DIACHRONOUS INDIA-EURASIA COLLISION IN RELATION TO SLAB BREAK-OFF: KINEMATIC AND RECONSTRUCTION CONSTRAINTS ON THE SIZES AND SHAPES OF GREATER INDIA AND GREATER TIBET

In a seminal contribution van der Voo et al.(1999) identified the E-W striking Tethyan Slab remnant beneath southern India and inferred that it had broken off. The timing of slab break-off is reasonably assumed to date from the more than halving of India’s convergence velocity wrt Eurasia at ~47Ma. The relationship of the age of slab break-off to the age of initiation of the India-Eurasia collision is uncertain. Based on recent field studies there is evidence of some diachroneity of the age of initial collision as dated by the juxtaposition of thicker (>~20km) Tethyan shelf crust with the Eurasian active margin. Earliest collision is dated as post-58 Ma in the vicinity of Sandanling, where Gangdese-derived zircons in turbidites with an interbedded tuff dated at ~58 Ma conformably overlie deeper water Tethyan off-shelf sediments (DeCelles et al. 2014). Collision initiation is estimated at 56±1.5Ma (1s). The arrival of Gangdese-derived sediments in the Zhepure Shan, at ~51±1 Ma, overlying shallow shelf carbonates (Zhu et al. 2005) must be associated with the progressive subduction of Greater Indian shelf beneath Eurasia implying a very wide (>~750±350 km) Tethyan shelf. Initiation of collision farther west on the Zanskar shelf is well dated at ~51±1.5 Ma (Gaetani and Garzanti, 1991). East of Zhepure Shan the age of initiation of collision must be older than Eo-Himalayan metamorphism at 44 Ma (Aikman et al. 2008). Integrating these estimates with plate kinematic data results in a best estimate of the total areal convergence since continent-continent collision began of 9.1^+2.4/_-2.0 *10⁶ km² (2s). This areal convergence correlates with local convergence of ~2440 ⁺³⁵⁰/_-320 km at the western end of the suture since 51.0±1.5 Ma, ~3800⁺⁵³⁰/_-510 km along the central segment since 56 ± 2 Ma, ~3440⁺⁸⁹⁰/_-720 km in the eastern Himalayas since 52 ± 4 Ma, incorporating both age and rotation-related 2s uncertainties. Eastern Gondwanan reconstructions are compatible with >~half of this converged area underlain by Greater India, implying that Greater Tibet comprised the remainder. Only ~50% of this area is reconstructable today implying substantial continental crustal loss into the mantle. These data imply that slab break-off post-dates initial collision by >5Ma after >750km of syn-collisional continent-continent convergence.