We interpret the sub-horizontal high-V structure (>2% increases) shallower than 250 km in the mantle as IL underthrusting beneath Tibet. The thickness of the underthrusting IL is between 100 and 150 km based on the 2% level contour of shear wave speed anomalies (Figs 3 and 4). This observation is consistent with a receiver-function study of the interfaces beneath the Indian subcontinent30, where the derived depths of the lithosphere–asthenosphere boundary vary between 70 and 140 km, and reach up to ∼170 km beneath the Himalayan region and Moho depths located between 30 and 56 km. Arc-normal cross-sections B and C show that underthrusting IL gently dips northward at an angle of ∼10° (Fig. 4b,e) without visible high-angle subduction in the deeper upper mantle. It is laterally continuous from the Main Frontal Thrust to its northern leading edge, which proceeds beyond the Bangong-Nujiang Suture (BNS) and as far north as the JS (Figs 1, 2b and 4, and Supplementary Fig. 2a–d). Our interpreted location of IL’s leading edge (Fig. 1), approximately coinciding with the JS, is different from previous interpretations from P-wave models based on traditional tomographic methods. P-wave tomography studies generally map the IL underthrusting/subduction front19,31,32,33,34 very close to (for example, Supplementary Fig. 2f) or to the south of the BNS (for example, Supplementary Fig. 2g,h) in Central Tibet between 87°E and 91°E. Along profile 83°E, a model comparison (Supplementary Fig. 2a,e) shows agreement on that the IL underthrusting front reaches as far north as JS in both model EARA2014 and the global P-wave model used in Replumaz et al.32 However, their P-wave model32 reveals a much thicker (∼300 km thick) craton-like structure beneath India (Supplementary Fig. 2e), whereas EARA2014 shows a normal thickness of ∼150 km without invoking underthrusting a very thick continental craton.

Seismicity is distributed along the interpreted upper interface of IL and terminates at depths shallower than ∼100 km (Fig. 4c,f). Except along profile B, one earthquake at a depth of 140 km located by the EHB catalogue35 (event ID: 12477266) occurs in the vicinity of the interpreted IL upper interface (Fig. 4c). There is a large gap between this deep earthquake and shallower crustal seismicity along the interface of IL. Therefore, it probably belongs to the southward subducting Pamir slab, annotated as Asian lithosphere (AL) (Fig. 4c). It is possible that the IL upper interface is steeper under Himalayan blocks between the Main Frontal Thrust and the IYS36 and flattens further north37, but such details cannot be resolved in this study due to resolution limits.

No large-scale low-V anomalies are discernable within the underthrusting IL, which does not support the hypotheses of IL being fragmented due to delamination and asthenosphere upwelling20,21. Low-V anomalies beneath Southern Tibet are only visible at depths shallower than 150 km. Such low-V anomalies (more than 2% reductions) imply possible partial melting. The low-V zones located at crust and uppermost mantle depths do not have a visible connection to any deeper mantle low-V zones. This suggests that partial melting is not a mantle driven process, but instead a crustal process either related to shear heating generated in ductile shear zones near the India-Himalaya lithospheric interface38 or to radioactive heating within the crust39.

Lateral heterogeneities do exist within the interpreted IL in the arc-parallel direction, where beneath the Southern Tibet rift region (83°E–95°E) ∼200 km wide strongly high-V zones (more than 4% increases) alternate with ∼100 km wide relatively weakly high-V zones (3–4% increases) (Fig. 3c). This suggests that underthrusting IL is probably intact, with local weaker zones representing either pre-existing, that is, before initial subduction, structures or locally modified regions due to melt and/or volatile injection after subduction. Absolute values of shear wave speeds in the underthrusting region range from 4.7 to 4.8 km s−1 (Figs 3c and 4c,f, and Supplementary Figs 3 and 4), comparable to those of the North American craton and much higher than in active tectonic regions (<4.5 km s−1) in the uppermost mantle40. If the underthrusting IL can be treated as the root of the present-day TL, then the lithospheric structure of Tibet resembles that of Archaean and Proterozoic cratons, except with a hotter and thicker crust at present, which may be gradually eroded at the top and become more similar in terms of crustal thickness to Archaean and Proterozoic cratons39.

