Paper No. 4
Presentation Time: 1:00 PM-5:00 PM
POLYDEFORMED PROTEROZOIC BASEMENT IN THE BLUE RIDGE THRUST COMPLEX OF NORTHEAST TENNESSEE-NORTHWEST NORTH CAROLINA PART I: DECIPHERING 1 BILLION YEARS OF SOUTHERN APPALACHIAN HISTORY PRESERVED IN AN OUTCROP OF MIDDLE PROTEROZOIC ROCKS IN THE WESTERN BLUE RIDGE, NORTH CAROLINA
A ~240 m long, westfacing roadcut near milepost 5 on Interstate 26 in northwestern North Carolina exposes an assemblage of ~1.1 Ga migmatite, amphibolite, biotite granitoid gneiss, and biotite granitoids in a complex fold mullion structure cut by sugary quartz veins and intruded by two gabbro bodies. Crosscutting relationships permit the development of a detailed deformational history, although, absolute timing is based on correlation with published data. Three Proterozoic deformations are responsible for the majority of structures, fabrics, and high-grade metamorphism. Biotite granitoid gneiss and granitoids intruded migmatitic inequigranular gneiss and amphibolite at ~1.15 Ma. The rocks were then penetratively deformed at upper amphibolite facies during D1 and D2. Early D1 structures include S1 foliations preserved in amphibolite and biotite granitoid gneiss boudins and F1 intrafolial folds. These structures are transposed by D2 structures, interpreted as peak Grenville deformation. S2 foliation defined by alignment of biotite, hornblende, and metamorphic and stromatic migmatite layering trend N to WNW, and dip moderately to steeply SW. Reclined to recumbent, closed to isoclinal F2 folds plunge gently NS. Anvilshaped structures cored by biotite granitoid gneiss, subparallel to F2 folds and L2 lineations, may represent outcropscale sheath folds. Intrusion of the Bakersville gabbro at ~734 Ma produced contact metamorphism, M2, and local deformation, D3, of the country rocks. The contact aureole ranges from 1-20 cm wide and consists of quartz lenses and medium to coarsegrained hornblende granofels that contain hornblende + plagioclase + quartz + chlorite + actinolite + biotite + epidote. Geometry, rim synforms, plutonupshearsense, and S3 parallel to the pluton contact suggest emplacement of the Barkersville gabbro as a diapir. A penetrative foliation, S4, is recognized in one Bakersville gabbro body and is tentatively correlated with the Taconic orogeny, D4. Retrogressive Alleghanian structures, D5, minor mylonitic fabrics, slickensided brittle faults, and filled joints, are observed only locally in the outcrop. Filled joints and associated centimeter wide alteration zones commonly contain pink potassium feldspar, chlorite, epidote, quartz, and minor fluorite. Mesozoic or Cenozoic joint formation is the youngest deformation, D6, observed in the outcrop.