Three-dimensional density structure of the lithosphere beneath the North China Craton and the mechanisms of its destruction

The lithosphere beneath the North China Craton (NCC) has been thinning since the Phanerozoic. Previous geophysical studies on NCC structures have mainly focused on the results of seismic wave propagation. In this study, we obtained 3D density structures of the NCC lithosphere by using the sequential inversion of observed gravity data and P-wave travel times. Analyses of the resulting density model and discussions of the destruction of the NCC are provided. Our density model shows that distinct horizontal and vertical density heterogeneities exist throughout the lithosphere beneath the NCC; in the west of the craton, the Ordos block is characterised by a relatively uniform high-density lithospheric mantle, while laterally heterogeneous densities are observed at shallower than 100 km depth. Distinct low-density anomalies are imaged in the Cenozoic Yinchuan-Hetao and Shaanxi-Shanxi rifts surrounding the Ordos block. Comparing with the thinned lithosphere and high Poisson's ratio, the low density in the rifts may indicate partial melting induced by thermo-mechanical erosion. Predominantly low-density anomalies, which represent an upwelling of the asthenosphere, are revealed in the south of the Taihang orogen, as well as at the junction of the Yinshan-Yanshan orogen (near Datong volcano) and the Taihangshan orogen, from 100 to 200 km depth. This supports the hypothesis of thermo-mechanical erosion in these regions. High-density anomalies are visible at the base of the lithosphere or in the upper mantle beneath the central North China Basin. Combined with high velocities at 300 km depth indicated by seismic tomography, these high densities suggest that the lithospheric destruction of this region may not be explained by thermo-mechanical erosion, as there is no evidence of deep, hot upwelling that could support thermo-mechanical erosion.