Plate kinematic and mantle seismic tomography constraints for the India-Asia collision

The India-Asia collision is one of the most well-studied orogenic events on Earth; it recorded the terminal stages of the eastern Tethys ocean basins and offers invaluable insight into the geological processes associated with continental collision. A variety of different models to explain the India-Asia collisions are still debated; distinctions between these models include the size and structure of Greater India, the timing of collision, and the occurrence of an earlier collision between India and an intra-oceanic arc. We consider the validity of these models with respect to plate kinematics and the deep mantle structure of subducted slabs beneath the Indian hemisphere.

Development of the user-friendly GPlates software for plate reconstruction modelling allows geological and geophysical datasets to be readily integrated within a plate kinematic framework. Likewise, comparison of global seismic tomography models of the mantle may be readily conducted using the recently developed SubMachine web-based tool. We combine these tools with a synthesis of geological and geophysical datasets from the Himalayan orogen and Tibetan plateau to investigate the validity of models for the India-Asia collision. Particular focus is given to the distribution of subducted slabs in the mantle inferred from seismic tomography that may be used to estimate the location of paleo-subduction trenches and the size and location of India at the onset of collision.

Our assessment of existing models for the India-Asia collision reveals inconsistencies between the observed and model-predicted distribution of seismically fast anomalies in the upper and lower mantle. Many of these models require slab sinking rates to vary significantly over lateral distances of <2000 km, equating to significant lateral heterogeneity in mantle viscosity. Based on these inconsistencies, we propose a forth model for the India-Asia collision in which interpretation of seismically fast mantle anomalies takes precedence over bedrock constraints. We discuss the validity of the constraints and assumptions used to build our model compared with other models, and their implications for our understanding of the bedrock record of continental collision, the geodynamics of sinking slabs, and the geological interpretation of seismically fast anomalies in the mantle.