LETTERS PUBLISHED ONLINE: 4 MAY 2015 | DOI: 10.1038/NGEO2418 Anomalously fast convergence of India and Eurasia caused by double subduction Oliver Jagoutz1*, Leigh Royden1, Adam F. Holt2 and Thorsten W. Becker2 Before its collision with Eurasia1–5, the Indian Plate moved The existence of this Trans-Tethyan subduction system is 1 20 rapidly, at rates exceeding 140 mm yr− for a period of 20 mil- consistent with the presence of two relict slabs below India . At lion years1,3–7. This motion is 50 to 100% faster than the maxi- the longitude of the Himalaya, palaeomagnetic data show that mum sustained rate of convergence of the main tectonic plates in the Cretaceous, rocks of the Trans-Tethyan subduction system today8. The cause of such high rates of convergence is unclear (Kohistan–Ladakh Arc and ophiolites of the Tsangpo suture zone) and not reproduced by numerical models9,10. Here we show formed near the equator18,21,22, whereas the magmatic rocks devel- that existing geological data11,12 support the existence of two, oped along the southern margin of Eurasia (Karakoram–Gangdese almost parallel, northward dipping subduction zones between Arc, labelled Km, G and M in Fig.1 ) formed at ∼20◦–25◦ N (ref. 23). the Indian and Eurasian plates, during the Early Cretaceous This indicates that the two subduction systems were separated by an period. We use a quantitative model to show that the combined approximately 1,500–3,000 km wide oceanic plate, here called the pull of two subducting slabs can generate anomalously rapid Kshiroda Plate (Fig.1 ). convergence between India and Eurasia. Furthermore, in our Before the initiation of spreading in the Indian Ocean simulations a reduction in length of the southern subduction (∼120–130 Myr ago (ref.5 )), a spreading ridge must have system, from about 10,000 to 3,000 km between 90 and existed between India and Eurasia to accommodate subduction 80 million years ago, reduced the viscous pressure between in the Neo-Tethys whereas little convergence occurred between the subducting slabs and created a threefold increase in plate the southern continents and Eurasia5. We place this ridge south convergence rate between 80 and 65 million years ago. Rapid of the Trans-Tethyan subduction system because spreading along convergence ended 50 million years ago, when the Indian Plate its eastern extension is needed north of Australia to prevent its collided with the southern subduction system. Collision of India northward motion until the Woyla and Sumatra arcs become with Eurasia and the northern subduction system had little inactive at 90 Myr. eect on plate convergence rates before 40 million years ago. The timing of subduction along the Trans-Tethyan subduction We conclude that the number and geometry of subduction system is crucial for understanding the convergence history of India systems has a strong influence on plate migration rates. and Eurasia. From the Semail ophiolite westwards, intra-oceanic The northward motion of India from the Early Cretaceous subduction ended at ∼90–80 Myr when the arc collided with the period to the Early Cenozoic era is related to the subduction of northern margin of Arabia15 (Fig.1 ). In Sumatra, intra-oceanic oceanic lithosphere north of India, which consumed the Neo- subduction ended at ∼90 Myr, with northward obduction of the Tethys Ocean13, and to seafloor spreading south of India, which Woyla Arc onto continental crust12. Andean subduction in Sumatra created the Indian Ocean14. During the Cretaceous to Early Tertiary also ended at about the same time12. period, multiple subduction systems operated within the Neo- In contrast, subduction along the central portion of the Trans- Tethys, including a north-dipping subduction boundary beneath the Tethyan subduction system continued along an approximately southern margin of Eurasia (for example, ref. 11) and, as proposed 3,000 km segment that formed the northern margin of the Indian here, a second, intra-oceanic `Trans-Tethyan' subduction system Plate (Fig.1 )17. Evidence for continued subduction until ∼50 Myr is that extended from the eastern Mediterranean to Indonesia, and well established in the western Himalaya17. New geochronological perhaps beyond (Fig.1 )12. and isotopic data from the western Himalaya17 show a ∼50 Myr The preserved geologic entities that make up this Cretaceous to collision of the Indian subcontinent with the Kohistan–Ladakh Early Tertiary `Trans-Tethyan subduction system' are, from west to Arc and a ∼40 Myr collision of the amalgamated arc-continent east: the Antalya nappes and Cyprus ophiolite of southern Turkey15; with Eurasia. This collisional history is also consistent with the peri-Arabian ophiolites in the Middle East (labelled PA in Fig.1 ) palaeomagnetic data from the Kohistan–Ladakh Arc18,21, and and the Semail ophiolite in Oman15 (labelled Sm); ophiolitic and with the timing of two major tectonic events observed in the arc sequences of western Pakistan (Bela, Waziristan ophiolites16); Indian Ocean2. the Kohistan–Ladakh Arc (labelled K) and associated ophiolites of In the central to eastern Himalaya, most