HR: 09:00h
AN: T31B-05 INVITED     [PDF]
TI: Constraints on the Thermochemical Structure of the Earth's Deep Mantle Using Seismic, Geodynamic and Mineral Physics Data
AU: * Mitrovica, J X
EM: jxm@physics.utoronto.ca
AF: Department of Physics University of Toronto, 60 St. George Street, Toronto, ON M5S 1A7 Canada
AU: Forte, A M
EM: forte.alessandro@uqam.ca
AF: GEOTOP, D\'epartement des Sciences de la Terre, Universit\'e de Qu\'ebec \`a Montr\'eal, C.P. 8888, Succ. Centre-Ville, Montr\'eal, QC H3C 3P8 Canada
AB: The integration of seismic, geodynamic and mineral physics data to constrain the large scale composition and dynamics of the Earth's mantle is a widely stated goal of global geophysics; however, the appropriate methodology for this integration is a matter of debate. In recent work [Forte and Mitrovica, Phil. Trans., 2002] we outlined, atleast for a simple compositional model of the mantle, an approach for combining joint (shear and bulk sound) seismic models, seismic velocity derivatives obtained from results in mineral physics, and convection related observations (plate motions, gravity anomalies and the excess ellipticity of the CMB). Our inversions yielded a mantle viscosity profile characterized by two viscosity maxima within the lower mantle. The deepest of the two, at 2000 km depth, suppresses all but the longest horizontal wavelengths of the present-day flow in the bottom 1000 km of the lower mantle, thereby providing a simple interpretation for the `red' spectrum of seismically-inferred heterogeneity in this region. The integration also suggested that while chemical anomalies in the lower-most mantle are required to explain seismic observations, these anomalies are unable to inhibit the dominant thermal buoyancy of the deep-mantle mega-plumes below the Pacific and Africa. In this talk we describe the results of a large set of new inversions that: (1) extend the viscosity inferences to include a suite of data related to glacial isostatic adjustment (GIA; these data include site-specific post-glacial decay times from Fennoscandia and Hudson Bay, and a relaxation spectrum which provides the decay time versus wavelength of Fennoscandian deformation); and (2) map out, using Monte-Carlo simulations, plausible variations in thermochemical structure (summarized by the so-called buoyancy ratio) associated with uncertainties in the seismic models, mineral physics data and mantle viscosity (including the presence of the lateral variations). The GIA data provide an important, independent constraint on absolute viscosity and significantly improve the resolution of the resulting viscosity inferences, particularly in the transition zone and top half of the lower mantle. However, the viscosity peak at 2000 km depth remains a robust feature of these new inversions, reinforcing our earlier conclusion that this structure plays a pre-eminent role in deep mantle flow dynamics. Furthermore, the Monte-Carlo simulations indicate a positive correlation between density and shear wave velocity in the deep mantle and thus a dominance of thermal buoyancy within that region.
DE: 8121 Dynamics, convection currents and mantle plumes
DE: 8122 Dynamics, gravity and tectonics
DE: 8124 Earth's interior--composition and state (old 8105)
DE: 8162 Rheology--mantle
DE: 8180 Tomography
SC: Tectonophysics [T]
MN: 2003 Fall Meeting