The Bangong-Nujiang Suture Zone (BNSZ) in central Tibet separates the Qiangtang and Lhasa terranes and records the closure of the Bangong-Nujiang Ocean, the central-Tibetan branch of the Mesozoic (Meso-)Tethys. A 2021 synthesis by Jiang and co-workers, built on wavelet-decomposed gravity data combined with geological and geochronological observations, concluded that the suture may be an important tectonic boundary and that the ocean closed scissor-style from east to west. Four linked issues arising from that synthesis remain unresolved in the literature that followed: whether the BNSZ is a continental-scale plate boundary (the northern limit of Gondwana-affinity terranes), the spatial and temporal evolution of subduction polarity and slab geometry (including the timing and locations of slab rollback, break-off, and tearing), the reliability of gravity-inversion-derived crustal structure and of the lithospheric delamination inferred from it, and the timing of terminal closure in the sparsely dated western segment. Later work through 2026 has added dense magnetotelluric, seismic-reflection, receiver-function, geochronological, and stratigraphic constraints and converges partially toward bidirectional subduction in the central segment, yet competing scenarios survive: published closure ages still span the latest Middle Jurassic (ca. 166-163 Ma) to the Late Cretaceous (ca. 100-92 Ma), divergent-double-subduction and single-polarity models both have recent defenders, and no consensus exists on the suture's boundary status. The problem is to resolve these linked questions into one coherent, uncertainty-quantified tectonic model of the suture and of the ocean it consumed.

Background

The Tibetan Plateau was assembled, before the Cenozoic India-Eurasia collision, by the progressive northward drift and accretion of microcontinental terranes rifted from the northern margin of the Gondwana supercontinent across the Tethyan ocean realm; terranes derived this way are said to have Gondwana affinity (Zhu et al., 2021). A suture zone is a belt of deformed rocks marking a consumed ocean, typically containing ophiolites - fragments of former oceanic crust and upper mantle thrust onto continental margins. The Bangong-Nujiang Suture Zone (BNSZ) is an east-west trending belt over 1000 km long in central Tibet that separates the Qiangtang terrane to the north from the Lhasa terrane to the south. It records the Bangong-Nujiang Ocean, widely regarded as the main central-Tibetan branch of the Meso-Tethys, the Mesozoic generation of Tethyan oceans intermediate between the older Paleo-Tethys (whose closure assembled the Qiangtang terrane) and the younger Neo-Tethys (closed along the Indus-Yarlung Zangbo suture when India collided with Eurasia). Jiang et al. (2021) use a different nomenclature for the same realm: they call it the 'eastern Neo-Tethys' (their title refers to the 'eastern Tethys Ocean') and equate the closure of the Bangong-Nujiang Ocean with its extinction. Throughout this statement, the ocean whose terminal closure is at issue is therefore the Bangong-Nujiang Ocean consumed along the BNSZ, not the whole multi-branch Meso-Tethys system. Closure of the Bangong-Nujiang Ocean by the Lhasa-Qiangtang collision is the key pre-Cenozoic event in the growth of the plateau.

Jiang et al. (2021) synthesized geological and geophysical evidence along the BNSZ. They analyzed the Bouguer gravity anomaly (the gravity field corrected for the attraction of topography above sea level), decomposed at multiple scales by wavelet analysis - a technique that separates gravity anomalies of different wavelengths, and hence arising from different source depths - and estimated the depth of the Moho (the boundary between the crust and the mantle) by gravity inversion. Such inversions require assumed parameters: a reference Moho depth D0D_0, a crust-mantle density contrast ρ0\rho_0, and a choice of wavelet orders to isolate depth bands, so the results carry structural non-uniqueness. They found a Moho step of more than 5 km across the suture, inferred from the relatively shallow Moho beneath the South Qiangtang terrane that the lithospheric mantle root there had been removed by delamination (detachment and sinking of dense lithosphere into the mantle), identified gravity lows south of the suture coinciding with Late Jurassic-Early Cretaceous volcanic outcrops that they read as evidence of a southward subduction episode beneath the Lhasa terrane, and concluded that the ocean closed scissor-style from east to west: the eastern segment in the Middle Jurassic, the central and western segments in the late Early Cretaceous, with collision-related deformation mainly in the Cretaceous.

