Numerical Study Of Geodynamic Effects Of Seamounts During Single-sided Continental Overthrusting At The Manila Trench

2D numerical modelings were used to investigate the geodynamic effects of seamounts during single-sided continental overthrusting at the Manila Trench.According to the constraints of the geological setting to Manila Trench,e.g.convergence rate of Philippine Sea Plate,the age of the subducting oceanic plate,the lithospheric rheology,the size of the subducting seamount etc.A reference model was conducted as well as different series of numerical modeling experiments using different parameters,including activation volume(Va)and cohesion(C),which influence lithospheric rheology,plate convergence velocity,age of subducting slab and without seamount subducting.Modeling results indicate that:(1)under oceanic crust active subducting mode,the slab behave continuous subducting with a high degree angle without slab break-off occurring;while under continental overthrusting,the slab break-off is more likely to occur at the break-off depth is between 100-300 km.(2)when continental overthrusting,varying the Va and C within an appropriate range have limited effects on the geodynamic process of subduction.A lower Va allows the slab to sink more easily and results in a steeper dip angle.(3)the break-off will occur no matter the seamount is exist or not when continental overthrusting,while the existence of a seamount further promotes the break-off process.The break-off position is focus on the front or back margin of the seamount,where the extensional stress accumulate.Meanwhile,without seamount subduction will result in a 2 Myrs delay in break-off timing.Further research indicates that there are two major factors controlling the slab break-off time and depth:the convergence rate of two plate and the oceanic crust age.A large rate overthrusting of the continent would increase the amount of extensional stresses within the slab by pulling the upper portion ocean-ward.Therefore,the tectonic stresses tend to accumulate within the downgoing plate,causing necking and subsequent slab break-off.A 500 km length of the subducted is required for the slab breaking-off,the position is near front margin or back margin of the seamount at the depth of 100-300 km.Decreasing the continental overthrusting rate,the break-off time is much later and even forming slab necking instead of slab break-off(1 cm/yr).Slab age is another factor controlling break-off,whereas break-off time increases with slab age(<60Ma).While the age of slab is older than 60 Ma,which is colder and stronger,will insult steady subduction.Compared the reference model with the Zhenbei-Huangyan seamount subduction at Manila Trench,the results indicate that the low-velocity zone under the Manila Trench at 170 N is the result of a break-off event due to subduction of the Zhen-Huangyan Seamount Chain.Further to the north,such as at 19°N,the absence of seamount subduction and an older oceanic crust should favor a delay in break-off.