On The Role Of Oceanic Plateaus In Subduction Zone Reorganization:Two-dimensional Thermo-mechanical Modeling

The oceanic plateau is widely distributed in the oceanic plate.It is an abnormally buoyant body with a larger area and higher terrain than the surrounding oceanic plates.The subduction,collision and accretion of oceanic plateaus is an important geological process,which is closely related to material circulation,continental accretion,subduction initiation,inland orogeny and magmatism,but the mechanism of the oceanic plateau in the reorganization of the subduction zone is still unclear.In this paper,we use two-dimensional numerical simulation to explore the role of oceanic plateaus in subduction zone reorganization,and have gained some new understandings.Flat subduction refers to low-angle(<10°)or sub-horizontal subduction of oceanic slabs.Flat subduction is only recognized in ?10% of present-day subduction zones,but its impact on the behavior of the overriding plate is particularly strong.For example,flat subduction zones are typically associated with stronger earthquakes.The deformation caused by a typical flat subduction will transfer from the trench to the overlying continental interior and form a broad magma belt.The formation mechanism of flat subduction has been linked to the relative buoyancy of subducted oceanic plateaus,overthrusting of the overriding plate,plate suction,and trench retreat.However,these mechanisms remain debated.This paper systematically analyzes and summarizes previous studies on flat subduction,and outlines the geological effects of flat subduction,such as intracontinental orogeny and magmatism.Using examples from numerical modeling,we discuss possible formation mechanisms.The most important factors that control the formation of flat subduction are associated with overthrusting of the overriding plate and the arrival of an oceanic plateau at the subduction zone.In addition,trench retreat is required to enable flat subduction.Plate suction contributes to the reduction of the slab dip angle,but is insufficient to form flat subduction.Future numerical modeling of flat subduction should carry out three-dimensional high-resolution thermo-mechanical simulation,considering the influence of crustal eclogitization(negative buoyancy)and mantle serpentinization(positive buoyancy)of oceanic lithosphere,in combination with geological and geophysical data.Whether the buoyancy of oceanic plateaus can produce flat-slab subduction is still unclear.We use two-dimensional thermo-mechanical modeling to quantitatively analyze the influence of the oceanic plateau thickness and scale on the subduction zone reorganization.Through systematic model tests,the model results show that the buoyancy of the oceanic plateau has a greater impact on the subduction style.Compared with the scale of the oceanic plateau,the subduction pattern is more sensitive to the thickness of the plateau's crust.Small-scale(<20km),low-buoyancy,and huge(?35km)oceanic plateau models will not form flat-slab subduction.Only the medium-thickness(?25km)oceanic plateau model will form flat-slab subduction under certain conditions.That is,the condition for flat-slab subduction is that while the oceanic plateau meets sufficient positive buoyancy(thickness ?20km),and the thickness of the oceanic plateau cannot exceed 30 km,because the huge crust of the oceanic plateau has strong resistance to subduction,which is not conducive to the subduction of the plate.Oceanic plateaus are characterized by overthickened crust and rheologically strong and chemically depleted mantle root with respect to normal oceanic lithosphere.The buoyant nature makes them have a great potential to resist subduction and initiate a new subduction zone behind the plateaus,yet the geodynamic process remains enigmatic.Here we use 2D thermo-mechanical modeling method to investigate the role of an oceanic plateau in subduction zone reorganization.We systematically examine the rheological strength and compositional density of oceanic plateau mantle and the size of oceanic plateau.Three distinct subduction styles are observed:(1)steep subduction without subduction zone reorganization,favored by a small-sized(?150 km)or lowly depleted oceanic plateau;(2)breakoff of subducting slab followed by flat-slab subduction,which is conditional on a moderate-sized(?300 km)oceanic plateau with high degree depletion but relatively low strength;and(3)initiation of a new subduction zone behind the plateau(subduction zone jump),facilitated by a moderate-sized oceanic plateau with a strong depleted mantle root.The model results demonstrate that the overriding lithosphere undergoes a tectonic force change from compression to tension in ?5 Myr as the subduction zone is reorganized.This can be reflected in the crustal deformation of the overriding plate and used as an indicator to identify past subduction zone jumps.The spatial relationship predicted by the subduction zone jump models well explains the Jurassic closure event of the accretion on southern Qiangtang at?175 Ma,and the intraoceanic infant arc magmatism in the Meso-Tethys at ?165 Ma.