Mapping the Earth's thermochemical and anisotropic structure using global surface wave data

L., Boschi and J. A. D., Connolly and Adnan, Shahid Khan (2011) Mapping the Earth's thermochemical and anisotropic structure using global surface wave data. Journal Of Geophysical Research, 116 (B01301). pp. 1-29. ISSN 0148-0227

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Abstract

We have inverted global fundamental mode and higher‐order Love and Rayleigh wave dispersion data jointly, to find global maps of temperature, composition, and radial seismic anisotropy of the Earth’s mantle as well as their uncertainties via a stochastic sampling‐based approach. We apply a self‐consistent thermodynamic method to systematically compute phase equilibria and physical properties (P and S wave velocity, density) that depend only on composition (in the Na2‐CaO‐FeO‐MgO‐Al2O3‐SiO2 model system), pressure, and temperature. Our 3‐D maps are defined horizontally by 27 different tectonic regions and vertically by a number of layers. We find thermochemical differences between oceans and continents to extend down to ∼250 km depth, with continents and cratons appearing chemically depleted (high magnesium number (Mg #) and Mg/Si ratio) and colder (>100°C) relative to oceans, while young oceanic lithosphere is hotter than its intermediate age and old counterparts. We find what appears to be strong radial S wave anisotropy in the upper mantle down to ∼200 km, while there seems to be little evidence for shear anisotropy at greater depths. At and beneath the transition zone, 3‐D heterogeneity is likely uncorrelated with surface tectonics; as a result, our tectonics‐based parameterization is tenuous. Despite this weakness, constraints on the gross average thermochemical and anisotropic structure to ∼1300 km depth can be inferred, which appear to indicate that the compositions of the upper (low Mg# and high Mg/Si ratio) and lower mantle (high Mg# and low Mg/Si ratio) might possibly be distinct.

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