Constraints on upper mantle Vp/Vs ratio variations beneath eastern North China from receiver function tomography

Abstract

To detect the thinning, modification, and replacement of the basement of the lithosphere is a key step in understanding the destruction mechanism of the North China lithosphere. The difference of the basement of the lithosphere is mainly displayed by the variation of the peridotite composition and its physical state. Vp/Vs ratio (hereafter referred to as velocity ratio) is more sensitive to this change than Vp or Vs alone. By means of the strong dependence of the travel-time of the wave converted at the 410-km discontinuity (P410s) observed in the receiver function (RF) on the velocity ratio in the upper mantle, we developed a new mapping method to constrain the velocity ratio between the Moho and 410-km discontinuity. Using the RFs extracted from 246 broadband stations beneath the North China Craton (NCC), we obtained a high-resolution velocity ratio image of the upper mantle. The abnormal velocity ratio indicates a strong lateral variation of the mineral composition in the upper mantle beneath North China. Two low-velocity-ratio patches are imaged at the top of the upper mantle and the 410km depth, respectively. The former may be related to the orthopyroxene enrichment in the lithospheric mantle, and the latter may reflect the stagnant Pacific slab in the mantle transition zone (MTZ). A prominent high-velocity-ratio anomaly is also imaged in the upper mantle beneath the Shaanxi–Shanxi rift system in the central NCC, with the highest anomaly reaching 10%. We speculate that the high velocity ratio of upper mantle is related to convective flow due to slab dehydration in the MTZ. The dehydration of the retained slab in the MTZ results in partial melting and upwelling of upper mantle materials. Such convective flow and their melting are closely related to the Cenozoic basalt eruption in the northern section of the Shaanxi–Shanxi rift system.