Was the Lithospheric Mantle of the Eastern North China Craton Removed or Just Stirred during the Mesozoic Reactivation?

Abstract:

The North China Craton (NCC), as an Archean craton, consists of two distinctly different units. The eastern NCC had undergone extensive reactivation during the Mesozoic and Cenozoic as seen from surface volcanism, magmatism, and tectonic deformation, while the western NCC has remained stable. Seismic studies show that lithospheric thicknesses are about 80 km and 200 km for the eastern and western NCC, respectively. A particularly important question is to what extent the chemically buoyant lithospheric mantle of the eastern NCC has been removed during the reactivation process and how much relict lithospheric mantle has remained after the reactivation. It is sometimes suggested that the reactivation removed the entire cratonic lithosphere in the eastern NCC and that the 80 km thick lithosphere at present is mainly comprised of the normal mantle materials. In this study, we seek to answer this question by determining residual topography for the NCC and comparing it with the model topography from dynamic models of gravitational instability of cratonic lithosphere. The residual topography after removal of crustal thickness effects shows that the eastern NCC is ~500 m higher than the western NCC. Our dynamic models with non-Newtonian rheology show that the instability for chemically buoyant cratonic lithosphere initiates at the shallow part of the lithosphere, propagates through and affects the entire lithosphere, thus leading to efficient thermal mixing of the lithosphere and thinning of thermal and mechanical lithosphere. However, due to its chemical buoyancy, most of the lithospheric mantle returns to and remains in its original vertical extent (i.e., the top 200 km), after its thermal mixing with the underlying asthenospheric mantle. The dynamic topography for the destabilized cratonic lithosphere is ~700 m higher than that for the stable lithosphere, consistent with the estimated residual topography for the NCC. This suggests that the reactivation process only thins the thermal and mechanical lithosphere for the eastern NCC, while its chemically distinct and buoyant lithospheric mantle largely remains where it used to be in the top 200 km. Our proposed thermal and chemical mixing processes have important implications for understanding the reactivation of the NCC.