The India-Eurasia collision and marginal basin evolution in the NW Pacific

The India-Eurasia collision is a first order event that dominated the regional tectonic evolution in the Cenozoic. The propagating stress field from the collision is thought to have induced Tibetan uplift, lateral extrusion of major continental blocks, and the opening of the South China and Japan Seas.However, the timing of the collision and the geometry of the pre-collision margins of Eurasia and Greater India remain controversial. Emerging evidence points to an early-Cretaceous back-arc along southern Eurasia in the central NeoTethys, rather than an Andean-type margin that is invoked by conventional models of the collision. Ophiolite belts in the Anatolian western NeoTethys have been linked to back-arcs, while extensive arc material is exposed in Pakistan corresponding to the Kohistan and Ladakh arc. This evidence suggests that the active southern Eurasian margin was characterized by a chain of back-arcs that likely formed during one or more episodes of back-arc creation. However, the preservation potential of back-arc material varies greatly along the margin.The existence of a back-arc along southern Eurasia would therefore fundamentally affect the nature and timing of the India-Eurasia collision, and the chronology of Tibetan crustal thickening. Such a scenario would invoke an initial collision between the leading edge of Greater India and an intra-oceanic island arc around ~55 Ma, followed by the subduction of the back-arc and the final continent-continent collision by ~40 Ma. This alternative model of the collision may help account for the ~20 Myr enigmatic time-lag between the initial contact with the island arc and major geological responses including crustal thickening and extrusion in the region.We aim to construct coupled global plate motion (GPlates) and mantle convection numerical models (CitcomS) to test the effect of varying Indian and Eurasian pre-collision margin geometries on the predicted present-day mantle structure, that can then be validated with mantle tomography. This approach will allow us to impose plate velocities and age-dependent crustal buoyancy on the uppermost thermal boundary layer, with a refined mesh resolution in the upper mantle to help track subduction zone evolution. Because our global mantle convection model is consistent with plate motion histories, we can isolate whether the chronology of the India-Eurasia collision or whether changes in the subduction style in NW Pacific better correlate with the timing of South China and Japan Sea opening.Preliminary results indicate that the back-arc scenario along southern Eurasia produces a better match between predicted and imaged present-day mantle structure. A continent-continent collision at ~40 Ma would correlate well with the distal effects of the propagating stress field leading to extrusion and rotation of east Asian blocks, and back-arc opening in the NW Pacific. Additionally, we aim to test if local effects in the NW Pacific such as changes in convergence rates, slab dip, trench rollback or slab interaction with various mantle transition zones offer a better explanation of the marginal basin evolution chronology.