HR: 14:15h
AN: T33E-03    [Abstracts]
TI: The seismic expression of deformation in the Australian lithosphere
AU: * Simons, F J
EM: fjsimons@alum.mit.edu
AF: Princeton University, Guyot Hall 321B, Princeton, NJ 08540 United States
AU: Bunge, H
EM: bunge@lmu.de
AF: Ludwig Maximilians University, Theresienstrasse 41, Munich, D-80333 Germany
AU: Becker, T W
EM: twb@ucsd.edu
AF: University of Southern California, 3651 Trousdale Pkwy, Los Angeles, CA 90089 United States
AB: The 3D azimuthal seismic anisotropy of the Australian lithosphere, as seen by multimode surface waves, shows a coherent alignment at depths below $\sim$150 km. Anisotropy above $\sim$150 km is related to the fossil strain field preserved in the relation of gravity anomalies to topography. The region below it, then, might reflect the active deformation of the upper mantle. We discuss four classes of models with increasing complexity. A first model of mantle strain is given by the direction of absolute plate motion. The alignment of the fast axes is better in the hot-spot reference frame than in the no-net-rotation frame. Our second class of models predict local instantaneous velocities from plate motions and/or from driving density anomalies inferred from 3-D tomographic models with radial viscosity structures. Third, we show modeling that uses the flow field to predict finite strain. The latter calculations are carried out in the no-net-rotation frame. We show results of strain calculations by backward advection over 10 Ma (constant time), and for logarithmic strain ratios of 0.5 (constant strain), a textural saturation level. None of the simulations in the no-net-rotation frame improve the fast axes alignment with absolute plate motion, whereas the velocities from the model with assimilated hot-spot referenced plate motions perform only slightly worse than the zeroth-order hypothesis. Such strain predictions, however, are subject to much uncertainty on the constitutive equations. In our last set of models, we focus more on the flow dynamics, by exploring the time-dependence of the alignment between the velocity field and the instantaneous strain field as yielded by three-dimensionsl convection calculations incorporating tomographic density anomalies and the history of plate motion. We are thus able to see (since) when and where upper mantle seismic anisotropy in Australia is likely to express mantle motion, by investigating the degree to which both predict each other. Varying the amount of core heating does not go unnoticed at the top of the mantle. The lithospheric seismic anistropy of our models is able to provide ``feasability'' constraints on such basic geodynamic parameters as the ratio of bottom to internal heating.
UR: http://www.frederik.net
DE: 8120 Dynamics of lithosphere and mantle--general
DE: 7218 Lithosphere and upper mantle
SC: Tectonophysics [T]
MN: 2004 AGU Fall Meeting