ALBERT

Description: The Dabie Orogen is a key part of the Central Orogen of China, and is well known because of its exposures of 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 major collapsed structure has been completed to find some possible clues to the mechanisms of the orogenic collapse. The inverted resistivity cross-section from 3D inversion of magnetotelluric profile data refines the post-collision lithospheric structure and in particular identifies domains of refertilized and depleted lithospheric mantle and isolated mid-crustal 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 refertilisation in the lower lithospheric mantle, promoted by upwelling of the asthenosphere and related fluids, during Paleo–Pacific subduction beneath the Euro–Asian continent.