The India-Asia collision zone is the archetype to calibrate geological responses to continent-continent collision, but hosts a paradox: there is no orogen-wide geological record of oceanic subduction after initial collision around 60-55 Ma, yet thousands of kilometers of post-collisional subduction occurred before the arrival of unsubductable continental lithosphere that currently horizontally underlies Tibet. Kinematically restoring incipient horizontal underthrusting accurately predicts geologically estimated diachronous slab break-off, unlocking the Miocene of Himalaya-Tibet as a natural laboratory for unsubductable lithosphere convergence. Additionally, three endmember paleogeographic scenarios exist with different predictions for the nature of post-collisional subducted lithosphere but each is defended and challenged based on similar data types. This paper attempts at breaking through this impasse by identifying how the three paleogeographic scenarios each challenge paradigms in geodynamics, orogenesis, magmatism or paleogeographic reconstruction and identify opportunities for methodological advances in paleomagnetism, sediment provenance analysis, and seismology to conclusively constrain Greater Indian paleogeography. The orogen at the India–Asia continental collision zone is the archetype to calibrate the relationships between collision, orogenic architecture, history, and dynamics, resulting magmatism and mineralization, as well as climatic and biological responses [ 2–6 ]. But long-standing paradoxes and controversies in tectonic history have led to an impasse, making using the full potential of the archetype difficult. Geophysical imaging has revealed that Indian continental lithosphere has horizontally underthrust the Tibetan upper plate [ 7–12 ]. Paradoxically, the youngest unequivocal geological records of plate-boundary-wide oceanic subduction between India and Asia are older than 60 Ma [ 16–18 ], after which >4000 km of India–Asia plate convergence occurred [ 19 , 20 ]. So between the geologically recorded collision and the onset of horizontal underthrusting of Indian lithosphere, thousands of kilometers of post-collisional subduction occurred. This paradox is not readily explained by dynamic models of continental collision. The first endmember predicts that all post-collisional subduction consumed continental lithosphere [ 18 , 22 , 23 ] and the second and third infer that after initial collision, oceanic lithosphere remained to the north [ 6 , 24–27 ], or to the south [ 9 , 28 ] of the initial In summary, the paradox of the India–Asia collision is the following: there is no geological record of oceanic subduction that spanned the width of the orogen after initial collision ∼60 Ma and the system is therefore widely believed to have been fully continental since this time [ 11 , 22 , 23 ]; yet thousands of kilometers of Indian plate lithosphere was consumed without leaving an accretionary record and subducted deeply into the mantle, which are both typically associated with oceanic subduction and not previously demonstrated for continents [ 30 ]. Only the Indian plate lithosphere that arrived in the collision zone in the Early to Middle Miocene did not steeply subduct, but instead horizontally underthrusted below the upper plate. The controversy: scenarios for Indian plate paleogeography and subduction history The above paradox has led to paleogeographic reconstructions for post-collisional Greater India that fall into three classes (Fig. 4 ). The first and most commonly portrayed scenario (Model C, for Continental) assumes that all post-collisional convergence consumed continental lithosphere [ 18 , 22 , 23 , 40 ]. The constraints: architecture and evolution of the Tibetan–Himalayan orogen Elements of the Himalayan and Tibetan orogen that play a key role in the interpretations of its tectonic history since 60 Ma are: (i) the accretionary fold-thrust belt of the Himalaya that was offscraped from now-underthrust/subducted Indian plate lithosphere; (ii) a belt of overlying ophiolites, and in the west of the collision zone, Cretaceous–Eocene intra-oceanic arc rocks that represent the upper plate of an overriding oceanic lithosphere above a subduction zone; and (iii) continental crust of the Tibetan Plateau that consists of pre-Cenozoic accreted terranes and intervening sutures, intruded by a Mesozoic–Cenozoic magmatic arc that also shows it was in an upper-plate position above a subduction zone (Fig. 4 ), only Model A (intra-oceanic arc) could allow for this scenario: in that case, Early Eocene burial of the Greater Himalaya follows upon obduction and activation of the MCT would reflect final collision of the obducted margin with Tibet—but this would require a diachronous Miocene collision age, instead of the proposed collision ages of 40 ± 5 Ma. All other scenarios require that a subduction plate boundary (intra-continental, or ocean-below continent) existed within the Himalaya. The transition from subduction to horizontal underthrusting in Model M is simply caused by the change from oceanic to continental subduction. But Model M invokes that the anomalous magmatic history of Tibet between 45 Ma and the 25-Ma onset of horizontal underthrusting occurred during oceanic (perhaps flat-slab [ 9 , 86 ]) subduction and would thus allow calibrating possible magmatic arc expressions of anomalous oceanic subduction. Between initial collision recorded in the Himalaya at 60 Ma and the onset of horizontal Indian underthrusting, thousands of kilometers of subduction consumed Indian plate lithosphere. Three endmember scenarios invoke that all or part of this lithosphere was continental, challenging geodynamic and paleogeographic reconstruction paradigms, or that most of this lithosphere was oceanic, challenging magmatic and orogenic architecture paradigms. But an impasse is reached because each of these reconstructions is argued for and against based on the same data types.

