HR: 0800h
AN: GP21A-0152    [Abstracts]
TI: Magnetic Fabrics in Shear Zones from the Southern Appalachian Mountain Belt
AU: Lebit, H
EM: hlebit@louisiana.edu
AF: University of New Orelans, Dept. of Geology and Geophysics, New Orleans, LA 70148 United States
AU: * Luenburg, C
EM: cluneburg@louisiana.edu
AF: University of New Orelans, Dept. of Geology and Geophysics, New Orleans, LA 70148 United States
AU: Hirt, A M
EM: hirt@mag.ig.erdw.ethz.ch
AF: ETH-Zurich, Institute of Geophysics ETH-Hoenggerberg Schafmattstr. 30, Zurich, CH-8093 Switzerland
AU: Bose, S
EM: shamikbose@louisiana.edu
AF: University of Oklahoma, School of Geology and Geophysics , Norman, OK 73019 United States
AB: Shear zones from the Piedmont Province in the Southern Appalachian Mountains have progressively modified a pre-existing fabric in biotite-hornblende gneisses. In the pre-deformed gneiss a sub-horizontal L-fabric of varying intensity is found. This fabric changes progressively to an S-fabric towards the center of the localized ductile shear zones. The anisotropy of magnetic susceptibility (AMS) was used to investigate how the mineral and magnetic fabric is modified by the ductile shear zones. The AMS was measured in low fields with an AGICO KLY-2 susceptibility bridge. High-field measurements were made with a torsion magnetometer to investigate the minerals responsible for the magnetic fabric. The magnetic fabric in the pre-deformed gneiss is characterized by a prolate ellipsoid with the maximum axes of the AMS ellipsoid lying sub-parallel to the mineral lineation, dominating the gneiss and which is to the NE-SW and sub-horizontal. The intermediate and minimum axes are distributed in a girdle with the intermediate axes lying predominantly in the SE quadrant and the minimum axes in the NW quadrant. Several sample profiles were taken across the shear zones. The magnetic fabric becomes progressively less prolate for samples closer to the shear zone and in some cases becomes oblate in the center of a shear zone, which is associated with a macroscopic foliation. The maximum axes of the AMS remain sub-horizontal but rotate progressively to a more N-S direction. High-field torsion magnetometry indicates that the mineral carriers of the magnetic fabric are not homogeneous. Some samples are dominated by the ferrimagnetic minerals, others are dominated by paramagnetic phases and still others have both, a ferromagnetic and paramagnetic contribution. The orientation of the maximum axes is in good agreement between the low- and high-field measurements. The intermediate and minimum axes are always in the plane normal to the maximum axes, and the low-field component coincides with the mineral component that dominates the high-field AMS.
DE: 8015 Local crustal structure
DE: 1500 GEOMAGNETISM AND PALEOMAGNETISM
DE: 1518 Magnetic fabrics and anisotropy
DE: 1519 Magnetic mineralogy and petrology
SC: Geomagnetism and Paleomagnetism [GP]
MN: 2004 AGU Fall Meeting