Large contrast observed in crustal composition and structure between the Ordos plateau and the northeastern margin of the Tibetan Plateau

The Ordos plateau located in the western side of the North China craton is a distinct aseismic block. The plateau is bounded on the south and east, respectively, by the Paleozoic-Mesozoic Qinling orogenic belt and the Taihangshan mountain range. A series of graben systems were developed late in Cenozoic around the southern and eastern margins of the plateau. The Ordos is also bordered on the southwest by the northeastern margin of the Tibetan Plateau formed by the Cenozoic India-Eurasia collision. The gentle topographic slope together with the moderately thickened crust along the northeastern margin of the Tibetan Plateau was modeled by an injection of lower crustal material into a relatively weak foreland. We have analyzed thousands of receiver-function data recorded by 182 national and regional stations of the broadband seismic network operated by the China Earthquake Administration to study the crustal structure beneath the Ordos plateau and the northeastern margin of the Tibetan plateau. Crustal thickness and average crustal Vp/Vs ratio were measured at each station. The Ordos plateau is underlain by a moderate thick crust with an average Moho depth ~40 km. The Trans-North China Orogen east to the Ordos is featured by a thin crust varying from 25-35 km. The Weihe Graben at the southern border of the Ordos plateau is lying above a ~30-km thin crust, while its southern neighbor, the Qinling organic belt has a thick root extending to as much as 45 km deep. Crust beneath the northeastern 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, lower crustal rocks are more mafic with a higher Poisson’s ratio than those found in the upper crust and mafic rocks are mechanically stronger than the felsic ones. The measured lower ratio thus indicate that the crustal column beneath the eastern margin of the Tibetan Plateau is rather felsic and is inconsistent with the lower-crust injection model. This lower crustal flow model also has difficulty in explaining the thin crust beneath the Weihe Graben. The observed contrast in the Poisson’s ratio, on the other hand, agrees with the observed surface deformation that indicates the Ordos plateau is mechanically stronger than the Tibetan plateau.