Underlying underthrusting IL, the T-shaped high-V structure beneath South-Central Tibet has a less obvious origin (Figs 4e and 6d). Its top part is located above the transition zone with a height of about 150 km and an arc-normal width of ∼750 km spanning from latitude 28°N (south of the IYS) to latitude 33°N (north of the BNS). Its bottom part resides in the transition zone with a height of ∼250 km and an arc-normal width of ∼200 km, situated between the IYS and BNS. In contrast to a narrow high-V structure observed from a depth of 250 km to the top of the transition zone beneath Western Pacific regions (for example, the Japan and Izu-Bonin convergent margins), which is associated with abundant deep-focus seismicity and interpreted as subducting oceanic lithosphere24, the deep mantle high-V structure beneath South-Central Tibet is a much broader feature and completely lacks seismicity. Such striking differences indicate that the T-shaped high-V structure is unlikely subducting oceanic lithosphere and the portion of Indian oceanic lithosphere probably already sank into the lower mantle16,32,41,42. This argument is further bolstered by a simple estimation of the total budget of consumed continental lithosphere after complete subduction of Indian oceanic lithosphere. The total budget of continental lithosphere, possibly from Indian, Tibetan, or Asian continental blocks, that entered the mantle since continental collision is conservatively estimated at ∼2,250 km in length, given an average convergence rate of 50 mm per year since 45 Ma1. However, the observed IL overriding the T-shaped high-V structure is ∼750 km in length (Fig. 6d), which accounts for only one-third of the total budget and leaves the remaining ∼1,500 km unaccounted for. If the imaged T-shaped high-V structure is interpreted as a foundering continental mantle lithosphere, that is, the majority of the thickened continental mantle lithosphere detached at the bottom but with some part of the top portion still left attached to the crust above, then unwrapping the area of the imaged anomaly to a 120 km30 thick pre-collision lithosphere gives a length estimate of about 1,354 km (equation 1 and Fig. 6d),

Figure 6: Inferred tectonic evolution of Tibet. The interpretation is based on the seismic image along profile C (Fig. 4e) as well as previous studies on magmatism46,53. (a) Between 30 Ma and 25 Ma: following lithospheric thickening due to continental collision, convective instability triggers removal of a lithosphere root and surface uplift. Asthenospheric return flow initiates ultrapotassic and adakitic volcanism in Southern Tibet. (b) Between 25 Ma and 15 Ma: magmatism persists in Southern Tibet while partial melt and heat modify the remaining thin uppermost mantle lithosphere. (c) Between 15 Ma and 10 Ma: further northward underthrusting of IL gradually shuts down the heat source of magmatism in Southern Tibet. (d) Present: Southern Tibet is completely underthrusted by IL up to the south of the JS. Magmatism in Northern Tibet is still an ongoing process. Full size image

which makes up the majority of the missing post-collision continental lithosphere. Therefore we argue that the T-shaped high-V structure is of continental lithospheric origin.

It is still yet to be determined if the detached T-shaped lithosphere is derived from Indian, Tibetan, or Asian continental blocks, because all three continental blocks have the possibility of entering and remaining in the upper mantle through different processes, such as subduction followed by slab breakoff16, or lithospheric thickening followed by convective removal43, that is, foundering in this discussion. If AL subducts southwards under Tibet and later breaks off, a south dipping slab structure would be expected under Northern Tibet from either the Tarim or Qaidam Basins. A previous receiver-function study images a prominent south-dipping interface down to 250 km beneath northern Tibet and interprets it as the top of south dipping AL44. However, there is no compatible seismic tomographic evidence showing positive wave speed jumps downward across the imaged interface (for example, their Fig. 4 in a previous P-wave tomography study42). Alternatively, the receiver-function interpreted south dipping AL interface44,45 can be reconciled with the strong wave speed contrast between our interpreted weakly high-V TL and the strongly low-V zone above (Fig. 4e), which we speculate as an internal interface within TL. Moreover, consistent with previous tomographic results20,21, no obvious evidence of south dipping AL under Northern Tibet is shown in the arc-normal cross-section (Fig. 4e), because weakly high-V anomalies (<1% increase) interpreted as TL are significantly weaker than strongly high-V anomalies (2 to 5% increases) interpreted as AL. In the W–E oriented cross section along latitude 36°N, AL is also outlined by strong high-V anomalies of 2 to 5% down to a depth of at least 250 km under the Qilian Shan fold-thrust belt and is seismically discernible from TL that has weakly high-V anomalies of less than 1% (Supplementary Fig. 5). Although our observation does not support the model involving AL southward subduction leading to growth of crustal accretionary wedges12,13, it is possible that AL subducts eastward at a dip angle of ∼25° from the Qaidam Basin and contributed to the high-elevation of the Qilian Shan fold-thrust belt (Supplementary Fig. 5).

Therefore, continental lithosphere more likely foundered from IL or TL or both, although their relative contributions depend on the pre-collision thickness and strength of both lithospheric blocks. As TL is considered to be hotter and, as result, most likely to be rheologically weaker than colder IL46, we speculate that Tibetan mantle lithosphere is more prone to thicken along with the crust right after continental collision starts (Fig. 6a). The colder and stronger Indian mantle lithosphere is more likely to undergo underthrusting without significant internal deformation. Continued penetration of IL is resisted by thickened TL and is likely to be limited to a few hundred kilometers of distance in the arc-normal direction (Fig. 6a). Owing to the Rayleigh–Taylor instability43,47, the viscous lower part of thickened Tibetan mantle lithosphere can initially ‘drip’ on a relatively small scale (∼200 km wide), followed by breakoff of the more rigid upper part (∼750 km wide) of IL and TL accommodated by faults or other weak zones47.

