Differences in both the structure and interaction of the crust and mantle on the eastern and western sides of the Ordos Block

The Ordos Block, the western part of the North China Craton (NCC), has preserved most of its cratonic lithospheric roots during the destruction and modification of the NCC, and nowconnects two distinctly different tectonic units, i.e., the Tibetan Plateau (TP) to the west and the North China Plain to the east. It is important to studythe detailed crust-mantle structure of the Ordos Block and surroundings to understand the interactions between the Ordos and adjacent regions and the dynamics of the NCC evolution. In this study, the S-wave velocity structure at depths of 0–100 km along an E-W profile at around 36.5°N from the Qilian orogen (QLO) to the Trans-North China Orogen (TNCO) within the NCC was investigated by joint inversion of the receiver functions (RFs) and surface wave dispersion data from 104 broadband seismic stations deployed in the region under the ChinArray project (Phase II and Phase III). The image of discontinuity structures along the profile was further constructed by common conversion point stacking of RFs. The main results are as follows: (1) The depth of the Moho in the study region gradually decreases from west to east, being the deepest in the QLO, the second in the Ordos Block, and the shallowest in the TNCO. Local depression and uplift of the Moho are also identified beneath the Haiyuan fault zone and the Shanxi-Shaanxi rift system (SSR), respectively. (2) The crust-mantle structure displays obvious lateral heterogeneities among tectonic regions. A pronounced low-velocity anomaly is observed at depths of 20–40 km beneath the QLO and becomes weakened and thinned to the east, indicating that thickening of the mid-to-lower crust probably occurred in the QLO during the growth and lateral extrusion of the northeastern TP under the resistance of the NCC. The lithosphere-asthenosphere boundary beneath the TNCO is imaged at ∼90 km depth, which is significantly shallower than that beneath the Ordos Block. This observation suggests that the lithosphere of the TNCO may have experienced a notable thinning, possibly by thermal erosion and modification due to the mantle upwelling associated with the western Pacific subduction in the Mesozoic. (3) The uplifted Moho under the SSR and its neighboring areas is spatially coincident with the thinned lithosphere in the TNCO, indicating a close relationship between the development of the SSR and the lithospheric thinning and modification in this region.