HR: 0800h
AN: T21B-0517 INVITED     [Abstracts]
TI: Multiple Layers of Anisotropy in the Chile-Argentina Subduction Zone, South America
AU: * Anderson, M L
EM: anderson@geo.arizona.edu
AF: Department of Geosciences, Gould-Simpson Building University of Arizona, Tucson, AZ 85719
AU: Zandt, G
EM: zandt@geo.arizona.edu
AF: Department of Geosciences, Gould-Simpson Building University of Arizona, Tucson, AZ 85719
AB: We examine shear wave splitting in teleseismic and local phases to observe seismic anisotropy in part of the South American subduction zone. The data is from the CHARGE network, which traversed the Andes Mountains of Chile and Argentina across two transects between 30$\deg$ and 36$\deg$ S. Beneath the southern part of our network, fast polarization directions from teleseismic phases are consistently trench-parallel, while in the northeastern part of the network fast directions are trench-normal. This trend appears to be correlated with a changing geometry of the subducted slab. Subduction zones may exhibit multiple layers of mantle anisotropy, corresponding to the mantle wedge, subducting slab, and the asthenosphere in the upper mantle below the slab. Based on previous work, we have suggested that the largest source of the anisotropy sampled by teleseismic phases is localized below the subducting slab. Preliminary measurements of anisotropy in the mantle wedge (sampled by local S-waves) reveal quite variable patterns in azimuth and magnitude, similar to other subduction zones. There is also an observed variation in splitting parameters in the teleseismic events with backazimuth that may be explained by a combination of wedge and below-slab anisotropy. However, preliminary two layer models with reasonable values for the wedge and below-slab components do not generally fit observed trends with backazimuth, therefore there may be three layers of anisotropy: the mantle wedge, the subducting slab lithosphere, and an asthenospheric layer. Because the magnitude of anisotropy due to the slab and to the wedge is relatively small compared to total anisotropy, removing it's effect from the total anisotropy leaves an asthenospheric component that is very similar to the total observed anisotropy by teleseisms. Calculations attempting this removal to date have made small adjustments to the telseismic values that actually make the asthenospheric component look more consistent across the network.
DE: 9360 South America
DE: 8123 Dynamics, seismotectonics
DE: 8150 Plate boundary--general (3040)
DE: 8162 Rheology--mantle
DE: 7218 Lithosphere and upper mantle
SC: Tectonophysics [T]
MN: 2004 AGU Fall Meeting