Divergent double subduction of Bangong-Nujiang Ocean revealed by high-resolution magnetotelluric data at 86°E in the northern Tibetan Plateau

Abstract

The Tibetan Plateau was mainly produced by the assembly of several terranes resulting from ocean closure and continental collision. The formation and evolution of the Bangong-Nujiang Ocean (BNO) played a prominent role in the Tibetan Plateau accretion before the Cenozoic collision between the Indian and Eurasian continents. The closure mode of the BNO between the Lhasa and Qiangtang terranes has been controversial, involving subduction polarity, i.e., regarding whether there was only northward subduction beneath the Qiangtang terrane or both southward and northward subduction beneath the Lhasa and Qiangtang terranes. To resolve this issue, a dense broadband magnetotelluric (MT) profile transecting the Bangong-Nujiang suture zone (BNS) at 86°E was completed. After standardized data processing and analysis, geological interpretation can be performed on the two-dimensional (2-D) inversion results of the MT data. Through a large number of inversion experiments, we obtained a reliable lithospheric electrical structure beneath the BNS and its adjacent regions. Our results show that the upper crust in our study area is featured by high resistivity overall, but it is cut by several fault zones, indicating that it may be influenced by the late strike-slip fault caused by the India-Asia collision. The middle-lower crust of the northern Lhasa terrane is highly conductive and may represent juvenile crust. The existence of a northward and southward inclined electrical gradient zone in the BNS may indicate that the BNO was closed by north-south bidirectional subduction.