Large contrasts in crustal structure and composition between the Ordos plateau and the NE Tibetan plateau from receiver function analysis

摘要

We analyzed thousands of receiver-function data recorded by 154 national and regional seismic stations to study the crustal structure beneath the Ordos plateau and the NE margin of the Tibetan plateau. Moho depth and average crustal Vp/. Vs ratio were measured at each station. The Ordos plateau and the Trans-North China Orogen east of the Ordos are underlain by a moderately thick crust of ~. 42. km. The Weihe Graben lying at the southern edge of the Ordos plateau has a thin crust of ~. 30. km, while its southern neighbor, the Qinling orogenic belt shows a thick crust extending to as much as 45. km deep. The Moho depth beneath the NE margin of the Tibetan plateau varies from 55 to 65. km. We found a remarkable contrast between the Tibetan and Ordos plateaus in the measured Poisson's ratio: the Ordos plateau is featured by a high Poisson's ratio while the Tibetan margin has a very low Poisson's ratio. In general, mafic lower crustal rocks have a higher Poisson's ratio than felsic ones. The measured low Poisson's ratio beneath the NE margin of the Tibetan plateau thus indicates that the crustal column beneath the margin is rather felsic, which seems to be inconsistent with a scenario of an inflated crust due to extrusion of lower crust material from the Tibetan plateau to the margin.