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The onset of timing and the model of the India-Eurasia collision are strongly debated based on continental works, without systematic marine tectonic constraints. Two aseismic ridges, the Laccadives-Maldives-Chagos Ridge (LMCR) formed after the Deccan volcanism in western India and the Ninetyeast Ridge (NER) formed after the emplacement of the Rajmahal Traps in eastern India are prominent features in the Northeast Indian Ocean. The onset of a major and steady increase in magma production rates along the aseismic ridge associated with the significant slowdown of the northward moving Indian Plate is a likely indicator of the onset of the India-Eurasia collision. Using gravity-derived crustal thickness, we calculated the magma production rates along the LMCR and the NER, respectively. The steady increasing magma production rate along the LMCR and a southwestward jump of the Central Indian Ridge constrained the "soft" (India-island arc) and "hard" (India-Eurasia) collisions between western India and Eurasia to 50–52 Ma and ~41 Ma, respectively. The steady increasing magma production rate along the NER and the formation of the Mammerickx Microplate constrained the soft collision between eastern India and Eurasia to 47–49 Ma, and the extinction of the Wharton Ridge constrained the hard collision between eastern India and Eurasia to 38 Ma. Both the soft and hard collisions between western India and Eurasia are ~3 Myr earlier than the soft and hard collisions between eastern India and Eurasia, respectively. With systematic marine tectonic constraints, a two-stage eastward diachronous model of the India-Eurasia collision was proposed. [Display omitted] • Soft collision between western India and Eurasia was constrained to 50–52 Ma. • Soft collision between eastern India and Eurasia was constrained to 47–49 Ma. • Hard collision between western India and Eurasia was constrained to ~41 Ma. • Hard collision between eastern India and Eurasia was constrained to ~38 Ma. • A two-stage, eastward diachronous India-Eurasia collisional model was preferred. [ABSTRACT FROM AUTHOR]

Field observations, petrology, zircon geochronology, whole-rock geochemistry, and Sr-Nd isotopes were used to reveal the lithology, age, and tectonic setting of the protoliths of the Sangsang mafic schists in the Yarlung Zangbo Suture Zone (YZSZ), central southern Tibet. The mafic schists occur as exotic blocks within the accretionary complex of the YZSZ. Relic amygdaloidal features indicate the schist protoliths were volcanic rocks. The mineral assemblage mainly comprises riebeckite + magnesioriebeckite + chlorite + sericite + albite + relic clinopyroxene. The youngest group of zircon ages constrains the formation time of the protoliths to 149.2 ± 2.2 Ma (i.e., latest Jurassic). Abundant Paleozoic and older zircons suggest the protolith volcanic rocks were erupted onto a continental terrane. Whole-rock geochemical and Sr-Nd isotopic data indicate the protoliths were of ocean island basalt affinity. The mafic schists mostly have high-Ti, alkaline, basaltic compositions with 43.57–46.93 wt% SiO 2 , 3.27–7.24 wt% Na 2 O + K 2 O, and 4.04–4.69 wt% TiO 2. The schists are enriched in light rare earth elements relative to heavy rare earth elements, and have (La/Yb) N = 5.85–8.53 and small positive Ta and negative Zr-Hf anomalies. (87Sr/86Sr) i values vary from 0.7044 to 0.7055, while (143Nd/144Nd) i ranges from 0.512670 to 0.512727, with εNd(t) values of +4.4 to +5.5. The protoliths of the mafic schists were probably formed in a within-plate extension setting associated with mantle plume upwelling and melting of continental lithosphere. This setting was related to the Late Jurassic continental breakup of Argoland off the northern margin of east Gondwana, and thus marked the paleoposition of the northern edge of Greater India before the breakup of India from east Gondwana during Early Cretaceous. The N-S extent of Greater India at that time was ~2400 km. This further indicates how the India–Eurasia continental convergence has been accommodated. Unlabelled Image • Sangsang mafic schists in southern Tibet are metamorphosed OIB-type rocks. • The protoliths formed during latest Jurassic continental breakup of east Gondwana. • The N S extent of Greater India is defined to be ~2400 km. [ABSTRACT FROM AUTHOR]

