Numerical Research On Oceanic Subduction And Continenal Collision Modes

Oceanic subduction and continental collision modes are thoroughly investigated based on sensitivity test to main geodynamic parameters, using high-resolution thermo-mechanical numerical modeling in this paper.According to the numerical results, parameters, which affect the oceanic slab dip and oceanic subduction pattern, can be classified into four main categories involving buoyancy of the subducting oceanic slab(e.g., the slab age, oceanic crustal thickness), viscous interplate coupling of the overriding with downgoing plate(e.g., the initial subduction angle, thermal structure of the overriding lithosphere), external kinematic conditions(e.g., absolute trenchward motion of the overriding plate), and rheological properties of the subduction zone(e.g., rheological properties of the asthenospheric mantle and overriding continental crust). Correspondingly, our numerical experiments indicate that the development of low-angle subduction can be attributed to four types of conditions involving positive buoyancy of the downgoing plate(resulting from subduction of the young oceanic lithosphere or thickened oceanic crust), strong plate coupling along the subduction interface(resulting from the low initial subduction angle or thick overriding continental lithosphere), high ratio of the absolute overthrusting velocity of the overriding plate to the absolute subduction velocity of the downgoing plate, and beneficial rheological properties of the subduction zone.It is revealed that every single parameter is neither sufficient nor necessary for flat subduction. In most cases, the development of flat subduction should be ascribed to a combination of “abnormal” thermo-mechanical conditions rather than one certain condition, which might account for infrequency of flat subduction on Earth. On the basis of the numerical results and the principal features of present-day zones of flat subduction that almost all regions of flat subduction occur near the spreading ridge where the oceanic slab age is less than 50 Ma, it is proposed that the positive buoyancy resulting from the subduction of either a young slab or thickened oceanic crust should be the primary controlling parameter on the development of flat subduction, although other parameters also can exert an effect on the formation of flat subduction. In order to investigate the fundamental reasons for slab dip variability, all the tested parameters were also evaluated in a mechanical framework, where slab dynamics is attributed as a balance of the gravitational torque and the hydrodynamic torque(suction torque). We find that any factors that can sufficiently reduce the gravitational torque or increase the hydrodynamic torque will exert a highly positive effect on the development of flat subduction. All the parameters we tested can be incorporated into the mechanical framework to account for the various subduction patterns through changing the gravitational torque or the hydrodynamic torque.Two types of continental subduction involving stable subduction and unstable subduction are identified based on tectonic evolution of the continental slab. Stable continental subduction is characterized by the maintenance of high effective viscosity and rheological strength of the continental slab; thus, the slab can preserve a geometric and mechanical integrity in the asthenospheric mantle during subduction. Stable continental subduction can be divided into two end-members: steep subduction and flat subduction, according to the continental slab dip angle. In contrast, since the slab viscosity decreases largely and the slab can’t preserve a geometric and mechanical integrity during unstable continental subduction, this subduction mode is featured with detachment of the subducting slab or the thickened lithospheric mantle around the subduction zone. Further, on the basis of slab deformation behaviour, unstable continental subduction can be divided into three end-members, which are mainly characterized by(1) multi-step slab breakoff,(2) flowing of the fluid-like slab into the asthenospheric mantle and(3) large-scale detachment of thickened lithospheric mantle in the collision zone. Moreover, the numerical results also show that fast continental convergence and cold thermal structure of the converging lithospheres are highly beneficial to the development of stable continental subduction, and which end-member happens is determined by the rheological strength of continental crust. Regarding the unstable continental subduction, the three end-members are positively correlated with(1) low continental convergence rate,(2) high convergence rate accompanying hot lithospheric thermal structure, and(3) high crustal rheological strength under hot lithospheric thermal structure, respectively. The unstable continental subduction always ends with large-scale detachment of thickened lithospheric mantle in the collision zone, regardless of the number of evolutionary stages and the type of earlier stages.Numerical results demonstrate that the development of continental underthrusting takes place under strict conditions, i.e. strong subducting continental lithosphere with higher convergence velocity, which may illustrate the rare underthrusting mode in the natural collision zones. The increasing strength of crust can lead to a larger amount of pro-continental crust entering the subduction channel, decreasing slab average density, and at the same time increases the slab rheological strength. In another aspect, higher convergence velocity prevents the continental slab from significantly heating and weakening, which may thereby contribute to the continental underthrusting; in contrast, low convergence velocity may easily lead to the development of Rayleigh-Taylor instabilities with the subducting slab dripping into the asthenosphere. The numerical results further provide significant implications for the India-Asia collision, which is characterized by long-distance continental underthrusting of the India plate beneath the southern Tibet. On the basis of numerical results, continental underthrusting requires strong Indian plate and high convergence velocity, both of which are consistent with the general geological observations. In addition, the numerical modeling also emphasizes that high thermal structure and resultant weak Tibetan lithosphere is required for significant shortening during Indian/Asian collision, which also conforms with acknowledged geological evolution of Asia before collided with India.It is also worth noting that two types of continental underthrusting can be identified on the basis of numerical models in this study. As for the first type, the downgoing continental lithosphere horizontally subducts for a long distance beneath the overriding lithospheric mantle. Regarding the second type, the downgoing continental lithosphere directly underlies the retro-side crust, which is decoupled from the subjacent lithospheric mantle by delamination or detachment. The former type is associated with strong overriding lithosphere, which can resist significant deformation and preserve its integration during the continental collision. The latter highly correlates with weak overriding lithosphere owing to low crustal rheological strength or hot thermal structure, which thereby can greatly contribute to the delamination of retro-side lithospheric mantle from the overlying crust and then facilitate development of continental underthrusting directly beneath the overriding crust. For both types, strong subducting continental lithosphere and high convergence velocity are required. Geophysical data also reveals that tectonic evolution of southern Tibet maintains broadly consistency with the second type of continental underthrusting, a scenario that the continuous northward drift of Indian continental lithosphere causes the lithospheric mantle of Asia to be delaminated from the overlying crust and subsequently makes the Indian plate underthrusted directly beneath the Asian continental crust. The processes are accompanied with the widespread develop of partial melting in the middle/lower crust, the significant thickening of Tibetan crust, as well as southward extrusion of GHS. Finally, based on various numerical results related to crustal strength, lithospheric thermal structure, convergence velocity and the tectonic structure of overriding lithosphere, it is proposed that the lateral variations of subduction-related mantle structure along the Himalayan belt might be correlated with various factors including the lateral variations of rheological properties of the Indian lithosphere, thermal structure of the overriding Asia lithosphere, convergence velocity and lithospheric structure in northern Tibet.