STRUCTURAL GEOLOGY AND METAMORPHIC PETROLOGY OF THE FRIES THRUST SHEET, BLUE RIDGE THRUST COMPLEX, WESTERN NC AND EASTERN TN

The Blue Ridge thrust complex of western 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. The goal of this project was to clarify structural relationships in part of the Blue Ridge thrust complex using geologic mapping and petrographic analysis of metamorphic rocks in the study area.

Neoproterozoic Bakersville dikes intrude basement rocks, predate Paleozoic orogenic events, and therefore are indicators of the grade of Paleozoic metamorphism in different thrust sheets. Bakersville dikes were sampled and analyzed from three thrust sheets in the study area in order to characterize their metamorphic mineral assemblages. Geochemical analyses of the Bakersville dikes show little variation in chemical composition, indicating that variations in mineral assemblages reflect Paleozoic metamorphic grade. Petrographic analysis of dikes from the Fries thrust sheet, the structurally highest tectonic unit in the thrust complex, reveals a difference in metamorphic assemblages within the thrust sheet. Trupe et al. (2004) proposed that the contact associated with the change in metamorphic grade is the Fries fault, with the structurally lower Sams Gap-Pigeonroost (SG-PR) fault forming a later branch off the Fries fault. Our recent work suggests that the SG-PR and Fries faults are correlative and that the contact separating the highest grade rocks from lower grade rocks is a shear zone informally termed the Cox Creek fault. 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. Geologic mapping, petrographic analysis of shear zone rocks, and mineral assemblages in Bakersville dikes support the hypothesis that the Cox Creek fault juxtaposes rocks with different Paleozoic metamorphic grade within the Fries thrust sheet.