Magnetotelluric signature of a crustal transition zone in northern Tibet associated with partial melt penetration across the Kunlun Fault

The overall objective of the last phase of the INDEPTH (International Deep Profiling of Tibet and Himalaya) project has been to develop a better understanding of the structure and evolution of the northern margins of the Tibetan plateau, namely the Kunlun and Altyn Tagh faults. For the Kunlun Fault, both INDEPTH Phase III and new Phase IV magnetotelluric (MT) data were investigated using 2D anisotropic inversion as well as 3D modelling. The resulting resistivity models highlight a penetrative extension of the partially molten Tibetan middle crust to the north, crossing the subvertical Kunlun Fault. Furthermore, the anisotropic observations highlighted by the INDEPTH MT modelling define progressive finger-like melt intrusion beneath the Kunlun Shan. However, this intrusion may not be homogeneous along the whole of the northern Tibetan border along the Kunlun Fault, as its depth and horizontal extension to the north are likely to vary. The low melt fractions required by the MT anomaly are sufficient to change the rheology of the Kunlun crust. Therefore, the weak middle crust crossing the Kunlun fault is decoupling locally the upper-crustal deformations from the lower crust and mantle. The anisotropic anomaly observed on the resistivity models characterizes a transition zone between the thicker weak Tibetan crust and the more rigid, shallower Qaidam crust. The melt penetration across the Kunlun Fault is likely to be accommodating crustal shortening in northern Tibet, but may also reflect a possible mechanism for the growth of the plateau to the north, with extension of the crustal thickening to the south of the Qaidam Basin.