Investigations of the Brevard Fault zone and Tugaloo Terrane adjacent to the Grandfather Mountain Window, North Carolina

Abstract

The Brevard Fault Zone adjacent to the Grandfather Mountain window, NC is interpreted to be a terrane boundary between Laurentian margin rocks and accreted rocks of the Tugaloo Terrane. Structural, geochemical, and lithological differences across the fault zone support this interpretation. The northeast trending Brevard Fault zone consists of mylonitic to cataclastic foliation and a backstop of a northwest-directed thrust stack that includes the Alligator Back metamorphic suite, and thrusts propagating into the Grandfather Mountain Formation and Grenville basement. Understanding the geometry, deformation, and strain history of the Brevard Fault zone and the lithotectonic/geochemical history of the Tugaloo terrane in this location can help answer questions regarding the evolution of the southern Appalachians, the relation between the Tugaloo Terrane and the Brevard Fault zone, how faults behavior varies with depth, and how structures are reused through time. A better understanding of the nature of these features in the southern Appalachians will provide context for interpretation of similar features in other orogenic systems. Rocks to the NW of the fault zone, in the Grandfather Mountain window, exhibit both ductile and brittle deformation with annealed quartz matrix and fractured potassium feldspar and large quartz porphyroclasts that sometimes have pressure shadows and are often visible at the mesoscale. Electron backscatter diffraction (EBSD) quartz crystallographic preferred orientation (CPO) pole figures support a high temperature, ~550-600 [degrees]C, deformation with a lower temperature overprint at~350 [degrees]C. The dominant mineral assemblage suggests deformation at greenschist facies. Any high temperature ductile deformation likely predates the amalgamation of Pangea and the transition from ductile to brittle deformation likely occurred during Alleghenian thrusting. In contrast, the Tugaloo Terrane rocks SE of the fault zone exhibit predominantly ductile deformation structures. Although quartz CPO pole figures show a similar high and low-temperature history as the data to the NW, brittle structures in thin-sections are lacking. Dominant microstructures include feldspar porphyroclasts with tails or strain shadows, and the quartz grains exhibit dynamic recrystallization with bulging grain boundaries and sub-grain boundary rotation. Fractured feldspar porphyroclasts are rare and primarily occur within ~500 m of the Brevard Fault zone. The metamorphic mineral assemblages support lower amphibolite facies with a greenschist facies overprint. The incipient Brevard Fault zone was first activated as a terrane boundary during the Taconic; it became the sole of an orogenic channel during the Acadian to Neoacadian orogeny, and then reactivated as a buttress/ramp for Alleghenian thrusting.