In this paper, we attempt to explain how the buoyant cratonic lithosphere may be transformed into a denser one, allowing delamination to take place on the basis of field observation, tectonic analysis and petrologic data on igneous rocks formed during the Yanshanian. We recognize four episodes of contractional deformation that resulted in significant crustal shortening and vertical thickening. The counter-clockwise Pressure–Temperature–time path of the tectonomagmatic events suggests that the underplating basaltic magma may have heated and weakened the existing cold and strong crust. This crustal change in rheology may have facilitated the contractional deformation and crustal thickening. Petrologic data of the contemporary igneous rocks and the lower crustal xenoliths suggest that the crust had reached 50–65 km in thickness. It suggests that input of large amount of asthenosphere-derived mafic magmas is required to cause crustal melting. Thus, a large amount of eclogite may be formed at the lowermost crust following the transient thickening events. The dense eclogite may trigger the lithosphere delamination.