New Insights Into Crustal and Mantle Flow Beneath the Red River Fault Zone and Adjacent Areas on the Southern Margin of the Tibetan Plateau Revealed by a 3-D Magnetotelluric Study

A number of geodynamic models have been proposed for the southeastern margin of the Tibetan plateau and include a range of deformation processes. One unresolved issue is whether crustal and mantle flow occurs, and if it does, how flow contributes to the mass balance of the India-Asia collision. To address this question, new magnetotelluric data were collected and used to derive a three-dimensional electrical resistivity model of the crust and upper mantle beneath the Red River Fault (RRF) zone and adjacent areas. The most prominent features of the model are (1) a resistor in the upper- middle crust directly beneath the trace of the RRF; (2) a major change in upper mantle resistivity across the RRF; and (3) a significant conductor in the upper mantle northeast of the RRF, which extends upward into the crust, and which requires a melt fraction of up to 3%. The model suggests that the lower crustal conductors may be due to melt/fluids derived from the mantle, rather than from outward flow from Tibet. The most likely source of fluids and melts could be upwelling mantle flow related to the Hainan mantle plume. The change in resistivity across the RRF implies a change in lithospheric strength may explain the present-day localization of deformation and uplift at this location. The resistivity model may also give insights into the distribution of ore deposits in Ailao Shan, since many mineral deposits are derived from magmatic fluids generated in the mantle at the edges of regions of thick lithosphere. Plain Language Summary Continent-continent collisions are an important tectonics process that have formed mountain ranges such as the Himalaya and features such as the Tibetan Plateau. Some recent models have proposed that deformation in such regions is partly accommodated by ductile deformation processes such as crustal flow. They proposed that flow occurs locally overs tens of kilometers beneath the Himalaya and are supported by geological and geophysical evidence. Much larger crustal flow systems have been proposed to occur beneath the eastern margin of the Tibetan Plateau, where the collision has extruded large regions of the crust with upper crustal deformation focused on a system of major transform faults. This study investigates these models by using geophysical data to image the properties of rocks beneath the Red River Fault area of southeastern Tibet. The magnetotelluric method was used to map the resistivity of the rocks, which is a parameter sensitive to the presence of fluids, which in turn control the strength. Regions of low resistivity were mapped and inferred to be due regions of partial melt. These appear to originate in the mantle, and not be connected to similar features to the North. This implies that large-scale crustal flow may not occur in this region.