Crustal-scale wedge tectonics at the narrow boundary between the Tibetan Plateau and Ordos block

Abstract

The Tibetan plateau is bounded in part by cratonic blocks, and in part by tectonothermally younger and weaker crust. How the Tibetan plateau interacts with bordering non-cratonic crust is a continuing debate, whether the plateau is currently enlarging its area, and if so, whether by thrusting or by lower-crustal flow, or both. The gently sloping northeastern margin of Tibet has been inferred to incorporate a lower-crustal channel flow outward towards the Ordos block that is a part of the North China Craton. We calculated receiver functions using teleseismic waveforms recorded by a dense nodal seismic array crossing from the Longxi basin of northeastern Tibet across a transition zone marked by the Liupan Shan (mountains) into the Ordos block. Our image shows the Longxi upper crust overthrusts onto the Ordos block building the Liupan Shan, and the Longxi lower crust underthrusts beneath the Ordos forming a 10-km thick crustal root. The Ordos crust forms a crustal-scale wedge inserted into the Longxi crust, and deformation is limited to a narrow zone of <50 km either side of the Liupan Shan. Convergence between the Longxi Basin and Ordos block is producing a narrow mountain range rather than broadening the plateau. The larger-scale gentle topography is likely an expression of negligible crustal deformation of the Longxi basin. Even if the Longxi basin of NE Tibet, between the Qaidam and Sichuan basins, is underlain by a weak lower crust, tunneling channel flow does not extend beyond the Liupan Shan, and the Tibetan Plateau has not grown outward beneath the Ordos block.