Upper-mantle velocity structure beneath the North China Craton: Implications for lithospheric thinning

The presence of high-velocity lithospheric 'keels'in the upper mantle is a ubiquitous feature of stable continental provinces. However, the upper-mantle structure of the reactivated craton, whether all such high-velocity (high-V) keels are eventually removed and the mechanism of removal, remain unclear. In this work, we determine 3-D seismic velocity structure beneath the eastern part of the North China Craton and the surrounding regions, using a large number of data from local, regional and teleseismic events. Our results show that the upper-mantle high-V layer, which represents the lithosphere, is only 60-100 km thick beneath the Huabei Basin, whereas it is at least 150 km thick under Liaodong. Over 100-km thick high-V anomalies are visible atop the 410-km discontinuity and right beneath the lithospheric-thinning area. We interpret the high-V anomalies as the remnants of the delaminated lithosphere, which caused the lithospheric thinning under the eastern North China Craton. The most significant feature of our tomographric results is that the lithospheric delamination occurred in localized areas rather than the entire eastern North China Craton. It is an important observation that provides geophysical constraints on the delamination mechanism.