Doctoral Dissertations

Date of Award

5-1993

Degree Type

Dissertation

Degree Name

Doctor of Philosophy

Major

Geology

Major Professor

Robert D. Hatcher, Jr.

Committee Members

William M. Dunne, Theodore C. Labotka, Carl J. Remenyik

Abstract

The geology of the western Inner Piedmont of North Carolina, from knowledge gained in an area called the Columbus Promontory, is characterized by a stack of crystalline thrust sheets. In this study the stratigraphic, structural, and metamorphic development of this crystalline thrust terrane was examined.

The lithostratigraphic framework of the Columbus Promontory is divisible into four distinct and mappable rock units that include the Henderson Gneiss, Sugarloaf gneiss, Poor Mountain Formation, and the Mill Spring Complex. This lithostratigraphic framework helps define three crystalline thrust sheets within the Columbus Promontory herein, called the Tumblebug Creek, Sugarloaf Mountain, and Mill Spring thrust sheets. Rocks of the Poor Mountain Formation and Mill Spring complex are similar to lithostratigraphic units recognized elsewhere in the southern Appalachian eastern Blue Ridge and Inner Piedmont. Similarities in physical stratigraphy indicate that the Poor Mountain Formation and Mill Spring complex rocks are representatives of two regionally extensive (Virginia to Alabama) lithostratigraphic sequences that record deep-to shallow-water deposition along the Laurentian margin and include: a lower sequence consisting of Lynchburg-Ashe-Tallulah Falls-Mill Spring-Sandy Springs/New Georgia-type rocks, and an upper sequence consisting of Evington-Alligator Back-Coweeta-Chauga River/Poor Mountain-Jackson's Gap/Ropes Creek-type rocks. By analogy to these rocks, the Poor Mountain Formation and Mill Spring complex are also interpreted as part of the deep-water facies rocks deposited along the Laurentian margin. Including the rocks of the Columbus Promontory into this regionally correlative lithostratigraphy further supports previous interpretations (Hatcher, 1978a, 1989) that the same lithostratigraphy occurs in the eastern Blue Ridge and Inner Piedmont. In addition, this correlation also supports the interpretation (Hatcher, 1978a, 1989) that the same rock units occur on both sides of the Brevard fault zone and suggests that this feature, although recognized as a major structural discontinuity, does not represent a terrane boundary.

Amphibolite comprises a significant component of the lithostratigraphy of the Columbus Promontory and is intercalated with other lithostratigraphic units of the Poor Mountain Formation and Mill Spring complex. Because relict igneous textures, sedimentary features, and contact relationships have been destroyed by high grade regional metamorphism and transposition, a whole-rock geochemical approach was undertaken to determine the protolith as well as fractional trends, and possible paleotectonic settings. Niggli trends, AFM relationships, and normative mineralogy suggest an igneous protolith for amphibolite in both stratigraphic units, which was tholeiitic basalt. Covariation diagrams indicate that both suites are fractionated and that the trends can be explained by fractionation of olivine, plagioclase, clinopyroxene, garnet, and magnetite. This assemblage is similar to the low-pressure fractionation sequence commonly observed in mid-ocean ridges and suggests the Columbus Promontory amphibolites are MORB. Zr/Nb, Y/Nb ratios further define the suite as N-type MORB, with a possible P-type MORB component. Other tectonomagmatic discriminant diagrams employed in this study indicate a correlation of the Columbus Promontory suite primarily with ocean-floor basalts, but also indicate some island-arc influence. The possibility of mixed N- and P-type MORB components suggests extrusion of these basalts along a mid-ocean ridge adjacent to a mantle plume, whereas the combination of MORB and island-arc characteristics indicate a back-arc basin setting. In either case, an oceanic setting is indicated for the Columbus Promontory suite. These observations further support the interpretation that the Poor Mountain Formation, Mill Spring complex, and correlative rock units in the eastern Blue Ridge and Inner Piedmont were deposited, at least partly, on oceanic crust.

The dominant structure of the western Inner Piedmont of North Carolina, South Carolina, and northeast Georgia is a stack of penetratively deformed ductile to semi-brittle crystalline thrust sheets. The structural development of this part of the Inner Piedmont is examined herein using data from crystalline thrust sheets in the Columbus Promontory in North Carolina, and the Tamassee area in adjacent South Carolina and NE Georgia. Structural analysis in both areas reveals a regionally consistent, five-phase deformation history. Although the western Inner Piedmont is polydeformed (D1 to D5), the D2 and D3 episodes were the most important and represent a deformation continuum. D2 was penetrative and synchronous with the principal (Acadian?) metamorphic even in the western Inner Piedmont. D3 generally represents late-to post-peak final emplacement of thrust sheets as coherent masses. The emplacement history and internal deformation of this crystalline thrust complex involved coeval D2-D3 orogen-parallel (SW-directed) displacement within the westernmost Inner Piedmont and Brevard fault zone and orogen-oblique (W-directed) displacement in thrust sheets in the Inner Piedmont. Importantly, this geometry indicates that early (middle Paleozoic) Brevard fault zone motion was kinematically linked to the crystalline thrust sheets in the adjacent Inner Piedmont. It is proposed that these dominant flow paths (W and SW directed) in the foreshortening crust were driven by large-scale transpression or oblique convergence during the amalgamation of the crystalline southern Appalachians. S2 mylonitic foliation is the most characteristic and kinematically important structural element within the Inner Piedmont. S2 also strongly controlled development of other D2-D3 structural elements in the western Inner Piedmont. Internal deformation and variations in orientation, kinematics, and geometry of D2-D3 structural features are interpreted to result from gradients of flow within S2. This resulted in a partitioned thrust-wrench transport parallel to the plane of S2 mylonitic foliation driven by larger-scale tectonic processes. These observations indicate that S2 is a regionally extensive shear surface along which extensive D2-D3 displacement occurred and suggests the Inner Piedmont represents a region of crustal-scale shear.

The metamorphic history, and the relationship between metamorphism and deformation in the thrust sheets of the Columbus Promontory are best recorded by pelitic schist within the Sugarloaf Mountain thrust sheet. The Sugarloaf Mountain sheet thrust rocks of the Poor Mountain Formation and upper Mill Spring complex over the Henderson Gneiss and other rocks of the western Inner Piedmont. Pelitic schist in the Sugarloaf Mountain thrust sheet contain a sillimanite-muscovite assemblage that is characteristic of thrust sheets throughout the western Inner Piedmont (e.g., Alto allochthon, Six Mile thrust sheet). Metamorphic textures and mineral zoning suggest sillimanite growth was the result of continuous reactions involving both garnet consumption and garnet growth following the metamorphic peak. These relationships also suggest that this sillimanite-muscovite assemblage is a post-peak rather than a prograde or peak metamorphic assemblage. Metamorphic textures and microstructural analysis indicate that growth of the sillimanite-muscovite was synkinematic with the development of microstructures related to emplacement of the Sugarloaf Mountain thrust sheet. The implication of these observations is that emplacement of the Sugarloaf Mountain thrust sheet occurred along the retrograde portion of the P-T path followed by these rocks. Qualitative constraints on the nature of this retrograde P-T path, gained from field criteria, petrographic observations, mineral zoning and geothermobarometric estimates, indicate a general path of decompression and cooling, but with episodes of near isobaric cooling.

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