The bending of a supra-subduction zone produced crustal thickening and arc migration of the Mongolian Orocline

Orogenic belts commonly display arcuate geometries (oroclines) formed by bending of quasi-linear belts around vertical axes. Although oroclinal bending profoundly influences paleogeography, magmatism, continental growth, and mountain building, the way large strains are partitioned and accommodated during bending remains unclear. Prior studies invoked flexural slip or orogen-perpendicular tearing to explain deformation in some oroclines, whereas others proposed contraction and crustal thickening at inner hinges. However, crustal-scale evidence directly linking oroclinal bending to contraction-induced crustal thickening has been lacking. To address this, the study estimates paleo-crustal thickness using LaN/YbN ratios of intermediate–felsic arc magmas and applies this approach to the Mongolian Orocline, a tightly curved, Andean-type Permian–Triassic subduction system in Central Asia. The research aims to test whether the inner hinge underwent contraction and crustal thickening during oroclinal bending and to assess implications for arc migration and orogenic architecture.

Multiple oroclines (eastern Australia, central Mediterranean) show evidence for flexural-slip mechanisms and orogen-perpendicular tear faults accommodating bending. In the Cantabrian Orocline (Western Europe), contraction and crustal thickening at the inner hinge have been inferred from structural observations. Despite these insights, crustal-scale proxies demonstrating thickening linked to oroclinal bending are scarce. LaN/YbN ratios in arc magmas have been developed as a proxy to reconstruct crustal thickness in supra-subduction zones and orogenic belts. For the Mongolian segment of the Central Asian Orogenic Belt, previous reconstructions indicate continuous subduction of the Mongol–Okhotsk Ocean through the Permian–Jurassic, development of a U-shaped Mongolian Orocline, and diachronous closure of the ocean, with earlier closure in the west. Paleomagnetic data suggest final closure by the Late Jurassic, but the onset of closure appears post-~220 Ma based on arc-related rocks. The present-day inner hinge (Hangay Mountains) exhibits thicker crust and elevated topography from gravity and geophysical studies, but the timing and mechanism of thickening have been debated.

- Study area and stratigraphic/tectonic framework: The Mongolian Orocline comprises Precambrian microcontinental blocks (e.g., Buteel, Tuva-Mongolian, Tarvagatay, Zavkhan, Baydrag, Ereendavaa, Idermeg) and Neoproterozoic–Paleozoic accretionary complexes (Bayanhongor zone, Zag zone, Hangay–Hentey complex). Permian–Triassic magmatism forms the Hangay Batholith in the inner hinge. - Sample classification: Hangay Batholith magmatic rocks subdivided into Group I (Permian; emplaced within Precambrian basement blocks: Tarvagatay, Zavkhan, Baydrag) and Group II (Triassic; emplaced within/along Paleozoic accretionary complexes: Bayanhongor, Zag, Hangay–Hentey). - Arc affinity confirmation: Lithologies (monzonite, quartz monzonite, granodiorite, monzogranite, syenogranite; also gabbro/gabbrodiorite) and geochemical signatures (calc-alkaline to shoshonitic series; LILE enrichment Rb, Ba, Th, K; HFSE depletion Nb, Ta, P, Ti). Tectonic discrimination (Th/Yb–Nb/Yb and Rb–Y+Nb) place samples in continental/volcanic arc granite fields. - Geochronology compilation: 145 zircon U–Pb ages (new + literature) indicate magmatism from ~298 to ~220 Ma with a west-to-east younging trend (Permian intruding basement, Triassic intruding accretionary complexes) implying arc migration toward the orocline core. - Crustal thickness proxy and data filtering: Compiled whole-rock geochemistry (n=129). Applied filtering to retain rocks whose LaN/YbN reflects crustal thickness: SiO2 between 55–70 wt%, MgO <4 wt%; excluded mafic and highly evolved high-Si rocks. Hangay granitoids are metaluminous to weakly peraluminous, minimizing peraluminous melt bias. After filtering, n=85, ages 298–230 Ma, spatially covering the area. - Thickness calculation: Used empirical regression LaN/YbN = (0.98 ± 0.19) × exp((0.047 ± 0.005) × H), where H is crustal thickness (km). Computed temporal evolution of LaN/YbN and converted to H. Employed Monte Carlo analysis with weighted bootstrap resampling to estimate average trends and uncertainties; noted data gaps (no data 285–270 Ma; sparse 240–230 Ma). - Field/structural and geophysical context: Considered present-day crustal thickness from gravity and Moho depth models (max ~60 km in inner hinge) and map-view widening of hinge zone from geological and magnetic maps. - Analytical methods (laboratory): LA-ICP-MS U–Pb zircon dating at GIG-CAS (laser 29 µm, 6 Hz; standards Zircon 91500, NIST 610; data reduction with ICPMSDataCal 10.9; Concordia and ages with Isoplot 4.15). Major elements by XRF at SKLIG GIG CAS (precision 1–5%). Trace elements by Agilent 7700e ICP-MS (external standards AGV-2, BHVO-2, BCR-2, RGM-2; precision better than 5%).

