GEOLOGIC MAP AND GEOCHRONOLOGY OF THE GREAT SMOKY MOUNTAINS NATIONAL PARK REGION, TN/NC

From 1993 to 2003, the U.S. Geological Survey re-examined the geology of the Great Smoky Mountain National Park region as part of a cooperative agreement with the National Park Service. The investigation focused on geologic mapping of previously unstudied areas and revising geologic interpretations in problem areas. A 1:100,000-scale surficial and bedrock geologic map (USGS SIM-2997) was produced from 1:24,000- and 1:62,500-scale mapping.

New geochronologic studies reveal that the rocks of the region were affected by multiple tectonic events from the Mesoproterozoic through the Cenozoic. Geochronologic methods utilized are: SHRIMP U-Pb ages of zircon, sphene, monazite, and xenotime, ⁴⁰Ar/³⁹Ar ages of hornblende and muscovite, fission-track (FT) ages of zircon and apatite, and ¹⁰Be analyses of quartz. The following tectonic history is documented:

1) Mesoproterozoic events include migmatization at 1202±10 to 1194±10 Ma, plutonism and deformation at 1165±8 to 1160±10 Ma, and crystallization of undeformed, post-orogenic granites at 1039±7 and 1022±5 Ma. Metamorphic overgrowths on igneous zircons formed between about 1.07 and 1.02 Ga.

2) Deposition of protoliths of paragneisses post-date the Grenvillian orogeny (youngest detrital zircon ages of 1000-950 Ma).

3) Deposition of the Great Smoky Group at about 590-565 Ma (ages of diagenetic xenotime and monazite).

4) Metamorphic events include lower greenschist-facies metamorphism from 690 to 630 Ma (monazite overgrowths prior to sedimentation within the Great Smoky Group), amphibolite-facies metamorphism at 449 to 423 Ma (⁴⁰Ar/³⁹Ar ages of hornblende and U-Pb age of sphene) and a second greenschist-facies metamorphism accompanied by deformation at about 350 to 340 Ma (⁴⁰Ar/³⁹Ar age of muscovite), and emplacement of the Great Smoky fault at 280 Ma (zircon FT age).

5) Truncation of 7 units by the polygenetic Greenbrier fault demonstrates that it is not a faulted stratigraphic contact. The fault must predate Taconian (Ordovician) isograds that transect it and was contractionally reactivated in the Devonian and Mississippian.

6) Post-Triassic unroofing, uplift, and erosion (rates between 28 and 18 m/m.y.), resulted in the formation of a Mesozoic and Cenozoic paleothermal high between two Paleozoic faults centered on Clingmans dome.