RUI: Modes of Dome Formation in the Southern Appalachians

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The Tallulah Falls and Toxaway domes are map scale structural features that interrupt the regional trend of the Southern Appalachian Blue Ridge mountains. In map pattern these two domes bare striking resemblance to domal geologic structures in western North America that have long been understood as the result of extension and thinning of the continental crust. However, the Tallulah Falls and Toxaway domes, along with a number of other Southern Appalachian domes are considered to have been thought to have formed major crustal shortening and thickening during plate convergence. Indeed, nowhere in the Southern Appalachians has evidence for major localized crustal extension been recognized despite its presence in every major mountain range and type of plate tectonic boundary world wide, including convergent (Himalayas, Alps), transform (Anatolia, Venezuela), and divergent margins (mid-ocean ridges). Consequently, the absence of major extension within the Southern Appalachians, a plate margin that has experienced several phases of collision and subsequent rifting, is notable either because it displays a unique set of dynamic processes, or these structures have yet to be documented. This project is aimed at distinguishing between competing models of mountain building processes as recorded in the Tallulah Falls and Toxaway domes, and to determine their origin. This research will accomplish many of the outcomes NSF seeks to achieve in terms of benefits to society, including: full involvement of women in science, improved science technology, engineering, and mathematics (STEM) education, increased public scientific literacy and engagement with science, and preparing and engaging a diverse STEM workforce. This project will maximize student involvement in all aspects of the project from field-work and laboratory analyses, to data analysis and presentation of results. These students will be trained in cutting edge scientific techniques, and consequently will be better prepared for careers in earth science. While in the field, undergraduates and the principal investigators will communicate live with students enrolled in introductory courses at Appalachian State University, allowing a dialogue both about how and what kind of geologic research is currently being conducted. Results of the study will be disseminated through peer-reviewed publications and presentations at professional meetings. The mechanism and timing of mid-crustal doming and exhumation is unknown in the Southern Appalachians, and here crystalline complexes are considered exclusively as examples of contractional features. This interpretation bares striking similarity to that of Cordilleran metamorphic core complexes prior to the recognition of low-angle normal faults, now widely accepted as the locations of major crustal extension. Extensional exhumation of mid-crustal rocks would represent a significant departure from extant models of Appalachian mountain building, which adhere to a framework wherein all major structures exhibit thrust, or strike-slip displacement. The study will focus on two supposed contractional domes, the Tallulah Falls and Toxaway Domes, which straddle the Georgia, North Carolina, and South Carolina borders. The objectives of this project are to investigate the criteria necessary to determine timing and distinguish between contraction and extension related dome formation and exhumational models. In order to accomplish the principal investigators will determine: 1) the nature of the boundaries between the Tallulah Falls and Toxaway Domes and their cover; 2) how the timing of exhumation relates to doming and structural emplacement; and 3) how metamorphic grade variation is distributed within these domes. This project will incorporate field and laboratory structural observations, and geo- and thermochronometric measurements. Shear zone fabrics will be analyzed in the field and in thin section in order to determine the kinematics and metamorphic conditions of dome exhumation, and correlated with specific tectonic events using monazite Uranium-Thorium-lead (U-Th-Pb) dating. Timing of exhumation will be based on relative cooling ages across the boundary using Argon-Argon (40Ar/39Ar) dating in white mica and amphibole, and fission track dating in zircon.