Numerical Simulation On Dynamic Process Of Double-slab Subduction Models With Opposite Dipping Directions

Lithospheric plates on the Earth's surface interact with each other,producing many distinctive structures.In the study of multi-plate subduction models,there are not many of them on the double subduction model with inward dipping.In order to do research on this type of subduction,a two-dimensional geodynamic model is established in this paper to simulate the dynamic subduction process of in-dip double subduction with different plate parameters and to analyze its influencing factors.It is found that the main concern is the subduction pattern of the subduction plate and the upwelling in the transition zone.Combining with the geological background of Hainan volcanic region,we propose the correlation between the subduction model and the genesis of the mantle plume in Hainan.This unique double-slab configuration limits trench motion and causes steep downward slab movement,thus forming fold piles in the lower mantle and driving upwards mantle flow between the slabs.By using underworld2,one numerical simulation software,the effects of lower mantle viscosity,overriding plate properties and subducting plate properties on stress in the middle of the overriding plate and upwelling velocity in the two-subduction model are discussed.It can be found from the numerical simulation that the viscosities of the lower mantle have a great influence on the upwelling.When the viscosities of the lower mantle are very high,the subsidence speed of the subduction plate will slow down at the boundary of the upper and lower mantles,and the upwelling speed will be relatively small.While varying the length and thickness of a long overriding plate(?2500)does not have a substantial effect on slab geometry,its viscosity has a marked impact on slab evolution and mantle flow pattern.When the overriding plate is strong,slabs exhibit an overturned geometry and hesitate to fold.Mantle transition zone upwelling velocity depends on the speed of descending slabs.The downward velocity of slabs with a large negative buoyancy(caused by thickness or density)is very fast,inducing a significant transition zone upwelling flow.The different settings of thickness,viscosity and density of subducting plates have a great influence on the dipping velocity and folding shape of subducting plates.In addition,the temporal variation of mantle upwelling velocity shows strong correlation with evolution of slab folding geometry.During the backwards-folding stage,upwelling velocity reaches its local maximum.In the double subduction system with inward-dipping directions,the mantle transition zone upwelling exhibits oscillatory rise with time.Our results provide new insights into the deep mantle source of intraplate volcanism in a three-plate interaction system such as the Southeast Asia region.