Date of Award
Master of Science
Robert D. Hatcher
Lawrence A. Taylor, Theodore C. Labotka
New field and laboratory data collected for the northeastern Inner Piedmont (IP) reveal the structure and composition of a portion of the southern Appalachian crystalline core, delimited the timing of orogenesis, and provide new insight into the nature of deep crustal processes during orogenesis. Previously recognized lithologies are continuous throughout the study area and comprise two distinct crystalline thrust sheets: the Marion and Brindle Creek thrust sheets, or eastern Tugaloo and western Cat Square terranes, respectively. Western IP Precambrian- Cambrian(?) metasedimentary lithologies and Ordovician metaigneous lithologies make up the Marion thrust sheet northwest of a Neoacadian suture, the Brindle Creek fault zone (BCFZ). To the southeast, the Brindle Creek thrust sheet contains Siluro-Devonian metasedimentary lithologies, which were intruded by Devonian anatectic metaigneous units.
Rocks in the study area preserve a polyphase thermal, metamorphic, and deformational history. Textural evidence suggests that two high-temperature events, followed by a retrograde event affected these lithologies. Peak metamorphic conditions of 745°C and 7.1 kbar are estimated for eastern IP assemblages using garnet core compositional data. Garnet rim compositional data indicate a clockwise P-T path for eastern IP lithologies. This is supported by textural observations of sillimanite replacing kyanite, further suggesting prograde conditions during metamorphism.
Northeastern IP lithologies retain evidence of six deformation events. The Mill Spring fault zone is recognized as a late dextral strike-slip fault that splayed from the Alleghanian Brevard fault zone. Map and outcrop-scale truncations of structures and lithologies of known ages provide relative and absolute timing constraints for IP orogenic pulses. Western IP D1 structures were produced by a pre-Neoacadian event, likely related to the 480-460 Ma Taconic orogeny. Eastern IP D1 structures are the product of initial subduction of Cat Square terrane lithologies beneath the Carolina terrane before BCFZ emplacement. D2 and D3 are responsible for regional structural trends, developed during the Neoacadian orogeny. SWvergent D4 dextral shear fabrics are related to dextral movement along the Alleghanian Brevard fault zone. Subsequent deformation features are brittle and result from Alleghanian uplift, and Mesozoic and Cenozoic extensional and rifting events.
The presence of two ductile fault zones in the study area, the Tumblebug Creek fault zone (TCFZ) and the BCFZ, provides a unique opportunity to study the development of two crustal-scale structures in an orogenic core. Here, the relationships between mineral reactions, deformation mechanisms, fluid flow, volume loss, and mylonitization are investigated for two granitic mylonites. Fluid flow paths are interpreted from SW-trending regional linear fabrics. Shear zone formation is characterized by the breakdown of quartz and feldspar, coeval with the formation of myrmekite and biotite. Crystal-plastic mechanisms operating at temperatures >400°C were responsible for the deformation during shear-zone development. Silica and alkalis were lost during deformation, while immobile major and trace element concentrations remained constant. Volume losses of 41 and 48% occurred for the TCFZ and BCFZ, respectively.
Zircon geochronologic data for northeastern IP lithologies provide new limits on the timing of orogenesis in the IP, and have produced a new developmental model for the Paleozoic development of the southern Appalachian crystalline core. Western IP Ordovician granitoids record the development of a volcanic arc system off of the Laurentian margin ~490 Ma. Taconic (480-460 Ma) arc accretion affected western IP lithologies, causing deformation, metamorphism, and magmatism. Eastern IP detrital zircon data indicate that the Cat Square terrane was being deposited after 430 Ma. Initial subduction of Cat Square sediments by the Carolina superterrane began by ~415 Ma. The Cat Square and Carolina terranes were accreted to the Laurentian margin together during the 360-340 Ma Neoacadian orogeny, causing the first shared thermal events between eastern and western IP terranes. Penetrative structures and dominant IP fabrics were developed during this event. Subsequent deformation occurred during dextral strike-slip motion along the Alleghanian Brevard fault zone.
Gatewood, Matthew Patrick, "Tectonics of the Northeastern Inner Piedmont, Northwestern NC, from Detailed Geologic Mapping, Geochronologic, Geochemical, and Petrologic Studies with Structural Analyses of Ductile Fault Zones. " Master's Thesis, University of Tennessee, 2007.