How did the Dabie Orogen collapse? Insights from 3‐D magnetotelluric imaging of profile data

Abstract

The Dabie Orogen is a key part of the Central Orogen of China and is well known because of its exposures to high‐ and ultrahigh‐pressure metamorphic rocks. It has been proposed to have formed by continent‐continent collision between the Yangtze and North China blocks in the Middle‐to‐Late Triassic and to have collapsed in the middle of the Early Cretaceous. However, there are few signatures in the lithospheric architecture that might constrain the scale of the orogenic collapse; as a result there has been a long‐term debate on its origin. A 300 km long broadband magnetotelluric transect perpendicular to a major collapsed structure has been completed to find some possible clues to the mechanisms of the orogenic collapse. The inverted resistivity cross section from 3‐D inversion of magnetotelluric profile data refines the postcollision lithospheric structure and in particular identifies domains of refertilized and depleted lithospheric mantle and isolated midcrustal low‐velocity and high‐conductivity zones. A correlation between voluminous surface magmatism in the Early Cretaceous and deep refertilized lithospheric mantle is illuminated by elevated electrical conductivity. We suggest that the lowermost crust was removed by delamination beneath the eastern Dabie during the orogenic collapse, while beneath the Tongbai‐Xinyang Terrain residual cratonic crust is underlain by depleted lithospheric mantle with relatively high resistivity. The isolated low‐velocity and high‐conductivity zones in the middle crust have been ascribed to shear‐promoted graphitic films which were destroyed by local granitic intrusions around 134 to 125 Ma. This lithospheric process can be attributed to far‐field effects, such as refertilization in the lower lithospheric mantle, promoted by upwelling of the asthenosphere and related fluids, during Paleo‐Pacific subduction beneath the Euro‐Asian continent.

Dynamic processes and mechanisms for collision to post‐orogenic extension in the Western Dabie Orogen: Insights from numerical modeling

“…In addition, the previous nappe thrust structure formed during the collision stage can be reconstructed by weak regional extensional deformation and subsequent voluminous magma, which is consistent with the geological data. It needs to be noted though that this unrooting process of the orogen is also similar with previous plate‐rift models and supported by other geophysical data (Xu et al, ; Zheng, ).…”

“…In the Western Dabie Orogen, the time interval from the timing of the original HP/UHP collision (about 220–200 Ma) to the post‐orogenic extension with voluminous magmatic activity (about 140–120 Ma) took about 80 Myr, which is obviously longer than the neighboring collision zone, such as the Qinlin Orogen (Qin, Lai, & Li, ; Yang et al, ) and Qaidam UHP metamorphism (Wang et al, ). Thus, some researchers suggest that the crust of the Western Dabie Orogen was strong and well welded to the underlying lithospheric mantle after the continent–continent collision (Xu et al, ). This means that the slab breakoff, which should play an important role in mountain collapse and magmatic intrusion, did not occur during the original UHP collision or occurred at least 80 Myr after the original UHP collision in the Western Dabie.…”

“…The Western Dabie Orogen as a product of the collision between the North China Block and South China Block, located in the middle segment of the Qinling–Tongbai–Dabie–Sulu orogenic belt (Figure ), has also undergone post‐orogenic extension and collapse during the Early Cretaceous, revealed by two episodes of magmatization (Li, Suo, et al, ; Liu et al, ; Zheng, ), typical extensional tectonics (Li et al, ; Li et al, ; Li, Kusky, et al, ; Li, Kusky, et al, ; Zhou et al, , ) and lithospheric architecture (Jiang et al, ; Xu et al, ). Most researchers have suggested that the mechanism of unrooting and collapse in the Dabie Orogen may be attributable to the Rayleigh–Taylor gravitational instabilities and subsequent anatexis triggered by subduction of the Paleo‐Pacific Plate (Li, He, & Wang, ; Xu et al, ; Zheng, ), because of the geodynamic setting of the Mesozoic magmatism in eastern Eurasia (Wu, Xu, Gao, & Zheng, ). However, as we know, collapse is the fate and intrinsic property of collisional orogens and maybe not directly related to other factors, such as plate subduction.…”

Early Cretaceous granitic dykes in the western Dabie Orogen, China: Geochronology, petrogenesis, and tectonic implications

“…130 to 120 Ma, suggest that the asthenospheric upwelling took place at that time (Figure c) (Jahn et al, ; Wang et al, ; Zhao et al, ). Lithospheric thinning might also have taken place during this process, as represented by magmatic origins and geophysical evidences (Chen, Chen, Zhang, Chen, & Zhang, ; Xu et al, ; Zhang et al, ). Therefore, we propose that these NW‐trending granites were formed in the asthenospheric upwelling and large‐scale crustal extensional setting, after the lower crustal delamination.…”

“…It is characterised by intensive Cretaceous magmatisms (Ma, Li, Ehlers, Yang, & Wang, ; Ma, Yang, Ming, & Lin, ; Mao, Pirajno, & Cook, ; Xu, Ma, & Ye, ; Xu, Ma, Zhang, & Ye, ; Zhao & Zheng, ; Zhao, Zheng, Wei, & Chen, ), which comprise 47% of the surface exposure in the Dabie Orogen (Ratschbacher et al, ). These Cretaceous igneous rocks mainly consist of intermediate to felsic granitoids and volcanics, with few mafic‐ultramafic intrusions, which provide clues to investigate the magmatic evolution and geodynamics process of tectonic collapse (e.g., Chen et al, , ; Huang et al, ; Jahn, Wu, Lo, & Tsai, ; Ma et al, , ; Xu et al, , , ; Zhao et al, , ; Zhao & Zheng, ; Zhao, Zheng, Wei, & Wu, ). Most studies have proposed that these Early Cretaceous granitoids were triggered by the tectonic collapse of the thickened crust in the Dabie Orogen (Xu et al, ) and were divided into two stages (pre ca.…”