The Paleo-Asian Ocean (PAO) closed along the Solonker Suture of Inner Mongolia by Permian divergent double subduction (oppositely dipping subduction of one oceanic plate) followed by Early Triassic soft collision, a collision that produced no thickened crust, high-grade metamorphism, or large foreland basins. Liu et al. (2020) documented the two greenschist- to blueschist-facies tectonic melanges that anchor this model, in a record whose "timing and processes" of PAO closure they describe as "still controversial issues"; the terminal geodynamics remain unresolved: after the inverted U-shaped oceanic slab was sandwiched between the Mongolian arc collage and the North China Craton, how did it detach — by wholesale foundering with mantle upwelling, by breakoff (rupture) of one or both slab limbs, by tearing induced by ridge subduction, or by another terminal process such as slab rollback or subduction reinitiation — and which process generated the diverse Triassic magmatic rocks? Seismic reflection and array imaging in 2023-2025 confirmed the fossil, bi-directionally subducted crustal geometry, and big-data and magmatic studies in 2021-2025 refined the diachronous, scissor-like closure history, but none identified the slab's post-closure fate; Jing et al. (2025) did attribute c. 250.8–248.2 Ma MORB-like diabase swarms in the southeastern CAOB to slab break-off with slab-window asthenospheric upwelling, yet that single-suite interpretation leaves the terminal geodynamics "unclear" among ridge subduction, break-off, roll-back and post-collision extension and does not fix the fate of the imaged inverted-U slab along the suture; explicit statements that the terminal processes 'remain hotly debated' persist into 2025. The problem is to determine the post-closure detachment process of the PAO slab beneath the Solonker Suture and its causal link to the terminal metamorphism and Triassic magmatism, discriminating among the candidate mechanisms with integrated geological, geochemical, and geophysical evidence.

Background

The Central Asian Orogenic Belt (CAOB) is the world's largest Phanerozoic accretionary orogen, built by the progressive closure of the Paleo-Asian Ocean (PAO) between the Siberian Craton and the Tarim–North China cratons. The PAO's final closure is recorded along the Solonker Suture Zone in Inner Mongolia (northeastern China), where the southern Mongolian arc collage was welded to the northern margin of the North China Craton in the Permian–Triassic. A tectonic melange is a block-in-matrix rock assemblage in which fragments of oceanic crust (ophiolite), seamounts, and sedimentary rocks are mixed in a sheared matrix; greenschist- and blueschist-facies metamorphism (the low-temperature metamorphic assemblages typical of subduction zones, blueschist bearing the diagnostic sodic amphibole glaucophane) indicate subduction-related pressures. Eizenhöfer et al. (2014) and Eizenhöfer and Zhao (2018) showed from geochronology, geochemistry, and sediment provenance that the eastern PAO closed by double-sided, or divergent double, subduction (DDS) — a single oceanic plate simultaneously subducting in opposite directions on its two sides, so that the sinking slab forms an inverted U — and that the collision was 'soft', lacking the thickened crust, regional high-grade metamorphism, syncollisional granite swarms, and deep foreland basins of a hard continental collision. Liu et al. (2020) supplied the structural anchor for this model: two Permian greenschist- to blueschist-facies melanges at Zhurihe (south of the suture, blueschist zircon U–Pb age — uranium–lead radiometric age measured on zircon — of c. 255 Ma, with enriched mid-ocean-ridge-basalt (E-MORB) affinity) and Erdaojing (north of the suture, c. 281 Ma normal mid-ocean-ridge-basalt (N-MORB) ophiolite blocks), defining two parallel Permian ophiolitic belts on either side of the suture, consistent with two oppositely dipping subduction zones converging on one ocean. Slab breakoff is the detachment and sinking of the denser subducted plate from the overriding plate after collision, typically triggering asthenospheric upwelling, transient high-temperature magmatism, and rapid exhumation; ridge subduction is the ingestion of an active spreading center into a trench, opening a slab window that similarly lets asthenosphere rise but predicts a different spatial-temporal pattern of magmatism and metamorphism. Liu et al. (2020) recognized that the timing and processes of closure remain controversial; the terminal fate of the inverted U-shaped slab — coherent whole-slab detachment and sinking versus rupture by breakoff or ridge-subduction-induced tearing — remains unresolved. Later work sharpened the geometry and timing without answering the mechanism question: crustal-scale seismic reflection profiling imaged opposite-dipping reflectors converging into an inverted U-shaped pattern beneath the Solonker Suture, confirming fossil divergent subduction in the crust (Tan et al. 2023; supported by dense short-period array imaging of bi-directional crustal subduction remnants, Pan et al. 2025), reflection evidence for a Xilinhot microcontinental block implies the PAO had two branches with separate sutures (Tan et al. 2024), and magmatic and big-data studies favor diachronous, 'scissor-like' west-to-east closure with Late Triassic A-type (anhydrous, high-temperature, extensional-type) and alkaline magmatism in a post-collisional extensional setting (Wu et al. 2021; Xue et al. 2025). What no study has established is the fate of the subducted oceanic slab itself after closure and the causal mechanism of the Triassic magmatism: whole-slab foundering with mantle upwelling, breakoff of one or both limbs of the inverted U, ridge-subduction-related tearing, and slab rollback or subduction reinitiation (Fu et al. 2025) remain competing alternatives, tested so far only for individual suites — Jing et al. (2025) read Early Triassic MORB-like diabase swarms as slab-window magmatism triggered by break-off of the Paleo-Asian slab — and complicated by the facts that the terminal closure was diachronous along strike, that geochemical fingerprints of these mechanisms overlap, and that the mantle beneath the region was later overprinted by Paleo-Pacific and Mongol–Okhotsk subduction.

Problem Statement

Determine the post-closure detachment process of the Paleo-Asian oceanic slab beneath the Solonker Suture and its causal link to the terminal geological record: after the Permian divergent double subduction welded the Mongolian arc collage to the North China Craton by Early Triassic soft collision, by what process did the inverted U-shaped oceanic slab detach? Candidate mechanisms include, but are not restricted to, wholesale foundering with attendant mantle upwelling, rupture by breakoff in one or both of its limbs, tearing by ridge subduction (slab-window formation), and slab rollback or subduction reinitiation. Determine the mechanism or sequence of mechanisms that operated, where and when along the suture, whether it is consistent with (i) the structural, metamorphic, and geochronological record of the greenschist- to blueschist-facies melanges and their documented ages (Zhurihe c. 292 Ma E-MORB ophiolitic blocks, c. 274 Ma OIB sequences, c. 255 Ma E-MORB-like blueschist matrix; Erdaojing c. 281 Ma N-MORB ophiolite blocks with c. 266 Ma actinolite-schist and clastic matrix), distinguishing for any invoked age whether it dates protolith crystallization, metamorphic crystallization, or exhumation cooling, (ii) the seismically imaged fossil bi-directional subduction geometry in the crust (inverted-U reflector pattern), including the along-strike variation implied by scissor-like, diachronous closure and the two-branch ocean geometry, and (iii) the ages and mantle-, crust-, and slab-derived geochemical spectrum of the Triassic igneous rocks — and why it, rather than the competing alternatives, produced that record.

References

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