Lithospheric structure of the Ordos block and its adjacent regions from P and S receiver functions and its implications for the Cenozoic lithospheric reworking

The Ordos block is the core of the North China Craton (NCC), one of the oldest cratons on earth. It is surrounded by the Weihe Rift and Qinling Orogen to the south as well as the Hetao Rift and Yinshan Orogen to the north. It is generally accepted that the lithosphere of eastern NCC was largely destroyed during the late Mesozoic. Whereas the Ordos block in the western NCC preserves a relatively intact cratonic keel, although its surroundings have experienced significant lithospheric reactivation since the Cenozoic. However, the actual mechanism that is responsible for the lithospheric reactivation around the Ordos is still largely unknown. Here, we obtain the lithospheric structure of the Ordos block and its adjacent regions along a roughly N-S profile with more than 1000 km long using P and S receiver functions with seismic data from a dense and broadband seismic array, the ChinArray-Himalaya II project. Both P and S receiver function migration images show highly consistent lithospheric features. We find an undulant Moho beneath the Hetao and Yinshan at depths between 40 and 55 km. Beneath the northern Ordos, a relatively flat Moho is observed at a depth of 45 km and it gradually deepens to a maximum of 50 km at the southern end of Ordos. Further to the south, an arc-shaped Moho is shown beneath the Weihe and the Qinling with its apex roughly beneath the Taibai Mountain, the summit of Qinling. The crustal thickness is ~50 km beneath the northern margin of Yangtze block. A relatively thinned lithosphere (120-150 km) is shown beneath the Hetao and Yinshan to the north of Ordos. However, the Ordos block has preserved a rather thick lithosphere (~180 km in average). The LAB deepens from north to south with the deepest LAB ~220 km occurring beneath the southern Ordos. The lithosphere abruptly thins to less than 120 km beneath the Weihe and northern Qinling, and the LAB deepens to ~150 km beneath the southern Qinling and the northern Yangtze block. The observed thin lithosphere here is consistent with our interpretation from body wave tomographic and surface wave anisotropic images that the Weihe and northern Qinling act as a pathway for the eastward asthenospheric flow from northeastern Tibetan Plateau. The asthenospheric flow may play an important role in controlling the lithospheric reworking around the southern Ordos during the Cenozoic.