Thermochemical heterogeneity of the upper mantle from a coupled, geophysical-petrological inversion of surface-wave, heat-flow, surface-elevation and satellite gravity data

Abstract:

We present a new global thermochemical model of the lithosphere and upper mantle (WINTERC-grav) constrained by state-of-the-art seismic waveform inversion, satellite gravity, surface elevation and heat flow data. WINTERC-grav is based upon an integrated geophysical-petrological approach where seismic velocities and density are computed within a thermodynamically self-consistent framework. The complementary sensitivities of our data sets allow us to constrain the geometry of the lithosphere-asthenosphere boundary, to separate thermal and compositional anomalies in the mantle, and to distinguish dynamic vs isostatic surface-elevation contributions. Our model shows that the thickest lithosphere is associated with cratons and, also, some tectonically active areas. We identify considerable differences in cratonic temperatures and compositions. The North American and Siberian Cratons are thick (>260 km) and compositionally refractory, whereas the Sino-Korean, Aldan and Tanzanian Cratons have a thinner, fertile lithosphere, similar to younger continental lithosphere elsewhere. WINTERC-grav shows progressive thickening of oceanic lithosphere with age, but with significant regional differences: the mantle lithosphere beneath the Atlantic and Indian Oceans is, on average, colder, more fertile and denser than that beneath the Pacific Ocean. Our results suggest that the composition, temperature and density of the oceanic mantle lithosphere are correlated to spreading rates up to values < 50-60 mm/yr. The 1D average profile of WINTERC-grav displays a mantle geothermal gradient of 0.55-0.6 C/km and a potential temperature of 1300-1320C for depths >200 km. The residual isostatic topography values, a proxy for dynamic topography, are large (>1 km) mostly in active subduction settings. The residual isostatic bathymetry from WINTERC-grav is remarkably similar to the lateral pattern independently determined based on oceanic crustal data compilations. The amplitude of the continental residual topography is relatively large (>500 m) in the East European Craton, Greenland, the Andes and Himalayas. Our results show that a substantial part of the topography signal previously identified as residual (or dynamic) is accounted for, isostatically, by lithospheric density variations.