Electrical resistivity structures northeast of the Eastern Kunlun Fault in the Northeastern Tibet: Tectonic implications

摘要

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.