13 Isotopic and Chemical Geochronology of Metamorphic Terranes in the Southern Appalachian Blue Ridge and Piedmont Environs: Implications for Tectonic Mapping and Modeling

Thursday, 10 April 2008
1:30 PM - 5:35 PM

VORTICITY, STRAIN SYMMETRY, AND MONAZITE GEOCHRONOLOGY OF THE BOWENS CREEK FAULT ZONE, NORTHEASTERN INNER PIEDMONT, SOUTHERN APPALACHIANS, VIRGINIA

Paper number: 13-7
Program: THEME SESSIONS
Time: 3:15 PM – 3:35 PM

Authors

Abstract

The Smith River Allochthon (SRA) represents the northeastern extension of the southern Appalachian Inner Piedmont (IP) and as such, it contains remnants of a refolded IP-derived nappe that was thrust northwest across the eastern Blue Ridge during the Taconic orogeny. Our current vorticity and strain studies have targeted this less well-understood region in an attempt to establish boundary conditions for southern Appalachian ductile flow models. Our primary focuses are the Brevard-Bowens Creek and Ridgeway fault zones, which define the limits of the SRA. SRA tectonites are polymetamorphosed upper amphibolite metasedimentary, metavolvanic, and igneous rocks. The Bowens Creek fault, which represents the northwestern boundary of the SRA, separates the aforementioned rocks from prograde upper greenschist facies Blue Ridge cover rocks in the footwall to the northwest. Post Taconic (Neoacadian and/or Alleghanian) transpression along the Brevard –Bowens Creek zone has deformed the Taconic fabrics and folded isograds in the SRA. Displacement vectors calculated from folds measured along the Brevard-Bowens Creek fault from the North Carolina border through the study area to the Roanoke River transect show a remarkable transition from a strong right lateral transpression to a predominantly dip-slip motion to the northeast. Right lateral transpression on the Brevard-Bowens Creek zone may lend credence to IP ductile flow models that have been proposed to the southwest.

Samples were collected along the Bowens Creek fault zone for kinematic and geochronologic (monazite) analysis. Hanging wall samples of Fork Mountain Formation ~10 km north of the NC-VA state line reveal preliminary kinematic vorticity numbers of 0.64-0.75, which indicate a strong component of pure shear (~50%). Sections were cut parallel to lineation and perpendicular to foliation, resulting in an orientation roughly parallel to orogenic strike. Preliminary monazite analyses indicate thermal events at ~460 ± 10 Ma, ~420 ± 10 Ma, and ~350 ± 10 Ma. Although the precise relationships between the SRA section of the Bowens Creek zone and the Brevard zone to the southwest have not been entirely resolved, there appears to be a strong temporal and kinematic link.