That synthesis sits inside a strongly contested field. On closure timing, published estimates include northward subduction beginning around 190-180 Ma with closure of a northern (Dongqiao-Amdo) ocean branch at 166-163 Ma and a southern (Beila-Nagqu) branch at 150-145 Ma at the onset of the Lhasa-Qiangtang collision (Hu et al., 2022); initial collision by ca. 122 Ma recorded by the Duba Formation of the northern Lhasa terrane (Lai et al., 2019); a 141-135 Ma ocean island within the suture implying the ocean was still open then, with the debate framed as ranging from the Middle Jurassic (ca. 166 Ma) to the Late Cretaceous (ca. 100 Ma) (Fan et al., 2021); and a bracket of ca. 113-92 Ma from Aptian trench-flysch deposition and the overlying non-marine Jingzhushan molasse (Luo et al., 2020). On subduction polarity, one school reconstructs northward subduction beneath Qiangtang only, while another infers divergent double subduction - one ocean floor subducting simultaneously beneath both flanks, the two slabs diverging from a common ridge - based on coeval opposite-facing magmatic arcs (Liu et al., 2020), and recent geophysical imaging has been recruited by both sides. On the status of the suture itself, the traditional view places the northern limit of Gondwana-derived terranes at the Longmu Co-Shuanghu suture north of the BNSZ, which would make the BNSZ a secondary rather than continental-scale plate boundary, whereas Jiang et al. (2021) read the gravity field as indicating that the suture may itself be an important tectonic boundary. The unresolved target is a tectonic model that settles these three intertwined questions - boundary status, subduction geometry through time, and terminal closure timing along strike - while accounting for an observational record that is only partly mutually consistent.

Problem Statement

Establish an uncertainty-quantified tectonic model of the Bangong-Nujiang Suture Zone - what kind of plate boundary it is, how its ocean was subducted, and when and how it closed - by resolving three linked questions that the Jiang et al. (2021) synthesis leaves open and that the later literature has not settled: (1) Plate-boundary status: does the BNSZ constitute an important, continental-scale plate boundary marking the northern limit of Gondwana-affinity terranes, or does that limit lie at the Longmu Co-Shuanghu suture to the north, making the BNSZ a secondary suture within the Cimmerian collage? (2) Subduction geometry and its evolution: what was the subduction polarity of the Bangong-Nujiang Ocean through time and along strike - northward beneath Qiangtang, southward beneath Lhasa, or bidirectional (divergent double) subduction - and when and where did slab rollback, break-off, and tearing occur? (3) Terminal closure: what is the timing of terminal closure of the ocean - here the end of the marine connection between the Lhasa and Qiangtang terranes and the onset of their collision - especially in the western segment where ophiolite and volcanic ages are spatially patchy and locally contradictory, and what was the along-strike sequence of collision-related deformation? The published ages date different observables - the last marine deposits (Aptian trench flysch), the onset of non-marine molasse (Jingzhushan), a reversal of sediment provenance (Duoni), the cessation of arc magmatism, the survival of an ocean island, the first collision-related deformation - which need not coincide and can lag the collision itself, so the terminal-closure age adopted for each segment has to be tied to the observable that dates it. The three questions are parts of one objective: a single internally consistent tectonic evolution model of the suture and the Bangong-Nujiang Ocean it consumed, spanning the eastern, central and western segments, that accounts for the published inventory of ophiolite ages, arc-magmatism distributions, stratigraphy, paleomagnetic constraints, and geophysical images, that states which of these constraints it treats as mutually incompatible and on what grounds, and that discriminates among the competing published scenarios - closure between the latest Middle Jurassic (ca. 166-163 Ma) and the Late Cretaceous (ca. 100-92 Ma), and unidirectional versus bidirectional subduction - with stated uncertainties. The fourth issue raised by the synthesis, the reliability of gravity-inversion-derived crustal structure and of the delamination inferred from it, is not a separate deliverable: because that structure was obtained by inversion under assumed parameters, its non-uniqueness conditions how much weight the Moho geometry and lithospheric delamination adopted by any such model can carry.

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