The Solonker suture in southeastern Inner Mongolia is widely accepted as the site of the terminal closure of the Paleo-Asian Ocean (PAO) between the North China Craton and the Mongolian terranes, yet the time and mode of that closure remain unresolved. Zhang et al. (2020) proposed a 275–262 Ma final-closure interval from a compression-to-extension switch in magmatism at Xinhure, attributed the post-collisional layered gabbros and strongly peraluminous granites to asthenospheric upwelling following slab break-off, and explicitly flagged the unresolved issues: the interval is established at one sector only and conflicts with Middle Permian, Late Permian–Early Triassic, and Mesozoic estimates elsewhere; slab break-off lacks direct evidence; and the suture's missing high-grade metamorphism and fold–thrust belts are unexplained. Literature through 2026 shows the controversy deepening rather than converging: competing syntheses argue for contemporaneous Middle–Late Triassic closure, scissor-like diachronous closure, or early Permian closure, while geophysical imaging of the deep suture is only beginning. This problem asks for a regionally corroborated determination of the closure time, its along-strike variability, the mechanism of the post-collisional transition, and an explanation of the absent collision features, with all four facets mutually reconciled.

Background

The Central Asian Orogenic Belt (CAOB) is the largest Phanerozoic accretionary orogen on Earth, assembled over roughly 800 million years by the progressive accretion of island arcs, back-arc basins, and microcontinents. It records the life and death of the Paleo-Asian Ocean (PAO), the ocean that separated the Siberian Craton from the Tarim and North China cratons. The PAO did not close along a single line; its residual seaways were eliminated along several sutures, of which the Solonker suture of southeastern Inner Mongolia (extending roughly from the Xar Moron River belt eastward toward the Changchun–Yanji zone) is widely accepted as marking the final, terminal closure, where the Mongolian terranes collided with the northern margin of the North China Craton (NCC). The timing and mode of this terminal closure are boundary conditions for the assembly of northeast Asia and the eastern margin of Pangea, for the transition from the Paleo-Asian to the Paleo-Pacific subduction regime, and for Permian biogeographic and climatic events along the northern NCC.

The classical answer has been Late Permian–Early Triassic closure (approximately 260–250 Ma). That estimate rests largely on indirect constraints, and the suture is notorious for what it lacks: unlike classic continent–continent collision zones, it shows no voluminous high-grade regional metamorphism and no well-developed fold–thrust belt (a belt of contractional folds and thrust faults expected where continental crust is strongly shortened), and outcrop is sparse beneath Mesozoic–Cenozoic basin cover. Eizenhöfer and Zhao (2018) synthesized the Paleozoic geology across the suture and proposed that divergent double-sided subduction — two opposed subduction zones consuming the last ocean basin from both the north and the south — explains both the arc magmatism on either side and the absence of collision-type features, with closure proceeding diachronously from west to east in the Late Permian–Early Triassic.

Zhang et al. (2020) approached the closure time through a geodynamic switch rather than a collisional feature. In the Xinhure area on the northern NCC margin they documented Early Permian foliated (syn-tectonic) gabbros and dikes dated 288–275 Ma with subduction-related geochemistry, followed by Middle–Late Permian undeformed layered gabbros, strongly peraluminous granites (S-type granites produced by melting of sedimentary upper crust), and I-type granites (granites derived from melting of older igneous crust) at 265–254 Ma. The ages come from zircon U–Pb geochronology (dating crystallization from the radioactive decay of uranium to lead in zircon), and whole-rock chemistry plus Sr–Nd–Hf radiogenic isotope tracers identify the magma sources. They interpreted the undeformed suite as products of asthenospheric upwelling — in their model, the consequence of slab break-off, the detachment and sinking of the dense subducted plate, which allows hot mantle asthenosphere to rise into the gap — marking a transition from terminal compression to post-collisional extension between 275 and 262 Ma, which they proposed as the final closure interval of the PAO along the Solonker suture.

The same study explicitly flagged what remains unresolved, and these flags define the open problem. First, the 275–262 Ma interval was established at a single sector of a suture that extends for a thousand kilometers and more; alternative closure timings reported elsewhere (Middle Permian, Late Permian–Early Triassic, and even Mesozoic constraints from distinct local samples) were not reconciled with the Xinhure record. Second, slab break-off and consequent asthenospheric upwelling were proposed as a testable model, not a demonstrated mechanism: direct evidence for the geometry and timing of slab detachment (regional seismic imaging, detailed metamorphic pressure–temperature–time paths, or systematic spatial patterns of diagnostic magmas such as adakites — high-Sr/Y melts of thickened basaltic crust — and sanukitoids, high-magnesium andesitic magmas produced by melting of mantle previously enriched by subduction components) was lacking at the regional scale. Third, the absence of extensive high-grade regional metamorphism and well-developed fold–thrust structures along many sectors of the suture remains unexplained: it is not known whether this is a primary feature of the collision (a localized, short-duration, low-crustal-thickening 'soft' collision) or a preservational artifact of limited exposure and subsequent erosion. Fourth, extrapolation from Xinhure to the whole suture requires more widely distributed, fabric-controlled U–Pb geochronology (dating tied to the deformation fabric of the rocks), systematic Sr–Nd–Hf isotopic mapping, and integrated structural, metamorphic, and thermochronological study (thermochronology uses temperature-sensitive isotopic systems to date the cooling and exhumation history of rocks).

Problem Statement

Determine, with regionally corroborated evidence, when and how the Paleo-Asian Ocean finally closed along the Solonker suture. The four linked questions left open by Zhang et al. (2020) are:

  1. Timing and along-strike variability. Does the 275–262 Ma final-closure interval inferred at Xinhure apply along the full length of the Solonker suture, or is closure diachronous (for example scissor-like from west to east) or discrete and younger (Late Permian–Early Triassic, or Middle–Late Triassic)? The conflicting local and regional estimates must be explicitly reconciled into a single defensible timeline for the suture.

  2. Mechanism of the post-collisional transition. Is the switch from compressional to extensional magmatism correctly attributed to slab break-off and asthenospheric upwelling, and how can slab break-off be distinguished from competing mechanisms including continuing bidirectional subduction, slab rollback (progressive rearward retreat of the subducting plate's hinge), and lithospheric dripping (foundering of dense lower crust and mantle lithosphere into the asthenosphere)?

  3. Collisional style. Why does the suture lack extensive high-grade regional metamorphism and well-developed fold–thrust structures? The answer must establish whether this reflects a primary feature of the collision (localized, short-duration collision with limited crustal thickening) or a preservational and exposure artifact.

  4. Integration. The timeline, mechanism, and structural-metamorphic explanation must be mutually consistent across the suture.

References

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