Anomalously fast convergence of India and Eurasia caused by double subduction

Before its collision with Eurasia1–5, the Indian Plate moved rapidly, at rates exceeding 140mmyr−1 for a period of 20 million years1,3–7. This motion is 50 to 100% faster than the maximum sustained rate of convergence of themain tectonic plates today8. The cause of such high rates of convergence is unclear and not reproduced by numerical models9,10. Here we show that existing geological data11,12 support the existence of two, almost parallel, northward dipping subduction zones between the Indian and Eurasian plates, during the Early Cretaceous period.Weuse a quantitativemodel to show that the combined pull of two subducting slabs can generate anomalously rapid convergence between India and Eurasia. Furthermore, in our simulations a reduction in length of the southern subduction system, from about 10,000 to 3,000 km between 90 and 80 million years ago, reduced the viscous pressure between the subducting slabs and created a threefold increase in plate convergence rate between 80 and 65 million years ago. Rapid convergence ended 50million years ago, when the Indian Plate collidedwith the southern subduction system.Collisionof India with Eurasia and the northern subduction system had little e ect on plate convergence rates before 40 million years ago. We conclude that the number and geometry of subduction systems has a strong influence on plate migration rates. The northward motion of India from the Early Cretaceous period to the Early Cenozoic era is related to the subduction of oceanic lithosphere north of India, which consumed the NeoTethys Ocean13, and to seafloor spreading south of India, which created the Indian Ocean14. During the Cretaceous to Early Tertiary period, multiple subduction systems operated within the NeoTethys, including a north-dipping subduction boundary beneath the southern margin of Eurasia (for example, ref. 11) and, as proposed here, a second, intra-oceanic ‘Trans-Tethyan’ subduction system that extended from the eastern Mediterranean to Indonesia, and perhaps beyond (Fig. 1)12. The preserved geologic entities that make up this Cretaceous to Early Tertiary ‘Trans-Tethyan subduction system’ are, from west to east: the Antalya nappes and Cyprus ophiolite of southern Turkey15; the peri-Arabian ophiolites in theMiddle East (labelled PA in Fig. 1) and the Semail ophiolite in Oman15 (labelled Sm); ophiolitic and arc sequences of western Pakistan (Bela, Waziristan ophiolites16); the Kohistan–Ladakh Arc (labelled K) and associated ophiolites of the western Himalaya17 (labelled X); supra-subduction ophiolites, forearc sequences and oceanic mélange sequences of the Tsangpo suture zone in the central and southern Himalaya18,19; ophiolitic and magmatic remnants in the Andaman Islands and Sumatra (Woyla Arc12 (W); Fig. 1). By at least the mid-Cretaceous, this subduction boundary seems to have been everywhere north-dipping12,15 (Fig. 2). The existence of this Trans-Tethyan subduction system is consistent with the presence of two relict slabs below India20. At the longitude of the Himalaya, palaeomagnetic data show that in the Cretaceous, rocks of the Trans-Tethyan subduction system (Kohistan–Ladakh Arc and ophiolites of the Tsangpo suture zone) formed near the equator18,21,22, whereas the magmatic rocks developed along the southern margin of Eurasia (Karakoram–Gangdese Arc, labelled Km, G andM in Fig. 1) formed at∼20–25 N (ref. 23). This indicates that the two subduction systems were separated by an approximately 1,500–3,000 km wide oceanic plate, here called the Kshiroda Plate (Fig. 1). Before the initiation of spreading in the Indian Ocean (∼120–130Myr ago (ref. 5)), a spreading ridge must have existed between India and Eurasia to accommodate subduction in the Neo-Tethys whereas little convergence occurred between the southern continents and Eurasia5. We place this ridge south of the Trans-Tethyan subduction system because spreading along its eastern extension is needed north of Australia to prevent its northward motion until the Woyla and Sumatra arcs become inactive at 90Myr. The timing of subduction along the Trans-Tethyan subduction system is crucial for understanding the convergence history of India and Eurasia. From the Semail ophiolite westwards, intra-oceanic subduction ended at ∼90–80Myr when the arc collided with the northern margin of Arabia15 (Fig. 1). In Sumatra, intra-oceanic subduction ended at ∼90Myr, with northward obduction of the Woyla Arc onto continental crust12. Andean subduction in Sumatra also ended at about the same time12. In contrast, subduction along the central portion of the TransTethyan subduction system continued along an approximately 3,000 km segment that formed the northern margin of the Indian Plate (Fig. 1)17. Evidence for continued subduction until∼50Myr is well established in the western Himalaya17. New geochronological and isotopic data from the western Himalaya17 show a ∼50Myr collision of the Indian subcontinent with the Kohistan–Ladakh Arc and a ∼40Myr collision of the amalgamated arc-continent with Eurasia. This collisional history is also consistent with palaeomagnetic data from the Kohistan–Ladakh Arc18,21, and with the timing of two major tectonic events observed in the Indian Ocean2. In the central to eastern Himalaya, most of the rocks related to the Trans-Tethyan subduction system are missing, but sporadic remnants of this subduction system remain19. In particular, an intraoceanic mélange sequence near Xigaze yields Palaeocene fossil ages that attest to intra-oceanic subduction into the Early Tertiary18,24,25. Figure 3 shows plate circuit data constraining the convergence history of India and Eurasia3,4. Because Eurasia and Antarctica