16 Undergraduate Research (Posters)

Friday, 13 March 2009
8:30 AM - 12:30 PM

STRUCTURAL GEOLOGY OF PALEOZOIC SHEAR ZONES IN THE BLUE RIDGE THRUST COMPLEX, WESTERN NC AND EASTERN TN

Paper number: 16-48
Program: Theme Sessions
Time: 8:30 AM – 12:30 PM

Authors

Abstract

The Blue Ridge thrust complex north of Asheville, NC is a stack of thrust sheets consisting of Grenville basement rocks that were subjected to multiple Paleozoic orogenic events. Paleozoic overprint of Grenville fabrics, metamorphic assemblages and structures has resulted in complex lithologic and structural relationships in this area. Neoproterozoic Bakersville dikes intrude the basement rocks, and thus record Paleozoic metamorphic grade in each thrust sheet. The Fries thrust sheet is the structurally highest tectonic unit, and contains the Devonian Burnsville fault. Bakersville dikes in basement gneisses adjacent to the Burnsville fault locally yield granulite-facies assemblages. Near the Sams Gap-Pigeonroost (SG-PR) fault to the NW, the dikes contain amphibolite-facies assemblages. Trupe et al. (2004) proposed that the contact between the granulite-facies rocks and amphibolite-facies rocks is the Fries fault, with the structurally lower SG-PR fault forming a later branch off the Fries fault. Subsequent work suggested that the SG-PR and Fries faults are correlative and the contact separating granulite-facies and amphibolite-facies rocks may be a shear zone informally termed the Cox Creek fault. This study was undertaken to resolve these relationships.

Recent geologic mapping has revealed a new exposure of the Cox Creek fault in the Bald Creek, NC quadrangle. This outcrop links previously mapped exposures of this fault and supports the hypothesis that the Cox Creek fault separates granulite-facies rocks in the hanging wall from amphibolite-facies rocks in the footwall. The Cox Creek fault is a ductile shear zone that strikes NE and dips SE; SE-plunging mineral stretching lineations indicate dip slip motion. Oriented samples of Cox Creek mylonites contain kinematic indicators consistent with top-to-NW sense of shear. Recrystallization of K-feldspar and the presence of myrmekite on feldspar grain boundaries suggest amphibolite-facies conditions during shearing. Below the Cox Creek fault, mylonites in the SG-PR-Fries fault exhibit mineral assemblages and microstructures indicative of greenschist-facies conditions during NW-directed thrusting. Further mapping will determine the extent of the Cox Creek fault and its relationship to other structures in this area.