The invisible double subduction can accelerate convergence rate

The Himalayan mountain range, lying between the Indian and Eurasian plates, is the symbol of inter-continental collisional belt. Paleomagnetism (Hall, 2012), the best option in order to track the past location of a land mass, suggests that the Indian continent lied on the southern hemisphere (~30° S) at 90 million years ago (Ma) and drifted northward until at around 40 Ma when the collision event begins.

Three major tectonic boundaries in plate tectonics, i.e. subduction, ridge and transform fault, are the essential tools to move a continent, and hence at least one subduction system must be operated during the drift of the Indian continent. The petrological, structural and geophysical evidence from various regions (e.g. Aitchison et al., 2000; Sengör and Stock, 2014) suggested that the actual system driven the Indian plate motion seems to be more complicated.

There are lots of things we have to consider. Existence of ridge can reduce rate of convergence, subduction zone can accelerate it. And plume also might have an effect on speed. Several factors, such as number and polarity of subduction zone or location and persistence of active ridge, are not yet consistent among the existing models (e.g. Hall, 2012; Van der Voo et al., 1999; Yin and Harrison, 2000).

Double subduction can explain anomalously fast convergence of Indian plate. Jagoutz et al. (2015) employed quantitative, three-dimensional models of double subduction. This model assumes that there is no active ridge between two subduction zones. To calculate speed of convergence, they use two quantitative modeling; by using FAST and CitcomCU. Because FAST includes more data such as three-dimensional asthenospheric flow, topographic loading, slab bending and so on.

Figure 1 shows that rates of India-Eurasia convergence of observed and model. Speed of initial stage (120~80 Ma) is very low because of high viscous pressure and existence of spreading ridge on the north of India. Second stage (80~65 Ma) have steep increase of speed by disappearing of ridge and narrowing of the subduction system. On third stage(65~Ma), increasing of viscous pressure reduces speed. And collision makes two radical point (50 Ma and 40 Ma).

This model can explain the rate of convergence of Indian plate above all. Location of active ridge, and existence of double subduction can be worth of close attention. We have to pay attention to invisible, unseen subduction on nowadays earth which can accelerate movement of plate of the past.