Electrical structure of the eastern segment of the Qilian orogenic belt revealed by 3-D inversion of magnetotelluric data: New insights into the evolution of the northeastern margin of the Qinghai-Tibet Plateau

摘要

The eastern segment of the Qilian orogenic belt, composed of the Linxia block and the Longzhong block, is located at the northeastern margin of the Qinghai?Tibet Plateau. The area has experienced multiple tectonic events, such as the closure of the Qilian Ocean, the convergence of the North China block and the Qilian terrane, and the collision of the Indian and Eurasian plates, forming a complex tectonic framework. To investigate the geological evolution of the area and the current location of the suture between the plates, we performed three-dimensional inversion of magnetotelluric data acquired along two profiles in the eastern segment of the Qilian orogenic belt using a nonlinear conjugate gradient algorithm to obtain electrical resistivity structure models to 60-km depth. The results include three key findings. (1) The Linxia block was originally a Japanese-type island arc with a mainly low-resistivity structure, which corresponds to deformed, fractured middle and lower crust that contains saline fluids. (2) The Longzhong block was originally a Mariana-type island arc and has a resistive structure with good integrity, consistent with crust composed of ocean island basalt. (3) The Maxianshan fault, shown as a belt with a steep gradient in electrical resistivity model, is a supracrustal fault that acts as a suture between the Japanese-type island arc (now the Linxia block) and the Mariana-type island arc (now the Long-zhong block).