Orogenic curvatures, known as oroclines, are common features along convergent plate boundaries, significantly impacting orogen evolution. While the bending of orogenic belts is well-recognized, the deformation mechanisms involved and their effect on crustal architecture remain unclear. Previous studies have proposed various models, including flexural-slip and orogen-perpendicular tear faulting, but crustal-scale evidence for contraction and thickening during oroclinal bending has been lacking. The Mongolian Orocline, a ~6000 km long, tightly curved Andean-type subduction system, presents an ideal case study to address this gap. Its U-shaped geometry and wider hinge zone, coupled with gravity data suggesting thicker crust around the Hangay Mountains, hint at a link between oroclinal bending and crustal architecture. This study aims to investigate this relationship by analyzing the spatiotemporal variations in magmatism and crustal thickness within the Mongolian Orocline during the Permian-Triassic period, providing crucial insights into the three-dimensional architecture and tectonic evolution of curved orogens.

Existing research on oroclines has focused on various aspects, including the mechanisms of bending and their influence on different geological processes. Studies in eastern Australia and the central Mediterranean suggest that flexural-slip mechanisms or orogen-perpendicular tear faults may play important roles in accommodating large-scale deformation. In contrast, other oroclines, such as the Cantabrian Orocline, suggest contractional deformation and crustal thickening in the inner hinge zone. However, large-scale, crustal-level evidence supporting contraction and thickening during oroclinal bending has been lacking. This paper aims to bridge this gap by applying the La/Yb ratio as a proxy for paleo-crustal thickness in arc magmas, a method successfully employed in other studies to quantify crustal thickness in continental collisional belts and magmatic arcs. This approach allows for a quantitative assessment of crustal thickness variations through time, providing crucial constraints on the deformation processes during oroclinal bending.

The study utilizes a multi-faceted approach combining geochronology and geochemistry to understand the tectonic evolution of the Mongolian Orocline. Firstly, zircon U-Pb dating of magmatic rocks from the Hangay Batholith, the major magmatic feature within the inner hinge of the orocline, is employed to determine the timing and spatial distribution of magmatism. This involved LA-ICP-MS analysis of zircon samples, using established methods and calibration standards. Secondly, whole-rock geochemistry, including major and trace element analysis using XRF and ICP-MS, is conducted to determine the geochemical characteristics of the magmatic rocks. This data is crucial for evaluating the La/Yb ratio, a proxy for crustal thickness in supra-subduction zones. Careful data filtering is applied to minimize the influence of factors other than crustal thickness on La/Yb ratios, such as the degree of magma differentiation and source rock composition. Specifically, samples with SiO2 contents outside the range of 55-70 wt% and MgO > 4 wt% are excluded. The filtered data are then used to reconstruct paleo-crustal thickness using an empirical regression equation established in previous studies, correlating La/Yb ratios with crustal thickness. Finally, a Monte Carlo analysis with a weighted bootstrap resampling approach is applied to evaluate the average trend and minimize sample bias in the crustal thickness reconstruction. The spatial and temporal variations in magmatism and crustal thickness are then analyzed to investigate the link between oroclinal bending, arc migration, and crustal thickening.

The geochronological data reveal that magmatism in the Hangay Batholith occurred between ~298 Ma and ~220 Ma, with progressively younger rocks towards the core of the orocline. This indicates arc migration towards the inner hinge during the Permian-Triassic. The geochemical analysis, particularly the La/Yb ratios, show a significant increase in crustal thickness from ~50 km at ~298 Ma to ~65 km at ~230 Ma. This increase in crustal thickness is consistent with the observed thicker crust around the Hangay Mountains today. The spatial distribution of the magmatic rocks shows that earlier (Permian) magmatism occurred mainly within Precambrian microcontinents, while later (Triassic) magmatism intruded into Paleozoic accretionary complexes. The inner hinge zone is wider than the limbs of the orocline, as evidenced by the spatial distribution of tectonic units. This widening, coupled with crustal thickening, strongly suggests contraction within the inner hinge zone during oroclinal bending. The study's findings indicate an interlinked relationship between arc migration, crustal thickening, and oroclinal bending within the Mongolian Orocline, supporting the idea that oroclinal bending can be accommodated by crustal-scale contraction in the inner hinge zone. The observed crustal thickening is likely due to the redistribution of crustal materials rather than new crustal growth via magma underplating.

The results of this study significantly advance our understanding of oroclinal bending and its impact on orogenic architecture. The demonstrated crustal-scale contraction in the inner hinge zone of the Mongolian Orocline provides compelling evidence for a new mechanism accommodating large-scale bending. This mechanism appears independent of the specific geodynamic process driving oroclinal bending, as suggested by previous studies that attribute the Mongolian Orocline's formation either to rapid slab rollback or to the convergence and rotation of the Siberian and North China cratons. The spatial correlation between the thicker crust and the elevated topography of the Hangay Mountains further supports this interpretation, suggesting a causal link between crustal thickening and high elevation. This phenomenon is mirrored in other curved convergent plate boundaries, indicating its potential as a general mechanism shaping orogenic landscapes. The study also highlights the potential for extensional tectonics, like rifting in the West Siberian Basin, to be linked to oroclinal bending, accommodating rotation of the orocline's limbs.

This study demonstrates that oroclinal bending in the Mongolian Orocline was accompanied by crustal-scale contraction in the inner hinge zone, leading to significant crustal thickening and arc migration. The findings provide crucial evidence for a new mechanism accommodating large-scale bending in orogenic systems and highlight the importance of considering crustal-scale deformation processes when studying oroclines. Future research should focus on applying similar methodologies to other oroclines to assess the generality of this mechanism and to further investigate the interplay between oroclinal bending, crustal architecture, and surface topography.

One limitation is the absence of geochemical data between 285 and 270 Ma, creating a gap in the crustal thickness reconstruction. Additionally, the dataset for the period 240–230 Ma is relatively small, which might affect the accuracy of the late Triassic crustal thickness estimation. While the study carefully filters the geochemical data to minimize the influence of factors other than crustal thickness on La/Yb ratios, some uncertainties may still remain due to the complexity of magma generation and differentiation processes. Further studies incorporating additional data and advanced modeling techniques could refine the understanding of these processes.