Mesozoic Decratonization of the North China Craton Triggered by Dehydration of the Pacifc Slab

Abstract:

Cratons are stable continental blocks with a cold and strong cratonic root that typically is at least ~200 km in thickness. Many cratons have undergone little internal tectonism and magmatism since their formation, but some of them, such as the eastern part of the North China Craton (NCC), the Dharwar Craton and the Wyoming Craton, have lost their thick cratonic root and become reactivated in recent geological history, leading to widespread Mesozoic-Cenozoic volcanism. The mechanisms responsible for such decratonization remain enigmatic. Here we show that the weakening of the lithosphere beneath the eastern NCC may have been caused by the release of water (in the form of structurally-bound H/OH) brought down to the mantle transition zone (MTZ) by subduction of the Paleo-Pacific slab. A depression of up to 37 km of the 660-km discontinuity (d660), which marks the bottom of the MTZ, is clearly observed beneath the eastern NCC, and is consistent with the existence of an anomalously large amount of water in the MTZ. Our study provides strong observational evidence for the geodynamic-modeling-based hypothesis that water in the MTZ can be driven out into the upper mantle by toroidal mantle flow induced by the subduction and retreat of subducted oceanic slabs. Continuous slab dehydration and the ascent of fluids have triggered the decratonization in the eastern NCC, as evidenced by the excellent spatial correspondence among the lower-than-normal upper-mantle seismic velocities, stronger P-to-S converted phases corresponding to a water-saturated layer above the MTZ, Mesozoic and Cenozoic basaltic volcanism, and thinning of the depleted cratonic lithosphere.