Structural Development, Strain History, and Timing of Deformation in the Eastern Great Smoky Mountains

Author

Jeffrey B. Connelly, University of Tennessee - Knoxville

Date of Award

5-1993

Degree Type

Dissertation

Degree Name

Doctor of Philosophy

Major

Geology

Major Professor

Robert D. Hatcher

Committee Members

William Dunne, Seven Driese, Eric Drumm

Abstract

The present investigation reveals that the Greenbrier and Dunn Creek thrust sheets preserve well-formed ramp-related folds within the Great Smoky Mountains area. The Greenbrier thrust sheet preserves a ramp anticline at klippes of the Greenbrier thrust sheet in the eastern Great Smoky Mountains that can be traced discontinuously to the western Great Smoky Mountains where this anticline has been modified by later displacement along the Rabbit Creek fault. A ramp-related fold is also preserved in the main Greenbrier thrust sheet. The main Greenbrier fault was subsequently folded by an underlying ramp anticline within the Dunn Creek thrust sheet. These earliest thrust systems have therefore been reconstructed based on foreland models of ramps and flats. The thrust faults form a folded imbricate fan structure with lower hanging-wall ramp anticlines folding higher thrust sheets. The foreland-style thrust system was internally deformed later in the Taconic during emplacement of a thrust sheet now floored by the Miller Cove fault. The Taconic package if imbricated Ocoee strata was emplaced onto the Valley and Ridge during the Alleghanian orogeny by the late Miller Cove and Great Smoky thrust systems. Faults in these late systems occupied various parts of the early ductile thrust zones, and almost certainly excised significant lower parts of the three early thrust sheets.

Internal strain within sandstones of the Miller Cove, Dunn Creek, and Greenbrier thrust sheets was also investigated. The three-dimensional finite strain geometry was determined for 69 samples using the R_f/ø and normalized Fry methods. Microstructural observations indicate that strains were accommodated by those deformation mechanisms typical of low grade metamorphic conditions including dislocation flow (undulatory extinction, deformation lamellae, deformation bands, patchy extinction, serrated grain boundaries), pressure solution (stylolites, sutured grain boundaries, overgrowths), and brittle fracturing (microfractures, fluid inclusion planes). Finite strains recorded within the sandstones are low and generally increase toward the hinterland (to the south). Mean X/Z strain ratios determined by the R_f/ø method for the Miller Cove, Dunn Creek, and Greenbrier thrust sheets are 1.29, 1.32, and 1.42, respectively. X/Z ratios determined using the Fry method are typically 5 to 20 percent higher. Principal strain axes within all thrust sheets exhibit subhorizontal strike-parallel X axes, subhorizontal transport-parallel Y axes, and steeply northwest plunging Z axes. Within hanging-wall ramp portions of the Dunn Creek thrust sheet, however, most X axes are parallel to transport and Y axes are parallel to strike. Two models were evaluated by strain factorization in an attempt to produce a sequence of strain events compatible with the finite strains observed in the two structural domains (hanging-wall flat and hanging-wall ramp). The first model involves compaction, layer-parallel shortening/extension, and simple shear. The second model is identical to the first with the exception of the addition of 90 degree rigid-body rotation following compaction to simulate sample from the hanging-wall ramp portions of the Dunn Creek thrust sheet. A sequence of strain events modeled by strain factorization, including 20 percent compaction, layer-parallel shortening of 5 percent, and thrust-parallel simple shear of 0.1, can produce the measured finite strains in the hanging-wall flat areas. The finite strains within the hanging-wall ramp portion of the Dunn Creek thrust sheet, however, require a different strain sequence, including 20 percent compaction by volume loss, 90 degree rigid body rotation following compaction, 20 to 30 percent horizontal extension, and a simple shear. The failure of a single model to account for observed finite strains in the two subdomains may be explained by: 1) Incorrectly assuming a single homogeneous strain across both subdomains; 2) The absence of compaction strains, although this would require another model to explain finite strains in the first subdomain; 3) Samples from the hanging-wall ramp area may yield unreliable results because of their fine-grained and matrix-rich compositions; or 4) The simplicity of the strain model, which assumes vertical bedding in the hanging-wall ramp where the average dip is 48 degrees and beds are overturned.

The importance of Ordovician tectonothermal activity in the western Blue Ridge of the southern Appalachians has been questioned by recent reports of Late Devonian-earliest Mississippian fossils within regionally metamorphosed rocks. In addition, metamorphism of fossiliferous Early Devonian rocks within the Talladega belt and suggested stratigraphic correlations with rocks of the Murphy belt suggest only post-Silurian metamorphism. The recent reports are contrary to most previous geochronology that suggests Ordovician metamorphism, as well as stratigraphic evidence indicating a Late Proterozoic age for most western Blue Ridge protoliths. To evaluate these contradictory results, eleven whole-rock samples (chlorite to garnet zones) and three muscovite concentrates (staruolite and kyanite zones) from the eastern Great Smoky Mountains of the western Blue Ridge were analyzed with ⁴⁰Ar/³⁹Ar techniques. Most chlorite-grade sample record plateau and intermediate temperature ages of 440 to 460 Ma. Illite crystallinity characteristics indicate that these samples attained metamorphic conditions sufficient for complete rejuvenation of whole-rock systems. Most biotite- and garnet-grade whole-rock samples yield plateau and intermediate temperature ages of 340 to 350 Ma. Muscovite samples record plateau ages of 360 to 380 Ma. It is unlikely that whole-rock samples collected several kilometers apart could have experienced contrasting cooling histories resulting in 100 Ma differences in apparent age. Therefore, the ⁴⁰Ar/³⁹Ar results most likely indicate a polymetamorphic history in which a 440 to 460 Ma (Middle to Late Ordovician) event was overprinted by a 360 to 380 Ma (Middle to Late Devonian) event. This interpretation is consistent with metamorphic textures observed in the western Blue Ridge.

Recommended Citation

Connelly, Jeffrey B., "Structural Development, Strain History, and Timing of Deformation in the Eastern Great Smoky Mountains. " PhD diss., University of Tennessee, 1993.
https://trace.tennessee.edu/utk_graddiss/1627