HR: 08:45h
AN: T21D-04    [Abstracts]
TI: Interaction Between Downwelling Flow and the Laterally-Varying Thickness of the North American Lithosphere Inferred from Seismic Anisotropy
AU: Behn, M D
EM: mbehn@whoi.edu
AF: Dept. Geology & Geophys., WHOI, Woods Hole, MA 02543 United States
AU: * Conrad, C P
EM: conrad@jhu.edu
AF: Dept. Earth & Planetary Sciences, Johns Hopkins Univ., Baltimore, MD 21218 United States
AU: Silver, P G
EM: silver@dtm.ciw.edu
AF: DTM, Carnegie Inst. Wash., Washington, DC 20015 United States
AB: Shear flow in the asthenosphere tends to align olivine crystals in the direction of shear, producing a seismically anisotropic asthenosphere that can be detected using a number of seismic techniques (e.g., shear-wave splitting (SWS) and surface waves). In the ocean basins, where the asthenosphere has a relatively uniform thickness and lithospheric anisotropy appears to be small, observed azimuthal anisotropy is well fit by asthenospheric shear flow in global flow models driven by a combination of plate motions and mantle density heterogeneity. In contrast, beneath the continents both the lithospheric ceiling and asthenospheric thickness may vary considerably across cratonic regions and ocean-continent boundaries. To examine the influence of a continental lithosphere with variable thickness on predictions of continental seismic anisotropy, we impose lateral variations in lithospheric viscosity in global models of mantle flow driven by plate motions and mantle density heterogeneity. For the North American continent, the Farallon slab descends beneath a deep cratonic root, producing downwelling flow in the upper mantle and convergent flow beneath the cratonic lithosphere. We evaluate both the orientation of the predicted azimuthal anisotropy and the depth dependence of radial anisotropy for this downwelling flow and find that the inclusion of a strong continental root provides an improved fit to observed SWS observations beneath the North American craton. Thus, we hypothesize that at least some continental anisotropy is associated with sub-lithospheric viscous shear, although fossil anisotropy in the lithospheric layer may also contribute significantly. Although we do not observe significant variations in the direction of predicted anisotropy with depth, we do find that the inclusion of deep continental roots pushes the depth of the anisotropy layer deeper into the upper mantle. We test several different models of laterally-varying lithosphere and asthenosphere viscosity. These models can be used to separate the contributions of asthenospheric flow and lithospheric fossil fabric in observations of continental anisotropy.
DE: 7208 Mantle (1212, 1213, 8124)
DE: 8120 Dynamics of lithosphere and mantle: general (1213)
DE: 8162 Rheology: mantle (8033)
SC: Tectonophysics [T]
MN: Fall Meeting 2005