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Abstract: Controversies exist on the tectonic models at the northern margin of the Qinghai-Tibetan Plateau. New magnetotelluric (MT) data, with a bandwidth of 0.001–5827.5s, were collected along one profile starting from the Northern Qilian Mountains in the southern end, crossing the Hexi Corridor, the Kuantan Shan-Hei Shan uplift, the Huahai Basin, and coming into the Beishan Block. Two-dimensional (2D) and three-dimensional (3D) resistivity models of the data are derived using magnetotelluric inversion codes. The 2D models, and the electrical structures along the profile extracted from the 3D models, are comparable and complementary in defining reliable scales and shapes of anomalies in electrical resistivity. The crust-upper mantle structures beneath the northern plateau are characterized by two imbricated southward underthrusted blocks of continental crust or lithosphere. The Northern Qilian Fault is interpreted as a gently south-dipping décollement in the upper crust along the top of a high resistivity body below the Northern Qilian Mountains. The two high resistivity bodies below the Kuantan Shan-Hei Shan uplift are disconnected; the upper crustal one may connect with the high resistivity basement below the Northern Qilian Mountains in that the attitudes and deformations of the strata at these areas are coherent; whereas the lower crust-upper mantle one may be the cold lithospheric materials of the block north to the Hexi Corridor and that forms a south-dipping boundary between the Hexi Corridor and its northern neighbors. The observed high electrical resistivity beneath the Beishan Block provides further support for the notion that the Beishan Block should be rigid as a whole; only in this manner could it transfer stress from south to north. The northern boundary of the Qinghai-Tibetan Plateau is below the Kuantan Shan-Hei Shan Fault and underthrusting of continental crust or lithosphere is the main cause for the thickening of the plateau crust in the north. [Copyright &y& Elsevier]

Abstract: Only sparse deep geophysical data have been obtained in the region to the north of the Eastern Kunlun Fault (EKLF) in the Tibetan Plateau, which leads to inconsistent and ambiguous descriptions of the area in various geodynamic models. The detailed geophysical issues in the region, including the northeast boundary of the plateau and the crust–mantle coupling mode northeast of the EKLF, are under dispute. To resolve these disputes, we have collected magnetotelluric (MT) data along five profiles in the region to the north of the EKLF since 2009. The interpretation of data from the latest profile, labeled L5, which starts from the eastern margin of the Qaidam Basin, crosses the Qilian Mountain and the Hexi Corridor, and finally reaches the Southern Alashan Block, is the focus of this paper. These new results are comparable with our previously determined structures. The two-dimensional (2D) resistivity model supports three divisions of the Qilian Mountain: a complex Northern Qilian, a relatively simple sandwiched Southern Qilian, and a transitional Central Qilian in between them. Our data indicate a relatively low-resistivity layer at the middle-lower crust below the eastern margin of the Qaidam Basin and the Southern Qilian Mountain. This result indicates a weak crustal zone (WCZ) existing northeast of the EKLF and is interpreted as a large north-dipping thrust at this depth. The presumed imbricated thrust in the crust in the northeastern plateau resulting from the indenting of the mantle wedge from the south is the primary process in the crustal thickening in the region. The MT results indicate that the northeastern boundary of the Tibetan Plateau is not below the EKLF; instead, we suggest that the northeastern boundary of the outgrowth of the Tibetan Plateau is along and below the northern rim of the Hexi Corridor. [Copyright &y& Elsevier]