Electrical resistivity structures of the Kunlun–Qaidam–Qilian system at the northern Tibet and their tectonic implications

Abstract

The northern Tibetan Plateau is characterized by northwest–southeast trending basin-range systems such as the Kunlun Shan Ranges–Qaidam Basin–Qilian Shan Ranges. The Cenozoic evolution and deformation of the Qaidam Basin and its neighboring ranges are important indications for the northward growth of the Tibetan Plateau. The latest magnetotelluric data were collected along a profile in N16.5°E from the Songpan-Ganzi Block, across the Kunlun Shan Ranges, east section of the Qaidam Basin, and ending in the Qilian Shan Ranges. Both two-dimensional and three-dimensional inversions of the data along the profile were performed and the results provided new evidence for lithospheric structures in the northern Tibetan Plateau. With three-dimensional isotropic inversion codes, we are able to model the Phase Roll Out of Quadrant (PROQ) magnetotelluric data, which exist at low frequencies in a number of continuous sites at the northern Qaidam Basin. Sensitivity studies of the three-dimensional models indicate that the Qaidam Basin with low-resistivity upper crust is the major contributor for the PROQ effect, while a south–north-striking low-resistivity gap intersecting with the Qaidam Basin at its northern part affects the range of the PROQ effect. In the magnetotelluric resistivity models, the Qaidam Basin is not in symmetric structure at upper-crust level, with its depocenter near the Northern Qaidam Fault. At mid-lower crust level, the compositions of both the Kunlun Shan Ranges and the southern Qaidam Basin are of generally high-resistivity, which does not support large-scale lower-crust materials of the Songpan-Ganzi Block flow below the Kunlun Shan Ranges or the Qaidam Basin. Instead, the unsymmetrical Qaidam Basin and the shovel-shape high-resistivity anomalies in crust below the Kunlun Shan Ranges and the southern Qiadam Basin indicate the regional northward push from the Songpan-Ganzi Block. The south-deepening low-resistivity anomalies in the crust of the Qilian Shan Ranges support the existence of an intracrustal detachment, which controls the thrusts in northern Qaidam Basin and the Qilian Shan Ranges. The crustal deformation in northern Tibetan Plateau is consistent with mantle convergence or collision between the Kunlun–Qaidam lithospheric mantle and the Asian lithospheric mantle.