Northward channel flow in northern Tibet revealed from 3D magnetotelluric modelling

Abstract

The Kunlun fault defines one of the major northern tectonic boundaries of the Tibetan plateau. Previous geophysical studies have detected a major change in rheology across this boundary, but it is not clear how this is related to models that have invoked crustal flow. The lithospheric resistivity structure of the Kunlun fault has been investigated by both the INDEPTH III and IV magnetotelluric (MT) transects. All the MT data were processed using modern statistically-robust methods, and have been analysed for directionality and dimensionality. In order to improve understanding of the anisotropic distribution of melt previously revealed by our remodelling of the INDEPTH III MT data, a variant approach on 3D inversion of 2D profiles was investigated to explore and improve lateral resolution. In addition to the apparent surficial deformation associated with the sinistral strike-slip Kunlun fault, the 3D modelling of the INDEPTH MT data reveals that complex deformation processes are occurring at mid-crustal depths in northern Tibet. The 3D MT inversion results, supported by synthetic modelling, particularly confirm and highlight the presence of separate north–south intrusions of conductive material crossing the Kunlun fault into the more resistive Kunlun–Qaidam block. These north–south intrusions are interpreted to be associated with the horizontal channel flow of partially molten Songpan–Ganzi crust into two (or more) separated channels moving northwards and crossing the surficial trace of the Kunlun fault at mid-to-lower crustal depths.