Crustal structure beneath the northeastern margin of the Tibetan Plateau constrained by receiver functions and gravity data

Abstract

The northeastern margin of the Tibetan Plateau is a key area for studies of the crustal thickening and outward expansion of the plateau and its interactions with surrounding blocks. Detailed crustal structure information can provide important constraints for understanding the tectonic evolutionary process in this region. We obtained the crustal thicknesses and average Vp/Vs ratios via joint estimation from teleseismic P-wave receiver functions and the complete Bouguer gravity anomaly. Our results show that the crustal thickness gradually decreases northward and eastward from ∼65 km beneath the Qilian Tectonic Belt to ∼40 km within the Alxa and Ordos blocks. A low Vp/Vs ratio (1.71–1.78) is found in the northeast Tibetan Plateau, which is indicative of its felsic crustal composition. A maximum 9.3 km deviation from the Airy isostatic crustal thickness is revealed beneath the Songpan–Garze Terrane and the West Qinling Orogen. The correlation between the elevations and corresponding crustal thicknesses within these two terranes is weak, suggesting that contributions of the isostatic compensation come not only from the crust but also from the lithospheric mantle. We found that the crustal thicknesses below the northeast Tibetan plateau are anti-correlated with the average crustal Vp/Vs ratios and densities, which is in favor of the previous hypothesis of upper-crust thickening or lower-crust delamination of the coherent thickening crust.