A plume-modified lithospheric barrier to the southeastward flow of partially molten Tibetan crust inferred from magnetotelluric data

摘要

Broadband and long-period magnetotelluric (MT) data were used to obtain a continuous image of the lithospheric electrical conductivity structure across the southeastern Tibetan Plateau margin, where long-wavelength crustal thickening and surface uplift have been attributed to the influx of weak crustal material from Tibet into the adjacent Yangtze Block. The resulting resistivity model reveals two major conductive features in the middle-lower crust beneath southeastern Tibet and the Chuxiong basin, both of which are consistent with the presence of fluids and likely enhanced by shear deformation along large faults. The horizontally extensive conductor beneath southeastern Tibet could be interpreted as a partially molten layer that represents topography-driven crustal flow from central Tibet, whereas the anomalously high conductivities observed beneath the Chuxiong basin require the addition of substantial amounts of saline aqueous fluids. These two conductors are separated by a lithospheric-scale, high-resistivity body that situated beneath the geologically inferred core of the Emeishan large igneous province (ELIP). Together with other geophysical and geological evidence, we interpret this anomaly as remnants of a plume-modified lithosphere that originally formed during the Permian Emeishan volcanism, and speculate it acts as an obstruction to the southeastward crustal flow from Tibet since Late Cenozoic. These results indicate that topography-driven crustal flow may contribute to crustal thickening and uplift in southeastern Tibet, but not in the adjacent foreland. Instead, magmatic underplating associated with the Permian plume activity could account for both the elevated topography and thickened crust of the adjacent Yangtze Block without the need for crustal flow. (C) 2020 Elsevier B.V. All rights reserved.