Analysis of Deformation History Using Anisotropy of Magnetic Susceptibility in the Brevard Zone, North Carolina

Posted May 28th, 2010 at 10:22 am.

JoAnn Gage
Mentor: Dr. Arlo Weil

The Brevard Shear Zone is a prominent tectonic feature in the Appalachian Mountains . A shear zone is an area that has undergone intense deformation due to motion along a fault deep in the Earth’s crust. As the rocks in the zone accommodate stress they deform in a ductile fashion such that they do not break or lose cohesion. Consequently, the deformation history of the area is recorded by changes in the internal shape and orientation of the initial rock fabric.

Anisotropy of Magnetic Susceptibility (AMS) can be used in shear zones to determine the intensity and orientation of deformation, which is often difficult to assess. AMS is based on the fact that all minerals are susceptible to acquiring a magnetization, and because each mineral has a unique crystallographic shape, a mineral’s magnetic susceptibility varies with orientation. AMS measurements record the variability of susceptibility with orientation, which can be strongly affected by deformation. Every rock forms with an initial magnetic susceptibility fabric that is the result of the original rock-forming environment. Different geologic processes can subsequently alter a rock’s primary fabric, producing a finite strain geometry that can be analyzed and interpreted using AMS.

The goals of this project are to assess the deformation history of the Brevard Zone and to examine the viability of using AMS to quantify deformation in shear zones, a methodology that has not been extensively examined. To accomplish this, changes in intensity of deformation with increasing proximity to the Brevard Zone will be investigated by measuring the AMS of rocks at different points across and within the shear zone. These data will then be compared to field and microscope observations to construct a general dynamic model for how magnetic fabrics relate to shear zone deformation.

Oriented hand samples were collected in the field at sites along a transect through the shear zone in Rosman , North Carolina . Cores from each sample will be drilled and cut to the proper size in preparation for analysis using a Kappabridge susceptibility instrument. Thin sections of various samples will be made in order to analyze shear sense and deformation microstructures on the microscopic level. Additional experiments will be conducted to determine the minerals providing the dominant magnetic susceptibility, an important component of all AMS analyses.

Filed under: 2004,Gage, JoAnn,Weil, Dr. Arlo by Ann Dixon

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