The 3D Thermochemical Structure of the Lithosphere and Upper Mantle beneath South China

Abstract:

We study the 3D thermal and compositional structure of South China by jointly inverting Rayleigh wave dispersion data, geoid height, topography and surface heat flow with a probabilistic (Bayesian) Monte Carlo method. We find that the lithosphere is thin (85-150 km) beneath the South China Fold system and thickens over the Yangtze Craton to maximum thicknesses of up to 280 km beneath the Sichuan Basin. In agreement with geochemical signatures from East China mantle xenoliths, our inversion predicts that the lithospheric mantle beneath the South China Fold system and Yangtze Craton is highly fertile (Mg# ~ 88-90). Such fertile compositions, together with the relatively thin lithospheric thickness in the area, point towards a widespread metasomatism/refertilization event. We suggest, as others have, that a flat-subduction episode and subsequent slab removal may have triggered both the delamination of the lowermost part of the subcontinental lithosphere and the generation of asthenospheric melts that metasomatized (refertilized) the remaining lithospheric mantle. Inconsistencies among geophysical observations and anomalously fertile compositions for the Sichuan Basin indicate that this region may be currently affected by small-scale convection or delamination processes. Alternatively, the anomalous observations may be associated with an eastward push of Tibetan lithosphere beneath the Yangtze Craton.