Inner Piedmont tectonics in the southwestern Brushy Mountains, North Carolina: field and laboratory data revealing 3-D crustal flow and sillimanite I and II metamorphism

Author

Arthur J. Merschat

Date of Award

12-2003

Degree Type

Thesis

Degree Name

Master of Science

Major

Geology

Major Professor

Robert D. Hatcher, Jr.

Committee Members

Lawrence A. Taylor, Theodore C. Labotka

Abstract

Field and laboratory data collected in a ~208 km² (~80 mi²) area at the southwestern termination of the Brushy Mountains in western North Carolina provide new insight into the crustal history of the Inner Piedmony (IP), part of the Neoacadian metamorphic core. Two large crystalline thrust sheets are present: the Mill Spring, and Brindle Creek thrust sheets. Migmatitic mafic and intermediate metaigneous rocks of the Mill Spring thrust sheet are exposed northwest of the Brindle Creek fault. The Brindle Creek fault is the boundary between the western and eastern Inner Piedmont, and the Tugaloo (to the west) and Cat Square (to the east) terranes. The Brindle Creek thrust sheet contains a lithostratigraphy that is different from rocks immediately to its northwest, consisting of two sillimanite schist units separated by a metagraywacke unit. These metasedimentary units were intruded by the Devonian Toluca (~378 Ma) and Walker Top (~366 Ma) Granites.

Rocks of the Brindle Creek thrust sheet are pervasively migmatitic but a 6 km wide zone of more intense migmatization occurs in the footwall. The occurrence of sillimanite and K-feldspar and the absence of muscovite in the hanging wall indicate these rocks locally attained second sillimanite grade metamorphic conditions. Second sillimanite grade rocks exist as lenses and pods within the assemblage of migmatitic, upper amphibolite facies rocks. P-T estimates of near-peak conditions range from 800 to 850° C and 5 to 7 kilobars followed by retrograde conditions of 500 to 560° C and 2.5 to 4.8 kilobars. These estimates define a retrograde P-T path similar to paths reported for western IP thrust sheets. Pervasive migmatite, ubiquitous prismatic sillimanite, lack of optical zoning of garnets, and similar mineral phases preserved in garnet cores in equilibrium with garnet rims suggest a single uniform high-temperature event affected the area. Timing of metamorphism is ~350 Ma delimited by U-Pb ages of zircon rims associated with migmatite formation and coeval emplacement of the Brindle Creek thrust sheet.

The region is polydeformed; six deformational episodes are recognized. D₁₉ D₂ and D₃ are ductile deformations possibly related to one protracted Neoacadian to early Alleghanian tectonothermal event of which D₂ is responsible for the majority of the penetrative fabrics and major structures. Recognition of several map-scale sheath folds and a curving mineral lineation pattern defined by sillimanite, hornblende, quartz ribbons and mica streaking indicate ductile flow at near peak metamorphic conditions. The curving lineation pattern and orientations of sheath folds indicate that NW- and W-directed flow rooted in the eastern IP was deflected SW by strong constrictional flow and buttressing by the primordial Brevard fault zone in the western IP. Three later episodes were superposed on the earlier fabrics and structures at considerably lower temperatures and pressures.

The tectonothermal history of the IP involved subduction, beginning at 400-390 Ma, of the Cat Square terrane beneath the Carolina terrane, to 15 km and 800° C by ~378 Ma, and melting of pelitic and psammitic sediments to form the Toluca Granite (D₁). Additional anatectic melting occurred at ~366 Ma to form the Walker Top Granite at the end of D₁₉ probably related to initial subduction of both the Cat Square and Tugaloo terranes (D₂). Ductile flow at peak first and second sillimanite conditions, transported plastic thrust sheets westward and southwestward and emplaced the Brindle Creek thrust sheet at ~350 Ma, D₂. D₃ represented continued high-temperature deformation follow by uplift. Burial again during the final pulses of emplacement of the Caroline terrane to >10 km produced the onset of Alleghanian deformation at ~330 Ma.

Recommended Citation

Merschat, Arthur J., "Inner Piedmont tectonics in the southwestern Brushy Mountains, North Carolina: field and laboratory data revealing 3-D crustal flow and sillimanite I and II metamorphism. " Master's Thesis, University of Tennessee, 2003.
https://trace.tennessee.edu/utk_gradthes/6358