HR: 17:45h
AN: U34A-08 INVITED    [Abstracts]
TI: Seismic Anisotropy as a Constraint on Global Mantle Flow and Plate Motions
AU: * Conrad, C P
EM: conrad@jhu.edu
AF: Johns Hopkins University, Department of Earth and Planetary Sciences, Baltimore, MD 21218, United States
AU: Behn, M D
EM: behn@whoi.edu
AF: Woods Hole Oceanographic Institute, Department of Geology and Geophysics, Woods Hole, MA 02543, United States
AU: Silver, P G
EM: silver@dtm.ciw.edu
AF: Carnegie Institute of Washington, Department of Terrestrial Magnetism, 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 fabric that can be detected by shear-wave splitting or surface wave observations. These observations thus provide a strong constraint on the pattern of asthenospheric shear that accommodates the relative motion between the surface plates and the flowing mantle. To utilize this constraint, we developed models of global mantle flow driven by either surface plate motions above a passive viscous mantle (plate-driven flow) or tomographically-inferred density heterogeneity in the viscous mantle beneath a rigid lithosphere (density-driven flow). In the no-net-rotation reference frame (NNR), the relative motions of the plates control the pattern and amplitude of plate-driven flow. For reference frames that incur net lithosphere rotation (e.g., the Pacific hotspot (HS3) frame induces strong westward motion), the NNR shear pattern is superimposed upon a global net shearing of the asthenosphere because some of the net shear between the plates and the deep mantle occurs within the asthenosphere. For density-driven flow, the amplitude of asthenospheric shear flow scales inversely with the absolute mantle viscosity. Thus, the combination of plate- and density-driven flow fields, which should represent the net mantle flow, is sensitive to both the absolute mantle viscosity and the plate motion reference frame. To constrain our models, we used anisotropy inferred from global surface wave tomography studies (e.g., Debayle et al. [2005]) and SKS splitting observations from mid-plate ocean island stations where lithospheric anisotropy appears to be small. We find that the orientation of anisotropy at these stations is best fit by a flow model with a viscosity structure consistent with constraints from postglacial rebound (asthenospheric viscosity of ~ 3-7 × 10²⁰ Pa s). We also use the data to evaluate the most appropriate deep mantle reference frame for plate motions by comparing the misfit among several commonly-used reference frames that include varying degrees of net rotation (e.g., NNR, HS3, HS2).
DE: 3902 Creep and deformation
DE: 8121 Dynamics: convection currents, and mantle plumes
DE: 8158 Plate motions: present and recent (3040)
DE: 8162 Rheology: mantle (8033)
DE: 8180 Tomography (6982, 7270)
SC: Union [U]
MN: 2007 Fall Meeting