Date of Award

12-1999

Degree Type

Thesis

Degree Name

Master of Science

Major

Geology

Major Professor

Robert D. Hatcher, Jr.

Committee Members

William M. Dunne, Harry Y. McSween

Abstract

This thesis examines the stratigraphic and structural relationships in the western Inner Piedmont of several orthogneisses and a paragneiss sequence. Detailed mapping was completed for an area slightly larger than one 7.5-minute quadrangle and is located southwest of Marion, North Carolina, at the northeast outcrop termination of the main Henderson Gneiss unit. From bottom to top, the stratigraphy of the study area consists of the Tallulah Falls Formation, Poor Mountain Amphibolite, and Poor Mountain Quartzite. Amphibolites and amphibole gneisses of the Tallulah Falls Formation, Poor Mountain Formation, and Dysartsville gneiss are interpreted as tholeiitic basalts related to rifting along the Laurentian margin. The entire stratigraphic sequence is interpreted to represent an early period of rifting in a marine basin that was filled with immature continent-derived material (Tallulah Falls Formation and Poor Mountain Quartzite) and rift-related volcanics and intrusions (Poor Mountain Amphibolite) prior to Paleozoic orogenic events. Based on the tonalite composition and high normative corundum content of the Dysartsville gneiss, it is interpreted to be part of the Late Cambrian to Early Ordovician western Inner Piedmont group of granitoids that intruded this sequence during the first Paleozoic orogenic event.

Metamorphism in the study area, as determined by mineral assemblage and comparison to previous work, is expressed in a single prograde event coeval with penetrative deformation during the Acadian orogeny. The Inner Piedmont is a stack of earlier Type-F (fold-related), Type-T (transitional), and later Type-C (composite) crystalline thrust sheets that comprise the polydeformed Acadian metamorphic core of the southern Appalachians. The transport direction of the thrust sheets is inferred from kinematic indicators and is parallel to a strong mineral stretching lineation. Immediately southeast of the Brevard fault zone, the mineral stretching lineations and transport direction are strongly oriented N E-SW. Structures in the western Inner Piedmont are grouped into one of five deformation events, D1 through D5. The development of most fabrics and structures observed in the study area are attributed to the penetrative D2 Acadian event. Folds in the Tallulah Falls Formation identified in the study area are the only macroscale evidence within the western Inner Piedmont of deformation prior to the Acadian event. The latest deformation events are attributed to the Alleghanian orogenic event and are expressed as open folds oriented orogen-parallel and orogen-oblique.

The Henderson Gneiss is a 509 Ma K-feldspar orthogneiss with an areal extent of ~5,000 mi2 southeast of the Brevard fault zone in North and South Carolina. Despite its areal extent, the Henderson Gneiss is interpreted to be less than 5 km thick, with the shape of a flattened prolate ellipsoid because it is structurally included within the footwall isoclinal synform of a Type-F thrust. Klippes and windows of Poor Mountain Amphibolite in the Henderson Gneiss outcrop belt are direct evidence of the thin nature of this unit. The regional map pattern of the Henderson Gneiss is complex, with a fold pattern that is difficult to explain in two-dimensions. The Henderson Gneiss is in contact with stratigraphically lower rocks at kyanite/sillimanite grade in North Carolina and with stratigraphically higher rocks at staurolite grade in South Carolina. The map pattern, stratigraphic relationships, and metamorphic relationships indicate that deformation directly southeast of the Brevard fault zone was southwest directed and accommodated by regional-scale sheath folding.

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