Abstract The Spongtang Massif is a remnant of Neotethyan ocean crust emplaced onto the Indian passive margin along the Indus-Yarlung-Tsangpo Suture in the NW Himalayan region of Ladakh. The age, tectonic evolution and timing of ophiolite obduction are critical to our understanding of the mechanisms via which entire oceans are formed, consumed and partly preserved before the onset of terminal continent-continent collisions. Geochemistry of the gabbro and basaltic units suggest the presence of both MORB-type and primitive arc-related mafic rocks. Zircons extracted from the Spongtang Massif gabbros yield U-Pb (SHRIMP) ages of 136–133 Ma with initial ε Hf values of +14 to +16, indicating Early Cretaceous juvenile, depleted mantle sources devoid of contamination by older continental crust. Previously, Middle Jurassic (~177 Ma) zircon ages were obtained from gabbro and we suggest these represent MORB-type Neotethyan oceanic crust through which a younger intra-oceanic island-arc (Spong arc) developed in response to subduction initiation during the Early Cretaceous (~136 Ma). Our zircon ages are consistent with Early Cretaceous ages obtained for radiolarian cherts within the Spong Arc complex. Subduction beneath the Spong Arc continued until its collision with the northern Indian continental margin during the early Eocene. We suggest that the Spongtang Massif is equivalent to the nearby Dras island arc terrane. Intra-oceanic subduction beneath this system was possibly initiated along NNE-SSW trending transform faults in the Neotethyan Ocean, along which different ages of ocean crust was juxtaposed, thereby development of the Early Cretaceous Spong Arc is superimposed on the older Jurassic Spongtang N-MORB crust. The juvenile ɛ Hf signature indicates the subduction system that spawned the Spong island arc was not related to the coeval Trans-Himalayan (Ladakh-Gangdese) arc that developed along the southern margin of Eurasia. The age, composition and nature of geological relationships with the underlying Indian rocks indicate the Spong Arc was a juvenile, intra-oceanic terrane that first collided with India before the onset of final continent-continent collision. Therefore, final late Eocene Neotethys closure was between the Kohistan-Ladakh (Eurasian) continental arc and the already inactive Indian + Spongtang margin. Graphical abstract Unlabelled Image Highlights • Gabbros from Spongtang ophiolite yield U-Pb zircon (SHRIMP) Early Cretaceous age of ~136 Ma. • Initial ε Hf zircon values of +14 to +16 indicate juvenile source devoid of older continental crust. • Early Cretaceous Spong Arc superimposed on older Jurassic N-MORB crust. • Spongtang ophiolite-Spong arc first collided with India before final continental collision. [ABSTRACT FROM AUTHOR]

The Late Cretaceous location of the Lhasa Terrane is important for constraining the onset of India-Eurasia collision. However, the Late Cretaceous paleolatitude of the Lhasa Terrane is controversial. A primary magnetic component was isolated between 580 °C and 695 °C from Upper Cretaceous Jingzhushan Formation red-beds in the Dingqing area, in the northeastern edge of the Lhasa Terrane, Tibetan Plateau. The tilt-corrected site-mean direction is D s / I s = 0.9°/24.3°, k = 46.8, α 95 = 5.6°, corresponding to a pole of Plat. / Plon. = 71.4°/273.1°, with A 95 = 5.2°. The anisotropy-based inclination shallowing test of Hodych and Buchan (1994) demonstrates that inclination bias is not present in the Jingzhushan Formation. The Cretaceous and Paleogene poles of the Lhasa Terrane were filtered strictly based on the inclination shallowing test of red-beds and potential remagnetization of volcanic rocks. The summarized poles show that the Lhasa Terrane was situated at a paleolatitude of 13.2° ± 8.6°N in the Early Cretaceous, 10.8° ± 6.7°N in the Late Cretaceous and 15.2° ± 5.0°N in the Paleogene (reference point: 29.0°N, 87.5°E). The Late Cretaceous paleolatitude of the Lhasa Terrane (10.8° ± 6.7°N) represented the southern margin of Eurasia prior to the collision of India-Eurasia. Comparisons with the Late Cretaceous to Paleogene poles of the Tethyan Himalaya, and the 60 Ma reference pole of East Asia indicate that the initial collision of India-Eurasia occurred at the paleolatitude of 10.8° ± 6.7°N, since 60.5 ± 1.5 Ma (reference point: 29.0°N, 87.5°E), and subsequently ~ 1300 ± 910 km post-collision latitudinal crustal convergence occurred across the Tibet. The vast majority of post-collision crustal convergence was accommodated by the Cenozoic folding and thrust faulting across south Eurasia. [ABSTRACT FROM AUTHOR]

