HR: 10:20h
AN: U52A-01 INVITED     [Abstracts]
TI: Asthenospheric Anisotropy Beneath North America
AU: * Fischer, K M
EM: Karen_Fischer@brown.edu
AF: Dept. of Geological Sciences, Brown University, Providence, RI 02912 United States
AU: McCarthy, C M
U52A-01 AF: Dept. of Geological Sciences, Brown University, Providence, RI 02912 United States
AU: Zaranek, S E
U52A-01 AF: Dept. of Earth and Planetary Science, University of California, Berkeley, CA 94720 United States
AU: Rychert, C A
U52A-01 AF: Dept. of Geological Sciences, Brown University, Providence, RI 02912 United States
AU: Li, A
U52A-01 AF: Department of Geosciences, University of Houston, Houston, TX 77204 United States
AB: We are studying how anisotropy varies from the lithosphere to the asthenosphere using teleseismic body and surface waves in eastern and central North America, and we are investigating the relationship of observed asthenospheric anisotropy to numerical models of asthenospheric flow. Surface wave inversions and migration of teleseismic scattered waves define a decrease in lithospheric thickness from more than 200 km in Proterozoic regions to less than 100 km at the eastern continental margin. Shear-wave splitting in SKS phases indicates significant anisotropy beneath the region, but alone these data cannot constrain the depth at which the anisotropy occurs. In an area of the northeastern United States and southeastern Canada that is relatively densely sampled by permanent broadband stations and past temporary broadband arrays, inversion of Rayleigh waves reveals very little azimuthal anisotropy at lithospheric depths; when the Rayleigh waves are integrated with observations of roughly 1 s of SKS splitting, significant asthenospheric anisotropy is required. Further insight on the asthenosphere in this area comes from inversions of Ps phases which reveal a very rapid drop in shear velocity across the lithosphere-asthenosphere boundary (3-11% over less than 11 km). This velocity gradient is too sharp to be explained by purely thermal models, but is consistent with an asthenosphere that contains a few per cent partial melt or that is enriched in volatiles relative to the lithosphere. Evidence for azimuthal anisotropy in the asthenosphere has also been obtained across a broader region of the eastern and central United States. Love and Rayleigh wave inversions along portions of the Florida to Edmonton (FLED) IRIS/PASSCAL array show that Love wave velocities are fast relative to Rayleigh wavespeeds, similar to the findings of Gaherty (2004) using data from the Missouri to Massachusetts (MOMA) IRIS/PASSCAL array. Given that these linear arrays are orthogonal with respect to each other, and that SKS splitting fast directions in the region are fairly consistent and parallel to the MOMA array, the surface waves are most simply explained by radial anisotropy (a fast horizontal plane and a slow vertical axis) in the upper 200 km of the mantle, again requiring azimuthal anisotropy in the sub-lithospheric mantle to produce the SKS splitting. These results suggest that deformation fabrics in the lithosphere and asthenosphere are fundamentally different. The orientations of observed SKS fast directions are in general consistent with the direction of asthenospheric flow around the base of the lithosphere predicted by models in which flow is dominated by plate motion.
DE: 7208 Mantle (1212, 1213, 8124)
DE: 7218 Lithosphere (1236)
DE: 8120 Dynamics of lithosphere and mantle: general (1213)
SC: Union [U]
MN: Fall Meeting 2005