the oceanic lithosphere and some continental lithosphere is recycled into the Earth's mantle at the convergent boundaries between tectonic plates. Where Subduction is a geological process in which the oceanic lithosphere and some continental lithosphere is recycled into the Earth's mantle at the convergent boundaries between tectonic plates. Where one tectonic plate converges with a second plate, the heavier plate dives beneath the other and sinks into the mantle. A region where this process occurs is known as a subduction zone, and its surface ex Subductio… Subduction zone physics: Sinking of the oceanic lithosphere (sediments, crust, mantle), by the contrast of density between the cold and old lithosphere and the hot asthenospheric mantle wedge, is the strongest force (but not the only one) needed to drive plate motion and is the dominant mode of mantle convection. Subduction zone chemistry: The subducted sediments and crust dehydrate and release water-rich (aqueous) fluids into the overlying mantle, causing mantle melting and fractionation of elements between the surface and deep mantle reservoirs, producing island arcs and continental crust. Hot fluids in subduction zones also alter the mineral compositions of the subducting sediments and potentially the habitability of the sediments for microorganisms. Subduction zones drag down subducted oceanic sediments, oceanic crust, and mantle lithosphere that interact with the hot asthenospheric mantle from the over-riding plate to produce calc-alkaline series melts, ore deposits, and continental crust. Subduction zones pose significant threats to lives, property, economic vitality, cultural and natural resources, and quality of life. The tremendous magnitudes of earthquakes and volcanic eruptions can also have knock-on effects with global impact. Subduction zones have also been considered as possible disposal sites for nuclear waste in which the action of subduction itself would carry the material into the planetary Subduction is a geological process in which the oceanic lithosphere and some continental lithosphere is recycled into the Earth's mantle at the convergent boundaries between tectonic plates. Where one tectonic plate converges with a second plate, the heavier plate dives beneath the other and sinks into the mantle. A region where this process occurs is known as a subduction zone, and its surface expression is known as an arc-trench complex. The process of subduction has created most of the Earth's continental crust. Rates of subduction are typically measured in centimeters per year, with rates of convergence as high as 11 cm/year. Oceanic lithosphere ranges in thickness from just a few km for young lithosphere created at mid-ocean ridges to around 100 km (62 mi) for the oldest oceanic lithosphere. Continental lithosphere is up to 200 km (120 mi) thick. The lithosphere is relatively cold and rigid compared with the underlying asthenosphere, and so tectonic plates move as solid bodies atop the asthenosphere. Individual plates often include both regions of the oceanic lithosphere and continental lithosphere. Subduction zones are where cold oceanic lithosphere sinks back into the mantle and is recycled. Both models can eventually yield self-sustaining subduction zones, as the oceanic crust is metamorphosed at great depth and becomes denser than the surrounding mantle rocks. The compilation of subduction zone initiation events back to 100 Ma suggests horizontally-forced subduction zone initiation for most modern subduction zones, which is supported by results from numerical models and geologic studies. Some analogue modeling shows, however, the possibility of spontaneous subduction from inherent density differences between two plates at specific locations like passive margins and along transform faults. There is evidence this has taken place in the Izu-Bonin-Mariana subduction system. Only with additional weaking of the crust, through hotspot magmatism or extensional rifting, would the crust be able to break from its continent and begin subduction. === End of subduction === Subduction can continue as long as the oceanic lithosphere moves into the subduction zone. However, the arrival of buoyant continental lithosphere at a subduction zone can result in increased coupling at the trench and cause plate boundary reorganization. The arrival of continental crust results in continental collision or terrane accretion that may disrupt subduction. Continental crust can subduct to depths of 250 km (160 mi) where it can reach a