Effects of Crustal Eclogitization on Plate Subduction/Collision Dynamics: Implications for India-Asia Collision

2D thermo-mechanical models are constructed to investigate the effects of oceanic and continental crustal eclogitization on plate dynamics at three successive stages of oceanic subduction, slab breakoff, and continental subduction. Crustal eclogitization directly increases the average slab density and accordingly the slab pull force, which makes the slab subduct deeply and steeply. Numerical results demonstrate that the duration time from initial continental collision to slab breakoff largely depends on the slab pull force. Specifically, eclogitization of subducted crust can greatly decrease the duration time, but increase the breakoff depth. The detachment of oceanic slab from the pro-continental lithosphere is accompanied with obvious exhumation of the subducted continental crust and a sharp uplift of the collision zone in response to the disappearance of downward drag force and the induced asthenospheric upwelling, especially under the condition of no or incomplete crustal eclogitization. During continental subduction, the slab dip angle is strongly correlated with eclogitization of subducted continental lower crust, which regulates the slab buoyancy nature. Our model results can provide several important implications for the Himalayan-Tibetan collision zone. For example, it is possible that the lateral variations in the degree of eclogitization of the subducted Indian crust might to some extent contribute to the lateral variations of subduction angle along the Himalayan orogenic belt. Moreover, the accumulation of highly radiogenic sediments and upper continental crustal materials at the active margin in combination with the strong shear heating due to continuous continental subduction together cause rising of isotherms in the accretionary wedge, which facilitate the development of crustal partial melting and metamorphism.

Differentiation of Continental Subduction Mode:Numerical Modeling

The convergence of the multi-layered continental lithospheres with variable and complex thermal and rheological properties results in various modes of continental collision with distinct deformation behavior of the lithospheric mantle. Using high-resolution thermo-mechanical numerical models,we systematically investigated the effects of crustal rheological strength and the convergence rate on the continental subduction mode. The model results reveal three basic modes of continental subduction,including slab break-off,steep subduction and continental flat-slab subduction. Whether lithospheric mantle of the overriding plate retreats or not during convergence enables the division of the first two modes into two sub-types,which are dominated by the crustal rheological strength. The mode of slab break-off develops under the conditions of low/moderate rheological strength of the continental crust and low convergence rate. In contrast,continental flat-slab subduction favors the strong crust and the high convergence rate. Otherwise,continental steep subduction occurs. The numerical results provide further implications for Geodynamics conditions and physical processes of different modes of continental collision that occur in nature.