Back to list of UTIG abstract submissions, Fall Agu 2003

08:15h AN: T31B-02 INVITED

AU: * Grand, S P
EM: steveg@geo.utexas.edu
AF: Jackson School of Geosciences University of Texas, Dept of Geological Sciences University of Texas in Austin Austin, TX 78712, Austin, TX 78712 United States
AU: Forte, A M
EM: forte.alessandro@uqam.ca
AF: Universitie de Quebec a Montreal, GEOTOP - Dept des Sciences de la Terre Universite de Quebec a Montreal C. P. 888, Succ. Centre-Ville Montreal, Quebec Canaa H3C 3P8, Montreal, QU h3c3p8 Canada
AU: Simmons, N
EM: nathan@geo.utexas.edu
AF: Jackson School of Geosciences University of Texas, Dept of Geological Sciences University of Texas in Austin Austin, TX 78712, Austin, TX 78712 United States

Abstract
Recent progress in global seismic tomography is yielding three-dimensional (3-D) images of mantle structure with greatly improved resolution on length scales ranging from a few hundred to several thousand kilometers. All seismic models agree on several large scale structures although seismic models from different groups still differ in many regions indicating issues of resolution remain. Furthermore, there is still debate on fundamental questions concerning the nature of mantle flow and chemical reservoirs in the mantle. Even as seismic models of the mantle improve, it is likely debate will remain concerning their interpretation in terms of mantle layering. Mantle flow has an observable signature in several geophysical fields including global gravity anomalies, plate motions, and dynamic surface topography. Given a density field of the mantle and a mantle viscosity structure, these geodynamic observables can be computed by calculating the instantaneous flow in the mantle. The dynamic response functions relating a given density anomaly to a given geophysical observable depend upon the existence of any chemical or phase change boundaries that impede vertical mass transport across the mantle. With knowledge of the three-dimensional variation in mantle density, we may thus discriminate among models with and without flow boundaries at different depths in the mantle using global geodynamic surface observables. Using various scalings between seismic velocity and density, we present joint inversions for 3D seismic velocity (density) mantle structure, plate motions, global free-air gravity anomalies, global dynamic surface topography, and dynamic ellipticity of the core-mantle boundary for a layered and un-layered mantle. The mantle flow models employ new mantle viscosity profiles that are optimized for each inversion. The seismic data consist of over 40000 S, multi-bounce S, ScS, multibounce ScS, SKS, and SKKS travel times. The joint inversions to date indicate that whole mantle flow is favored over a model with a barrier to flow near 660 km depth. We will also present results showing how these joint inversions can explicitly test the hypothesis of a negative or null correlation between anomalies of density and seismic shear velocity in the bottom half of the lower mantle. Such hypothesis tests provide insight on the relative importance of thermal and chemical contributions to lower-mantle density and seismic anomalies.