point of no return. Below ~1700 km, there might be a limited acceleration of slabs due to lower viscosity as a result of inferred mineral phase changes until they approach and finally stall at the core–mantle boundary. Here the slabs are heated up by the ambient heat and are not detected anymore ~300 Myr after subduction. === Orogeny === Orogeny is the process of mountain building. Subducting plates can lead to orogeny by bringing oceanic islands, oceanic plateaus, sediments and passive continental margins to convergent margins. The material often does not subduct with the rest of the plate but instead is accreted to (scraped off) the continent, resulting in exotic terranes. As a passive margin is pulled into a subduction zone by the attached and negatively buoyant oceanic lithosphere, the sedimentary and volcanic cover is mostly scraped off to form an orogenic wedge. An orogenic wedge is larger than most accretionary wedges due to the volume of material there is to accrete. The continental basement rocks beneath the weak cover suites are strong and mostly cold, and can be underlain by a >200 km thick layer of dense mantle. After shedding the low density cover units, the continental plate, especially if it is old, goes down the subduction zone. From the Seafloor to the Volcano's Top Video about the work of the Collaborative Research Center (SFB) 574 Volatiles and Fluids in Subduction Zones in Chile by GEOMAR I

Oceanic lithosphere is typically about 50–100 km thick - Continental lithosphere, which is associated with continental crust. Continental lithosphere has a range in thickness from about 40 km to perhaps 200 km, of which about 40 km is crust. The lithosphere is divided into tectonic plates, which move gradually relative to one another. Oceanic lithosphere thickens as it ages and moves away from the mid-ocean ridge. This thickening occurs by conductive cooling, which converts hot asthenosphere into lithospheric mantle, and causes the oceanic lithosphere to become increasingly dense with age. Oceanic lithosphere is less dense than asthenosphere for a few tens of millions of years, but after this, it becomes denser than asthenosphere. When a continental plate comes together with an oceanic plate at a subduction zone, the oceanic lithosphere always sinks beneath the continental. New oceanic lithosphere is constantly being produced at mid-ocean ridges and is recycled back to the mantle at subduction zones. The lithosphere can be divided into oceanic and continental lithosphere. Oceanic lithosphere is associated with oceanic crust (having a mean density of about 2.9 grams per cubic centimetre or 0.10 pounds per cubic inch) and exists in the ocean basins. Continental lithosphere is associated with continental crust (having a mean density of about 2.7 grams per cubic centimetre or 0.098 pounds per cubic inch) and underlies the continents and continental shelves. Oceanic lithosphere consists mainly of mafic crust and ultramafic mantle (peridotite) and is denser than continental lithosphere. Young oceanic lithosphere, found at mid-ocean ridges, is no thicker than the crust, but oceanic lithosphere thickens as it ages and moves away from the mid-ocean ridge. The oldest oceanic lithosphere is typically about 140 kilometres (87 mi) thick. This thickening occurs by conductive cooling, which converts hot asthenosphere into lithospheric mantle and causes the oceanic lithosphere to become increasingly thick and dense with age. In fact, oceanic lithosphere is a thermal boundary layer for the convection in the mantle. The thickness of the mantle part of the oceanic lithosphere can be approximated as a thermal boundary layer that thickens as the square root of time. Here, h {\displaystyle h} is the thickness of the oceanic mantle lithosphere, κ {\displaystyle \kappa } is the thermal diffusivity (approximately 1.0×10−6 m2/s or 6.5×10−4 sq ft/min) for silicate rocks, and t {\displaystyle t} is the age of the given part of the lithosphere. The age is often equal to L/V, where L is the distance from the spreading centre of the mid-ocean ridge, and V is the velocity of the lithospheric plate. Oceanic lithosphere is less dense than asthenosphere for a few tens of millions of years but after this becomes increasingly denser than asthenosphere. While chemically differentiated oceanic crust is lighter than asthenosphere, thermal contraction of the mantle lithosphere makes it denser than the asthenosphere. The gravitational instability of mature oceanic lithosphere has the effect that at subduction