The timing of lithospheric foundering beneath Southern Tibet can be constrained by the timing of ultrapotassic and adakitic magmatism that initiates at about ∼30 Ma and lasts until ∼9 Ma (Figs 1 and 6a–c)46. Post-collision adakitic magmatism suggests the occurrence of thickening of TL and subsequent lithospheric root foundering. Lithospheric foundering significantly thinned Southern TL that was thickened before 30 Ma due to continental collision. The loss of lithospheric root can drive surface uplift during the Oligocene48 and observed ultrapotassic and adakitic magmatism, fueled by the ascent of asthenospheric return flow. The continued sinking of foundering lithosphere in the upper mantle can also generate lateral pressure gradients in viscous asthenosphere that can drive shear traction at the base of overlying mantle lithosphere49. This shear traction drives northward underthrusting of IL and thickening of remaining TL in the north (Fig. 6b–d). The northward advance of underthrusting IL gradually shuts off sources of heat and melting and causes waning of ultrapotassic and adakitic magmatism in Southern Tibet46.

We conclude that the leading edge of IL has moved northwards over an arc-normal distance of about 750 km (Figs 4e and 6d) since the acceleration of underthrusting at ∼25 Ma (Fig. 6b), when the lower part of the pre-thickened lithosphere becomes detached completely. This interpretation gives an estimated average underthrusting rate of about 30 mm per year in the past 25 million years. It is slightly higher than the current ongoing convergence rate of ∼20 mm per year between India and the IYS, but remains a reasonable estimate as convergence is expected to have slowed down due to resistance associated with thickened lithosphere50.

Northern TL is probably being heated by asthenospheric upwelling. The S-N contrast in shear wave speed perturbations in our model (Supplementary Fig. 3) is compatible with other results independent from seismic tomography. Based on our observed 3% of S–N V S difference and a relation between V S perturbation and temperature of 1.3±0.30% per 100 K at 200 km51, TL under Northern Tibet is estimated to be 200–300 K warmer than underthrusting IL beneath Southern Tibet. Such temperature difference in the uppermost mantle agrees with heat flow modelling52. The S–N difference of the average shear wave speed between the surface and 410 km depth (about 3% along profile D; Supplementary Fig. 3) is also consistent with receiver function observations of the 410- and 660-discontinuities being parallel and relatively depressed in the south44. Thickened Northern TL may be gradually eroded or thermally modified by hot asthenospheric upwelling (Fig. 6c,d). The good spatial correlation between the strongly low-V zone in the uppermost mantle and more recent potassic magmatism in Northern Tibet (∼15–0 Ma)46,53 (Figs 1, 5b,c and 6c,d) further support the hypothesis of asthenospheric upwelling. The low-V anomalies are, however, limited to the uppermost mantle (<125 km) overlying weakly high-V TL that extends down to a depth of ∼200 km. Contrary to more dramatic lithospheric foundering and thinning during the Oligocene in Southern Tibet, Northern TL more likely to be experienced ‘diffused’ root removal or thermal modification and is still largely intact. Thermal modification can lead to a more buoyant lithospheric mantle that isostatically supports uplift of Northern Tibet.

Our results are consistent with the following conceptual model. Distributed thickening of TL and underthrusting of IL accommodate the bulk of mantle lithosphere convergence since India-Eurasia collision. Convergence leads to shortening and thickening of TL, including both crust and mantle. Subsequent foundering of thickened lithosphere during the Oligocene contributed to the rise of Southern Tibet. The foundering lithosphere is continental in origin and, as a result, is less negatively buoyant than oceanic lithosphere. This can lead to a long residence time (∼30 Ma) of foundering continental lithosphere in the upper mantle. In addition, the 660-discontinuity can act as rheological and density barrier preventing foundering continental lithosphere from sinking into the lower mantle (Fig. 6d).

Different from pure subduction settings, where lithosphere subducts without much thickening, the India-Eurasia continental collision zone involves thickening of the continental mantle lithosphere (TL) and low-angle underthrusting of stronger IL. Deformation and thickening on the Tibetan side is not confined to the crust and is more vertically distributed throughout the entire column of crustal and mantle lithosphere. Wholesale thickening of TL can initiate a Rayleigh–Taylor instability and subsequent foundering (convective removal) of the lithospheric root. Convective removal and associated lithospheric foundering creates an additional plate driving force, an asthenospheric drag force, resulting in continued thrusting of IL under Tibet. This is different from the principal driving force of plate tectonics at oceanic subduction zones, which is created by negative buoyancy of dense oceanic mantle lithosphere. The direct impact of such convective removal is a more pulsed surface uplift of Southern Tibet over a time scale of <10 m.y. (∼30–25 Ma) rather than over the entire 45 m.y. of India-Eurasia collision.

If northward penetration of IL slows down exponentially due to resistance from viscous mantle lithosphere50, it is likely to be that India-Tibet convergence will terminate by the time IL occupies the entire uppermost mantle underneath Tibet. The strength and buoyancy of Indian continental lithosphere might keep it in place beneath Tibet for a substantial amount of time, possibly long enough to be considered the root of a stable craton. This might provide a mechanism for the formation of a modern craton in the Tibet-Himalaya continental collision margin, consistent with geodynamic modeling54 and similar to a previously proposed mechanism of craton formation through underthrusting and imbrication of oceanic lithosphere55, however, through under-accretion of Indian continental lithosphere instead.