Date of Award
Master of Science
Robert D. Hatcher
Bill Dunne, Hap McSween
The geology of the Inner Piedmont in a part of southwestern North Carolina, called the Columbus Promontory, is characterized by a complex history involving high grade metamorphism and penetrative ductile deformation. This study involved detailed (1:24,000) geologic mapping, petrography, whole rock geochemical analysis, and structural analysis to study the stratigraphy, structural geology, and metamorphism of a part of the Columbus Promontory.
Rocks in the study area can be divided into three major lithotectonic units, each separated by a ductile thrust fault, and strike NE and dip gently to the SE. Unit 1, theStructurally lowermost and northwesternmost unit in the study area is composed entirely of Henderson Gneiss, a 509 Ma granitic augen gneiss. It also contains a 438 Ma intrusion of granitic gneiss. Unit 2 contains rocks of the upper Mill Spring complex with biotite gneiss and pelitic schist and the Poor Mountain Formation, dominated by amphibolite and quartzite. Unit 3, the structurally highest and southeasternmost unit, contains a lower unit of biotite gneiss associated with the upper Mill Spring complex and an upper unit of undifferentiated biotite gneiss, amphibolite, granitic gneiss, and pelitic schist. Based on the ordering of lithologies, the Poor Mountain and Mill Spring complex are interpreted to be correlative with rocks in other parts of the Inner Piedmont and eastern Blue Ridge of Virginia, North Carolina, South Carolina, Georgia, and Alabama.
The structure of the Columbus Promontory is dominated by a series of SE-dipping, penetratively deformed, ductile, thrust sheets. Five episodes of deformation (D1 to D5) have been recognized but the two most significant were D2 and D3. Of these two episodes, D2 accounts for virtually all the shear strain observed in the Inner Piedmont and is synchronous with peak M2 metamorphism. D2 deformation is associated with a regionally penetrative mylonitic C-fabric (S2), mineral lineation, folds, and S-C fabrics. The penetrative nature of these structures, the parallel orientations of the foliation, thrust faults, fold girdles, and great circle defined by mineral lineations, the parallel orientation of fold axes and mineral lineations, and the presence of sheath folds indicate that D2 involved very high shear strains ([see original document] >10) and that the Inner Piedmont represented a crustal-scale ductile shear zone. D3 occurred during the cooling stages of M2 metamorphism and forms a continuum with D2 deformation. Shear sense during D2, as indicated by mineral lineations, winged porphyroclasts, S-C fabrics, and snowball garnets, varied gradually from W-directed in areas to the SE to SW-directed in areas to the NW. Hansen analysis of F2 and F3 folds suggest that shear sense did not change significantly between D2 and D3 deformation. This change in the orientation of shearing from W to SW-directed transport is probably related to a dextral component of shearing along the primordial Brevard fault zone.
M2 metamorphism reached the lower sillimanite zone in pelitic schist and upper amphibolite facies in amphibolite. Textural relationships and optical zoning in garnet indicate that sillimanite grew by a series of continuous reactions involving garnet growth and garnet consumption. The observations are consistent with relationships observed in other parts of the Columbus Promontory suggesting that sillimanite growth occurred along the retrograde portion of the P-T path.
Whole rock geochemical analysis was conducted on Poor Mountain Amphibolite using X-ray fluorescence. Covariation diagrams and Niggli trends indicate that the amphibolite was metamorphosed from a tholeiitic basalt. Tectonic discrimination diagrams and spider diagrams suggest they evolved from a mid-ocean ridge basalt.
Yanagihara, Gregory M., "Structure, Stratigraphy, and Metamorphism of a Part of the Columbus Promontory, Western Inner Piedmont, North Carolina. " Master's Thesis, University of Tennessee, 1994.