Thermal-fluid-melt Activities In The Subduction Zone And The Overriding Lithosphere Responses:Numerical Modeling

The subduction channel processes are crucial for understanding the material and energy exchange between the Earth's crust and mantle.The materials in the subduction channel may experience complex pressure,temperature,stress and strain evolution,as well as strong fluid and melt activity.A certain amount of these materials may subduct to and later exhume from>100km depth,forming high to ultra-high pressure rocks on the surface as widely discovered in nature.It is an important method to study the subduction channel process by numerical modeling.In this study,we investigate the detailed structure and dynamics of both oceanic and continental subduction channels,by conducting high-resolution petrological-thermo-mechanical numerical simulations taking into account fluid and melt activities.The numerical results demonstrate that tectonic melange rocks in the subduction channel may either extrude sub-vertically upward through the mantle wedge to the crust of the upper plate,or exhume along the subduction channel to the surface near the suture zone.In order to study the conditions and dynamics of these contrasting exhumation modes,systematic petrological-thermo-mechanical numerical models are constructed with variable thicknesses of the overriding and subducting continental plates,ages of the oceanic subducting plate,as well as the plate convergence rate.The model results indicate that thermal structures of the converging plates control the fluid-melt activity in the subduction channel,which further strongly affect the material transportation and exhumation mode.The thick overriding continental plate and the low-angle subduction style induced by young subducting oceanic plate both contribute to the formation of relatively cold subduction channel,in which no diapir extrusion is predicted;instead,the material exhumation occurs exclusively along the channel.In contrast,the thin overriding lithosphere and the steep subduction style induced by older subducting oceanic plate are the favorable conditions for hot subduction channel,which lead to significant hydration,melting and weakening of the overriding mantle wedge and thus contribute to the material exhumation by diapir through the mantle wedge.In addition,plate convergence rate can also affect the exhumation mode by changing the amount of sediments carried into the subduction channel which further regulate the fluid-melt activity and thermo-rheological properties.Further,we also study the response of overriding continental plate from the fluid/melt activities in the subduction channel.In this study,we have made a review on the processes and mechanisms of thinning and destruction of cratonic lithosphere according to previous studies of geological/geophysical observations and numerical simulations,with specific application to the North China Craton(NCC).Two main models are suggested for the thinning and destruction of the NCC,both of which are related to subduction of the oceanic lithosphere.These two models have some similarities,in that both take the subducting oceanic slab and relevant fluid migration as the major driving mechanism for thinning or destruction of the overriding cratonic lithosphere.The key difference between the two models is the effective depth of the subducting oceanic slab.One is stagnation and flattening in the MTZ,whereas the other is flat subduction at the bottom of the cratonic lithosphere.In the NCC,the eastern lithosphere was likely affected by subduction of the Izanagi slab during the Mesozoic,which would have perturbed the asthenosphere and the MTZ,and induced fluid migration beneath the NCC lithosphere.The upwelling fluid may largely have controlled the reworking of the NCC lithosphere.In order to discuss and analyze these two models further,it is crucial to understand the role of fluids in the subduction zone and the MTZ.Here,we systematically discuss phase transformations of hydrous minerals and the transportation processes of water in the subduction system.Furthermore,we analyze possible modes of fluid activity and the problems to explore the applied feasibility of each model.In order to achieve a comprehensive understanding of the mechanisms for thinning and destruction of cratonic lithosphere,we also consider other four additional possible dynamic models,e.g.,extension-induced lithospheric thinning,compression-induced lithospheric thickening and delamination,large-scale mantle convection and thermal erosion,and mantle plume erosion.Compared to the subduction-related models,these four models are primarily controlled by the relatively simple and single process and mechanism,which could be the secondary driving mechanisms for the thinning and destruction of lithosphere.