Middle Crustal Partial Melting Triggered Since the Mid‐Miocene in Southern Tibet: Insights From Magnetotelluric Data

Abstract

AbstractA weak middle crust induced by fluid phases is generally considered to exist beneath the Himalayan‐Tibetan orogen; however, there remains controversy regarding the interpretation of high‐conductivity and low‐velocity regions imaged in various geophysical studies. To better understand middle crustal rheological behaviors and associated triggering mechanisms beneath southern Tibet, we present a geoelectrical model derived from a three‐dimensional magnetotelluric inversion in this study. An effective conductivity of 0.3 S/m at depths of 10–28 km indicates a high melt fraction of ∼35 vol% in the Lhasa terrane. In the Tethyan Himalayan orogen, an effective conductivity of <0.2 S/m corresponds to a melt fraction <15 vol%, which is lower than previous estimations. We obtain very low viscosities either from laboratory data (1010–1012 Pa s) or empirical models (1016–1018 Pa s). A combination of radiogenic heat and strain heating may account for the weakened middle crust. We prefer a weak coupling between crustal east‐west extensions and mantle lithospheric deformations.