It Takes Two to Tango: The Timing of the India –Eurasia Collision and the Origin of the Super-Fast India-Eurasia Convergence Rates.

Abstract:

The pre-collisional convergence history of India and Eurasia displays highly variable convergence rates, including anomalously high rates of ~120-180 mm/a between ~70 and 50 Ma. As the subduction zone forces play a key role in driving plate tectonic understanding the subduction zone configuration in the Tethyan ocean is essential to understand the observed variation in convergence rates. It has long been established that two subduction systems existed between India and Eurasia since the break up of Gondwana: An intra oceanic subduction system (in the equatorial part of the ocean) and a continental subduction system on the southern Eurasian margin. As relicts of both subduction systems are now incorporated in the Himalayan and Tibetan collision zone, reconstruction of the pre-collisional geometry and the timing of the collision between India and the different subduction system is challenging. However, in recent years it has become increasingly more obvious that India likely originally collided with the intra oceanic subduction system first and only subsequently with the Eurasia margin (e.g., Aitchison et al., 2007 JGR, Bouilhol et al, 2013 EPSL). In this presentation, we summarize the geological evidence that allows us to reconstruct the configuration of the subduction system in the Tehtyan ocean and the timing of the India-Arc and India Eurasia collision. These constraints guide 3D subduction zone models that are used to evaluate the role of two subduction zones for the variable India-Eurasia Convergence rates. Model convergence rates of up to 140 mm/yr are in agreement with observed rates from 120 Ma to present, except for a short-lived spike in rate at ~62-66 Ma, which may be attributed to the Reunion plume. The end of anomalously fast convergence is due to the entry of buoyant Indian continental lithosphere into this intra-oceanic subduction zone at 50 Ma and not to the collision of India with Eurasia, which had little effect before 40 Ma.