- Spatio-temporal arc migration: Permian plutons (Group I) emplaced mainly into Precambrian microcontinents in the west, whereas Triassic granitoids (Group II) intruded farther east into Paleozoic accretionary complexes, indicating arc migration toward the orocline core between ~298 and ~220 Ma. - Increasing LaN/YbN through time: Filtered arc rocks show LaN/YbN ratios generally increasing from Permian to Triassic, implying progressive thickening of the overriding crust. - Quantified crustal thickening: Monte Carlo–based trend from the LaN/YbN–thickness regression indicates average crustal thickness increased from ~50 km at ~298 Ma to ~65 km at ~230 Ma. Present-day crust beneath the inner hinge (Hangay Mountains) reaches up to ~60 km, consistent with long-lived thickening. - Spatial thickening pattern: Triassic Group II granitoids within accretionary complexes record crustal thicknesses exceeding Permian values of adjacent Precambrian microcontinents, demonstrating significant thickening of the accretionary complexes from Permian to Triassic. - Hinge-zone architecture: The inner hinge zone is wider in map view than the limbs and coincides with thicker crust and elevated topography, indicating substantial crustal-scale contraction localized in the hinge.

The results link oroclinal bending of the Mongolian Orocline with contemporaneous arc migration and crustal thickening in the inner hinge. Progressive widening of the supra-subduction hinge zone likely forced oceanward trench migration and passive arc retreat, analogous to rollback-driven systems (e.g., Calabrian and Banda arcs) but here strongly influenced by thickening and widening of the overriding orogen. The observed contractional deformation and thickening are best explained by redistribution of crustal material in the hinge zone rather than by significant new crustal growth via underplating. Coeval extensional tectonism in the West Siberian Basin may have accommodated relative rotation between limbs of the orocline. The deformation pattern—crustal-scale contraction at inner hinges—appears applicable across oroclines regardless of the exact geodynamic driver (rollback versus craton convergence/rotation) and may represent an end-member mechanism, potentially operating alongside flexural slip and orogen-perpendicular tearing. The spatial correspondence between thickened crust and elevated Hangay topography suggests an isostatic link, echoing topographic highs near hinges of other curved convergent margins (e.g., Bolivian Orocline, Cascadia). These insights address the long-standing question of how strain is accommodated during oroclinal bending and elucidate its control on 3D orogenic architecture and temporal-spatial evolution of arc magmatism.

This study demonstrates that the bending of a supra-subduction zone during formation of the Mongolian Orocline was accompanied by crustal-scale contraction in the inner hinge, driving arc migration toward the orocline core and progressive crustal thickening from ~50 km (ca. 298 Ma) to ~65 km (ca. 230 Ma). The arc magmatism’s west-to-east younging, increasing LaN/YbN ratios, and wider hinge zone collectively indicate significant hinge-focused shortening and orogenic reorganization, with present-day thick crust and elevated Hangay topography reflecting this legacy. These findings provide crustal-scale evidence that oroclinal bending can be accommodated by hinge-zone contraction and offer a framework to interpret analogous curved plate boundaries. Potential future directions include applying the LaN/YbN-based crustal thickness approach to other oroclines for comparative analysis, integrating additional geophysical constraints (e.g., receiver functions, seismic tomography) to refine thickness reconstructions, and high-resolution temporal sampling to bridge current data gaps and further resolve the dynamics of trench/arc migration during bending.

- Data coverage: No geochemical data for 285–270 Ma and relatively few samples for 240–230 Ma, increasing uncertainty in temporal trends during these intervals. - Proxy assumptions: LaN/YbN can be influenced by high-pressure fractionation (garnet/amphibole), magmatic differentiation, and source composition; although filtering (SiO2 55–70 wt%, MgO <4 wt%) and metaluminous to weakly peraluminous character mitigate biases, residual effects may persist. - Thickness calibration: Reliance on an empirical LaN/YbN–thickness regression introduces model uncertainty; results represent averaged trends from Monte Carlo resampling rather than site-specific measurements. - Geological inference: Attribution of thickening primarily to contractional redistribution versus new crustal growth is based on indirect lines of evidence; direct crustal-scale imaging or mass-balance quantification is limited.