjected to upper amphibolite facies metamorphic conditions at 360 to 350 Ma. Recent detailed geologic mapping in the South Mountains, near Marion, NC, reveals the Brindle Creek fault (BCF), a type-F thrust that juxtaposes migmatitic biotite gneiss/ metagraywacke and pelitic schist (Tallulah Falls Fm), megacrystic biotite gneiss (Walker Top gneiss) and peraluminous granite (Taluca Granite) over biotite-amphibole gneiss and quartzite (Poor Mountain Fm). Reconnaissance work suggests the tectono-stratigraphic relationships along the BCF are traceable throughout the IP of NC and SC, making it a major structural feature of the southern Appalachian crystalline core.

Detailed 1:24,000-scale mapping of some 340 km² (132 mi²) near Lenoir, NC, in the Brushy Mountains (NW, and along-strike with the South Mountains) recognizes a similar stratigraphy and confirms the presence of the BCF in northern NC. Mesoscopic and macroscopic fold patterns proximal, and on both sides of the BCF indicate complex, multiply folded relationships dominated by (F₂-D₂) NE-trending foliations. Further to the SE hanging-wall foliation patterns become more variably oriented, but dominantly strike NW and dip NE. Mineral lineations and fold axes mimic this change rotating to a SE plunge. This change in orientation may be the result of a buttressing effect caused by the BCF. Though relatively few mylonitic rocks crop out in the area, two bands of mylonitic megacrystic biotite gneiss (sheared Walker Top gneiss ?) are interpreted as minor shear zones within the hanging wall of the BCF.

U-Pb SHRIMP ages of zircons from the dominant footwall unit, a migmatitic biotite-amphibole gneiss/ amphibole quartz-diorite, and pressure- temperature estimates from detailed electron microprobe analysis of pelitic units will constrain the age of thrusting and metamorphic conditions in the study area. Comparison of these data with similar studies along-strike in the South Mountains will further constrain lateral variations in P-T conditions along the thrust trace.