Abstract 51338

THE HENDERSON AUGEN GNEISS OF THE WESTERN INNER PIEDMONT, NC & SC

Presented by Bream, Brendan R..
Authors:
      Hatcher, Robert D. Jr.,
      Hill, Joseph C..

Key words: Henderson gneiss, Inner Piedmont, Acadian

In Session 51     Central/Southern Appalachians Tuesday, 27-Oct-98 AM in Room: 701B at 9:30 AM for 15 minutes.

Abstract: The Henderson augen gneiss is a 509 Ma K-feldspar orthogneiss that has an areal extent of approximately 5,000 sq. km southeast of the Brevard fault zone in the North and South Carolina Inner Piedmont. Despite its areal extent, the Henderson gneiss could be <5 km thick, with the shape of a flattened prolate ellipsoid because it is the footwall isoclinal synform of a Type-F thrust, the Walhalla-Sugarloaf Mountain thrust. High-temperature mylonitic Henderson gneiss is common and reflects the intense noncoaxial deformation within the Acadian orogen. Younger, 438 Ma, foliated granitoid plutons intrude the main body at its greatest width in North Carolina. Based on contacts with a recognized stratigraphy and grain-size reduction at the contacts we have concluded that the Henderson gneiss was emplaced by NW-directed thrusting prior to penetrative deformation and SW-directed transport. It is possible that the present geometry of the Henderson gneiss is due to shearing of a large granitic diapir. Strain is dominantly flattening (S-L tectonite with k approximately equal to 0.75), but is also much higher (L tectonite with k approximately equal to 10.1) in some localities, especially in the hinges of folds. Previous work has recognized a strong NE-SW-oriented mineral stretching lineation throughout the body, truncation of cover rock contacts, and intensely folded contacts where the Henderson gneiss terminates. Recent detailed mapping of the northeast termination of the Henderson gneiss has led to the following observations: the Henderson gneiss truncates contacts in the enclosing rocks at both ends; the Henderson gneiss climbs stratigraphic section to the southwest from the lower Tallulah Falls Formation to the overlying Poor Mountain Formation; faults within the Walhalla nappe (Sugarloaf Mountain-Six Mile thrusts) terminate within a few kilometers along strike of the Henderson gneiss; augen in the Henderson gneiss may prove to be an important strain marker for deformation in the westernmost Inner Piedmont; and macroscale map relations, lineations, and other mesoscale features indicate SW-vergent sheath fold emplacement.