of the rocks related the western Himalaya17 (labelled X); supra-subduction ophiolites, to the Trans-Tethyan subduction system are missing, but sporadic forearc sequences and oceanic mélange sequences of the Tsangpo remnants of this subduction system remain19. In particular, an intra- suture zone in the central and southern Himalaya18,19; ophiolitic and oceanic mélange sequence near Xigaze yields Palaeocene fossil ages magmatic remnants in the Andaman Islands and Sumatra (Woyla that attest to intra-oceanic subduction into the Early Tertiary18,24,25. Arc12 (W); Fig.1 ). By at least the mid-Cretaceous, this subduction Figure3 shows plate circuit data constraining the convergence boundary seems to have been everywhere north-dipping12,15 (Fig.2 ). history of India and Eurasia3,4. Because Eurasia and Antarctica 1Department of Earth, Atmospheric and Planetary Sciences, MIT, Cambridge, Massachusetts 01890, USA. 2Department of Earth Sciences, University of Southern California, Los Angeles, California 90089, USA. *e-mail: [email protected] NATURE GEOSCIENCE | VOL 8 | JUNE 2015 | www.nature.com/naturegeoscience 475 © 2015 Macmillan Publishers Limited. All rights reserved LETTERS NATURE GEOSCIENCE DOI: 10.1038/NGEO2418 a Km G Today K X PA M India Africa Sm Arabia W b c 90 Myr 80 Myr Eurasia Eurasia Km 30° N 30° N G M Kshiroda PA Kshiroda Plate Plate K Africa Africa Sm X 0° 0° W Neo-Tethyan Ocean Latitude Latitude ° ° India 30 S India 30 S WR Indian Ocean Australia Australia Antarctica 0° 60° E 120° E 0° 60° E 120° E Longitude Longitude d e 50 Myr 40 Myr Eurasia Eurasia 30° N 30° N India Africa India Africa 0° 0° CIR Latitude Latitude WR CIR WR 30° S 30° S SWIR SEIR Australia SEIR Australia SWIR Antarctica Antarctica 0° 60° E 120° E 0° 60° E 120° E Longitude Longitude Figure 1 | Present day remnants of two subduction zones, and plate tectonic reconstructions for 90–40 Myr. a, Rocks related to subduction active after 80 Myr are red (Trans-Tethyan intra-oceanic system) and yellow (arc rocks of the Cretaceous subduction system of southern Eurasia). Rocks related to subduction that terminated before 80 Myr are white (both belts). Arrows show the sense of motion along the eastern and western margins of the Indian Plate after 80 Myr. b–e, Reconstructions of Neo-Tethyan plate boundaries; active boundaries shown in red, boundaries becoming extinct in grey. Red triangles indicate subduction, plain lines indicate spreading ridges and transform boundaries. remained relatively stationary, rates of opening of the Indian To investigate the effects of double subduction and the reduction Ocean are nearly identical to rates of India–Eurasia convergence, in trench length of the subduction plate, we employ quantitative, further constraining their pre-collisional history1. Before 80 Myr, three-dimensional models of coupled double subduction (that convergence rates are not constrained by reliable sea floor is, no spreading ridges present between the subduction systems, magnetic anomalies, so we use palaeolatitude data to constrain see Supplementary Fig. e1). Methods gives brief descriptions and convergence rates23. benchmarking exercises for double subduction computed using two Figure3 shows that, contemporaneous with the reduction in methods: an extension of the semi-analytic subduction technique trench length of the Trans-Tethyan subduction system, India– described by ref. 26 (fast analytical subduction technique (FAST), Eurasia convergence rates began to increase rapidly. Simultaneously, described in detail in the Supplementary Information) and a fully the Wharton Ridge (labelled WR in Fig.1 ) transform system numerical computation (an extension of CitcomCU; for example, initiated with a dominantly right-slip sense, forming the eastern ref. 27). The results of these techniques are very similar, indicating boundary of the Indian Plate (Fig.1 ) whereas spreading and left- that the physics governing slab and plate motion is captured slip on the Southwest Indian Ridge (SWIR) and Central Indian consistently. We choose to model the India–Eurasia convergence Ridge (CIR) accommodated divergence between India, Africa using FAST, which includes three-dimensional asthenospheric flow, and Madagascar5. topographic loading, slab bending and thrust interface coupling, 476 NATURE GEOSCIENCE | VOL 8 | JUNE 2015 | www.nature.com/naturegeoscience © 2015 Macmillan Publishers Limited. All rights reserved NATURE GEOSCIENCE DOI: 10.1038/NGEO2418 LETTERS 120 Myr Kshiroda Plate Indian Plate Eurasia India 90 Myr 60 Myr Age of oceanic lithosphere (Myr) 04020 60 80 100 5,000 km Figure 2 | Cross-sections of double subduction in the Tethys region. Slab geometries and distances between subduction systems and continents correspond to results of the model discussed in the text. The sea floor ages shown are those used as initial conditions in the model (at 120 Myr) and illustrate ageing of the oceanic lithosphere as derived from the model results. The yellow and red volcanoes indicate the volcanic arcs of the Southern Eurasian Margin and the Trans Tethyan subduction system, respectively.