zones, oceanic lithosphere invariably sinks underneath the overriding lithosphere, which can be oceanic or continental. New oceanic lithosphere is constantly being produced at mid-ocean ridges and is recycled back to the mantle at subduction zones. As a result, oceanic lithosphere is much younger than continental lithosphere: the oldest oceanic lithosphere is about 170 million years old, while parts of the continental lithosphere are billions of years old. Geophysical studies in the early 21st century posit that large pieces of the lithosphere have been subducted into the mantle as deep as 2,900 kilometres (1,800 mi) to near the core-mantle boundary, while others "float" in the upper mantle. Yet others stick down into the mantle as far as 400 kilometres (250 mi) but remain "attached" to the continental plate above, similar to the extent of the old concept of "tectosphere" revisited by Jordan in 1988. Subducting lithosphere remains rigid (as demonstrated by deep earthquakes along Wadati–Benioff zone) to a depth of about 600 kilometres (370 mi). Continental lithosphere has a range in thickness from about 40 kilometres (25 mi) to perhaps 280 kilometres (170 mi); the upper approximately 30 to 50 kilometres (19 to 31 mi) of typical continental lithosphere is crust. The crust is distinguished from the upper mantle by the change in chemical composition that takes place at the Moho discontinuity. The oldest parts of continental lithosphere underlie cratons, and the mantle lithosphere there is thicker and less dense than typical; the relatively low density of such mantle "roots of cratons" helps to stabilize these regions. Because of its relatively low density, continental lithosphere that arrives at a subduction zone cannot subduct much further than about 100 km (62 mi) before resurfacing. As a result, continental lithosphere is not recycled at subduction zones the way oceanic lithosphere is recycled. Instead, continental lithosphere is a nearly permanent feature of the Earth.

We review the present state of our understanding of mantle convection with respect to geochemical and geophysical evidence and we suggest a model for mantle convection and its evolution over the Earth's history that can reconcile this evidence. Whole-mantle convection, even with material segregated within the D" region just above the core-mantle boundary, is incompatible with the budget of argon and helium and with the inventory of heat sources required by the thermal evolution of the Earth. We show that the deep-mantle composition in lithophilic incompatible elements is inconsistent with the storage of old plates of ordinary oceanic lithosphere, i.e. with the concept of a plate graveyard. Isotopic inventories indicate that the deep-mantle composition is not correctly accounted for by continental debris, primitive material or subducted slabs containing normal oceanic crust. Seismological observations have begun to hint at compositional heterogeneity in the bottom 1000 km or so of the mantle, but there is no compelling evidence in support of an interface between deep and shallow mantle at mid-depth. We suggest that in a system of thermochemical convection, lithospheric plates subduct to a depth that depends - in a complicated fashion - on their composition and thermal structure. The thermal structure of the sinking plates is primarily determined by the direction and rate of convergence, the age of the lithosphere at the trench, the sinking rate and the variation of these parameters over time (i.e. plate-tectonic history) and is not the same for all subduction systems. The sinking rate in the mantle is determined by a combination of thermal (negative) and compositional buoyancy and as regards the latter we consider in particular the effect of the loading of plates with basaltic plateaux produced by plume heads. Barren oceanic plates are relatively buoyant and may be recycled preferentially in the shallow mantle. Oceanic plateau-laden plates have a more pronounced negative buoyancy and can more easily founder to the very base of the mantle. Plateau segregation remains statistical and no sharp compositional interface is expected from the multiple fate of the plates. We show that the variable depth subduction of heavily laden plates can prevent full vertical mixing and preserve a vertical concentration gradient in the mantle. In addition, it can account for the preservation of scattered remnants of primitive material in the deep mantle and therefore for the Ar and (3)He observations in ocean-island basalts.