Plate kinematic and mantle seismic tomography constraints for the India-Asia collision, Part II: Insights from Myanmar to Sulawesi

The India-Asia collision is one of the most globally significant tectonic events of the Cenozoic era. It is widely cited as providing a unique natural laboratory for studying collisional tectonics, offering invaluable insights of processes associated with continental collision across a multitude of scales. Yet despite its importance, significant debate continues to surround the validity of three mutually exclusive models to explain the India-Asia collision. These include: (1) the single subduction model; (2) the double subduction model; and (3) the Greater India Basin hypothesis. In our recent review (Parsons et al. 2020, Earth-Science Reviews) we demonstrated that available constraints from the Himalayan orogen and Tibetan plateau, including tomographic analysis of subducted slabs beneath these regions, are unable to robustly define the relative likeliness of each model. In this contribution, we expand upon the work of Parsons et al. (2020), with geological, geophysical, and plate kinematic constraints from the southern Eurasian margin between Myanmar and Sulawesi.

Our analysis focuses on the interpretation of subducted oceanic lithosphere beneath Myanmar to Sulawesi and includes a cross sectional area-based restoration of actively subducting India-Australia plate oceanic lithosphere. Our results provide a new restoration for the southern Eurasian margin and the India-Australia plate boundary (the Wharton ridge) during the India-Asia collision. Our integration of plate kinematic constraints with tomographic interpretation of subducted slabs suggests that the plate boundary between the Indian continent and southern Tibet migrated ~1000-2000 km northwards during collision. This includes ~1000 km lateral migration of subducted Indian plate oceanic lithosphere, now imaged beneath northern India and southern Tibet.

Our reconstruction proposes that northward migration of the India-Tibet suture and subducted Indian plate oceanic lithosphere initiated at ~43 Ma and reflects a major plate network reorganisation event. At this time, “hard collision” of the Indian continent with southern Eurasia occurred synchronously with (1) reduction in Indian plate velocity; (2) cessation of the Wharton ridge and coupling of the Indian and Australian plates; (3) subduction initiation of Australian plate oceanic lithosphere beneath southeast Eurasia, and onset of northeast migration of the Australian continent; (4) accelerated ocean spreading between Australia and Antarctica; and (5) southwest ridge jump of the Central India spreading ridge. Buoyancy of the Indian continent kept it afloat, whilst oceanic lithosphere to the east continued to drive wholesale motion of the coupled India-Australia plate. This forced the Indian continent to migrate northwards, dragging the subducted Indian plate oceanic slab with it, which effectively unzipped the coupled India-Australia plate along the extinct Wharton ridge, during subduction.

Our findings are most consistent with models (2) and (3), which are characterised by two collisions, the latter of which occurred between India and the Eurasian margin at ~45-40 Ma. More generally, our study demonstrates how changes in the balance of forces within a plate network, caused by events such as continental collision, can lead to significant plate network reorganisations. Such events can have dramatic effects on the position and geometry of subducted slabs and should be considered when interpreting plate restorations from deep-mantle structure.