The effects of the thermal state of overriding continental plate on subduction dynamics: Two-dimensional thermal-mechanical modeling

摘要

The dynamic process of ocean-continent subduction depends on not only the properties of the subducting oceanic plate,but also the characteristics and state of the overriding continental plate.Numerical models conducted to date have mostly focused on the oceanic lithosphere in this regard;research on the properties of overriding continental lithosphere remains relatively limited,especially the influence of its thermal state on subduction dynamics.Here we explored the performance of continental lithosphere with different thermal states during the subduction process using two-dimensional thermal-mechanical modeling and systematically investigated the effects of the thermal state of overriding continental plate,the age of subducting oceanic plate,and relative convergence rate on subduction dynamics.Modeling results show that:(1)When the geothermal gradient of continental crust is low(between 10 and 15℃km^-1),the oceanic plate first subducts at a low angle.As subduction continues,the slab dip gradually increases and the slab begins to retreat rapidly driven by its negative buoyancy,opening an ocean basin ranging from 600 to 1100 km in width.This leads to the decoupling between the overriding continental plate and oceanic plate.As the trench retreat continues,the horizontal deviatoric stress inside the overriding continental crust alternates between being positive and negative in a local area.Thinning of the overriding lithosphere mainly occurs at the region adjacent to the subduction zone,where the surface experiences significant subsidence.(2)When the geothermal gradient of continental crust is higher(greater than 15℃km-1),oceanic plate retreat causes the overriding continental plate to be strongly stretched.In this case,the trench retreat distance decreases and the width of the ocean basin also reduces by between 100 and 1000 km.The horizontal deviatoric stress inside the whole overriding continental crust first manifests as compression and then changes into extension,which causes the surface to first uplift and then slowly subside.(3)Increasing the age of oceanic lithosphere accelerates trench retreat and promotes overriding plate thinning.(4)An advancing overriding continental plate slows down trench retreat.In cases where the geothermal gradient of continental crust is greater than 17.5℃km-1,the hot continental crust experiences gravitational collapse and is overthrusted onto oceanic lithosphere,resulting in slow trench retreat.We analyzed the subduction process of the western Paleo-Pacific Plate in the Early Cretaceous based on our modeling results and discussed its possible control on the tectonic evolution of the rift basins in east Asia.We suggest that the development of a wide rift basin system on the Amurian Superterrane in the Early Cretaceous was likely related to slow trench retreat and the collapse of the hot crust,and the formation of a series of passive rift basins in the North China Craton was likely caused by the relatively cold thermal state of the lithosphere and the rapid retreat of the Paleo-Pacific Plate.