Bulldoze and rebuild: Modifying cratonic lithosphere via removal and replacement induced by continental subduction

Abstract

Establishing the mechanisms for craton modification is critical for understanding cratonic stability and architecture. Cratons are intrinsically strong with long-term stability, but plate tectonics or mantle plumes cause craton weakening, mechanical decoupling, and lithospheric removal. By comparison, craton modification—craton destruction accompanied or followed by rejuvenation—has received less attention. Oceanic plate subduction dominantly destroys the craton, with a lesser degree of rebuilding. Mantle plumes can facilitate decratonization, by weakening and peeling off the lithospheric mantle, or recratonization, by healing the craton with refractory mantle residues. Compared with the effects of oceanic plate subduction and mantle plumes, the role of continental subduction in craton modification remains an open question. The North China Craton (NCC), a previously stable continent with a lithospheric thickness of >200 km since the Paleoproterozoic, was reworked and partially destroyed due to lithospheric delamination triggered by Early Cretaceous Paleo-Pacific oceanic subduction. In eastern NCC, lithospheric thickness decreased from 200 km to 35 km in the Early Cretaceous in only 10 m.y. The NCC experienced an early Mesozoic continent–continent collision (as the overriding plate) with the South China Block (SCB). The collision provides an opportunity to understand the potential for craton modification due to deep continental subduction induced by continental collision. In the NCC, combined structural geology, magnetic fabrics, zircon U-Pb dating, and Hf-O isotopes, we report the presence of martial derived from a partially melted SCB’s crust. We proposed a three-stage model to interpret the material sourced from the subducted plate into the overriding carton: (1) SCB bulldozed and rebuilt NCC during 250–220 Ma; (2) during 220-200 Ma, the subducted SCB exhumed along the exhumation channel to underplate beneath the NCC, associated with partial melting; (3) finally, Late Jurassic granite derived partial melting of the SCB entrained Latest Triassic reworked SCB’s crust to emplace. Combining our new results with previous geophysical observations, we estimate the extent of the bulldozing and rebuilding. We argue that a 200-km-long tract of the NCC lithosphere was bulldozed and rebuilt by the subducted SCB, resulting in a lithospheric suture far from the suture zone at the surface. This lithospheric removal occurred at middle-lower crustal levels (16–20 km depth)—much shallower than previously thought possible. The bulldozed NCC lithosphere was replenished by the subducted SCB continental lithosphere rather than the asthenosphere, thus terminating the lithosphere modification. With essentially no net loss of lithosphere during deep continental subduction, the NCC maintained its stability until Early Cretaceous paleo-Pacific oceanic subduction. This “bulldoze and rebuild” model can thus account for how a craton maintains its stability during a collision with another continental plate.