Despite several decades of investigations, inferences on the timing and nature of collisions along the Mesozoic–Cenozoic Eurasian margin remain controversial. We assimilate geological and geophysical evidence into a plate tectonic model for the India–Eurasia collision that includes continuously-closing topological plate polygons, constructed from a time-dependent network of evolving plate boundaries, with synthetic plates constructed for now-subducted ocean floor, including back-arc basins that formed on the southern Eurasian margin. Our model is regionally-constrained and self-consistent, incorporating geophysical data from abyssal plains offshore West Australia and East Antarctica, including Jurassic age data from offshore Northwest Australia, limiting much of northern Greater India to a ~ 1000 km-long indenter, originally reaching to the Wallaby–Zenith Fracture Zone. Southern Eurasia and Southeast Asia are riddled with dismembered oceanic arcs indicating long-lived Tethyan intra-oceanic subduction. This intra-oceanic subduction system was well-established from Cretaceous time in the India–Eurasia convergence zone in the NeoTethys, which was consumed during Greater India's northward trajectory towards Eurasia from the Early Cretaceous. Fragments of obducted oceanic crust within the Yarlung-Tsangpo Suture Zone, between India and Eurasia, predominantly date to the Late Jurassic or mid Cretaceous (Barremian–Aptian). The various ophiolites along strike and a hiatus in subduction-related magmatism during the Tithonian–Aptian suggest that there was at least one generation of intra-oceanic arc formation, whose plate boundary configuration remains uncertain. Paleomagnetic and magmatic studies suggest that the intra-oceanic arc was at equatorial latitudes during the Early Cretaceous before subduction resumed further north beneath the Eurasian margin (Lhasa terrane), with another hiatus in subduction-related magmatism along southern Lhasa during ~ 80–65 Ma, possibly as the back-arc spreading centre approached the active Andean-style margin. In our model, Greater India collided with the Tethyan intra-oceanic arc in Paleocene–Eocene time, finally closing the Tethyan seaway from Mid-Late Eocene time, which is consistent with the age of the youngest marine deposits found between India and Eurasia. Geological evidence from the collision zone indicates an age of initial arc–continent collision by ~ 52 Ma, followed by the “soft” (initial) continent–continent collision between India and Eurasia by ~ 44 ± 2 Ma. This timing is supported by marine geophysical data, where the spreading centres in the Indian Ocean record a drastic decline in seafloor spreading rates and changes in spreading directions first at ~ 52 Ma, followed by another reorganisation at ~ 43 Ma. The abandonment of spreading in the Wharton Basin and the onset of extrusion tectonics in Asia by ~ 36 Ma are likely indicators of “hard” (complete) continent collision, and highlight the multi-stage collisional history of this margin. Our continuously evolving network of mid-ocean ridge and subduction zone geometries, and divergence/convergence vectors through time, provide a basis for future refinements to assimilate new data and/or test alternative tectonic scenarios. [ABSTRACT FROM AUTHOR]

Evidence for episodic magmatism is found within large batholiths of ancient and modern convergent margin settings and is often attributed to regional geodynamic changes such as an increase in plate convergence rate, lithospheric extension and/or delamination, and intra-crustal/lithospheric shortening. The nature and timing of collision between India and Eurasia remain contentious as many models suggest that the “hard” collision occurred during the Early Paleogene (65 to 45 Ma) whereas other models suggest younger ages (~ 34 Ma) or a diachronous collision. New zircon LA-ICPMS U/Pb ages from rocks of the Ladakh Batholith range from 47.7 ± 0.7 Ma to 57.6 ± 0.7 Ma and correspond to two main episodes of magmatism. The two major episodes of magmatism are correlative to changes in the convergence rate of the Indian and Eurasian plates. The formation of adakitic rocks within the Ladakh Batholith at 49.2 ± 1.2 Ma is facilitated by partial melting of a garnet-bearing thickened lower crust. Therefore we suggest that crustal thickening was initiated by the “hard” collision between India and Eurasia. The convergence slowdown is concurrent with an increase in magmatism within the Ladakh Batholith. Thus the collision is constrained by the decreasing convergence rates at ~ 52 Ma and formation of the adakitic rocks at ~ 49 Ma. [ABSTRACT FROM AUTHOR]