T74 The Archean of North America: The Core of a Continent

Monday, 10 October 2011
8:00 AM - 12:00 PM
Room 200H-J

EVALUATING THE EFFECTS OF SHEAR ZONE DEVELOPMENT ON SEISMIC ANISOTROPY IN THE DEEP CRATONIC CRUST: NATURAL EXAMPLES AND COMPUTATIONAL METHODS

Paper number: T74-3
Program: Technical Sessions
Time: 8:30 AM – 8:45 AM

Authors

Abstract

Understanding seismic anisotropy in cratonic crust requires an understanding of how the grain-scale interactions of the different minerals affect the bulk elastic properties, and how the bulk stiffness will change with changing fabric parameters. Owing to the abundance of highly anisotropic minerals in the crust, the Voigt and Reuss bounds on the seismic velocities can be separated by more than 1 km/s. These bounds are determined by modal mineralogy and crystallographic preferred orientations (CPO) of the constituent minerals, but where the true velocities lie between these bounds is determined by other fabric parameters such as the shapes, shape-preferred orientations, and spatial arrangements of grains. These additional parameters can contribute up to 25% of the anisotropy caused by modal mineralogy and CPO. Thus, the calculation of accurate bulk stiffness relies on explicitly treating the grain-scale heterogeneity, and the same principle applies at larger scales, for example calculating accurate bulk stiffness for a crustal volume with varying proportions and distributions of shear zones. We achieve these calculations by using asymptotic expansion homogenization (AEH) combined with finite element methods (Naus-Thijssen et al., 2011).

We explore the seismic anisotropy imparted by deep crustal shear zones to a crustal volume. Our results show that shear zones can either enhance, or modify, the seismic anisotropy present in the unsheared host rocks depending on the relative orientations of the two fabric and kinematic reference frames. Methods to be discussed include the following. (1) EBSD-derived data from sheared and surrounding unsheared rocks. (2) Synthetic microstructures generated by computational methods allowing sensitivity analyses around fabric parameters. (3) Numerical methods for sensitivity analysis whereby we populate crustal volumes with shear zones of varying thickness, abundance and geometry, and calculate the bulk stiffness of the crustal volume. (4) Synthetic wave propagation experiments that allow direct visualization of how shear zones might affect propagating seismic waves.

References

Naus-Thijssen, F. M. J., Goupee, A. J., Vel, S. S., Johnson, S. E., 2011. Geophysical Journal International, 185: 609–621. doi: 10.1111/j.1365-246X.2011.04978.x