HR: 11:20h
AN: T32A-05 INVITED     [Abstracts]
TI: Seismic Anisotropy and Flow in the Oceanic and Continental Upper Mantle: Inferences from SKS Splitting Observations
AU: * Behn, M D
EM: mbehn@whoi.edu
AF: Dept. of Geology & Geophysics, Woods Hole Oceanographic Institution, 360 Woods Hole Road -- MS #22, Woods Hole, MA 02543 United States
AU: Conrad, C P
EM: cpconrad@umich.edu
AF: Dept. of Geological Sciences, University of Michigan, Ann Arbor, MI 48109 United States
AU: Silver, P G
EM: silver@dtm.ciw.edu
AF: Dept. of Terrestrial Magnetism, Carnegie Institution of Washington, Washington, DC 20015 United States
AB: Seismic anisotropy provides a direct estimate of flow in the Earth's upper mantle. In this study, we examine measurements of anisotropy inferred from shear-wave splitting at continental and oceanic island stations. In the ocean basins, the splitting fast polarization directions are extremely well fit by a global flow model that is driven by a combination of plate-motion and mantle density heterogeneity. Global mantle flow is determined from instantaneous flow calculations that assume a radially variable, but laterally homogeneous viscosity structure. In general, the misfit between the observed fast polarization direction and the predicted direction of maximum shear in the asthenosphere is very small: on the order of the error in the splitting measurements for both fast- and slow-spreading ocean basins. In particular, a flow model that incorporates active upwelling from the lower mantle associated with the African superplume is required to explain the splitting data in the southern Atlantic and Indian Oceans. However, while this global flow field can account for the anisotropy in the ocean basins, it is a very poor fit in many continental regions. For example, although observed anisotropy in western North America is well fit by the global flow model, the splitting observations in eastern North America are nearly orthogonal to the model predictions. We hypothesize that this poor fit is caused by either the influence of continental roots on the mantle flow field, a component of fossil lithospheric anisotropy, or some combination of these two effects. These competing models will be evaluated by 1) estimating mantle flow from finite element calculations that incorporate radial and lateral variations in viscosity associated with the depth- and temperature-dependence of mantle viscosity, and ocean-continent and age-dependent variations in lithospheric thickness, and 2) examining the continental splitting data for evidence of a fossil lithospheric component of anisotropy.
DE: 8149 Planetary tectonics (5475)
DE: 8155 Plate motions--general
DE: 7218 Lithosphere and upper mantle
SC: Tectonophysics [T]
MN: 2004 AGU Fall Meeting