Thick crust beneath the Ordos plateau: Implications for instability of the North China craton

Surrounded by seismicity and other manifestations of active deformation, the Ordos plateau, or the western portion of the North China craton (NCC), is a uniquely stable terrane in Asia. Results from virtual deep-seismic sounding and crustal receiver functions suggests that the crust under the eastern Ordos is thicker (at least 60 km) than expected from previous studies and from its modest elevation (∼1500 m above sea-level). Receiver functions also reveal a pronounced elastic impedance contrast within the crust (at ∼40 km depth), which we interpret as the Conrad discontinuity. The presence of a 20 km thick layer of mafic lower crust between the Conrad and Moho discontinuities would maintain crustal isostasy. The ∼1000 km long seismic profile from the Ordos plateau in the west to the North China basin in the east reveals that crustal thickness changes by almost a factor of two across the active Shanxi rift in central NCC (over a distance of only about 100 km). Insofar the current configuration of the lithosphere under the Ordos plateau might serve as a proxy for the initial condition prior to reactivation of the eastern part of NCC—where a cratonic keel no longer seems to exist—our results support the hypothesis that lower crust foundering was due to transformation of a thick mafic lower-crust to a garnet-rich assemblage (possibly caused by hydration associated with subduction during and/or before mid-Mesozoic times).