Fault system, deep structure and tectonic evolution of the Qilian Orogenic Belt, Northwest China

The Qilian Orogenic Belt (QLOB), located on the northern margin of the Tethyan tectonic domain, suffered from the development and evolution of the Early Paleozoic Proto-Tethyan Ocean (i.e., the North Qilian Ocean, NQO), and was the key orogenic belt for the uplift and expansion of the Tibetan Plateau on its northeastern margin. Based on regional geological survey and structure geological mapping and combined with previous geophysical data and newly completed super-broadband magnetotelluric (MT) sounding, this paper expounds the characteristics of the fault system in Northwest China and the QLOB. Through superbroadband MT data acquisition, processing, and inversion, and structural interpretation of shallow, medium and deep electrical structures along the MT profile across the QLOB, the distribution of Mesozoic and Cenozoic basins related to the fault system, and the deep crustal structure underneath the QLOB are analyzed. The deep electrical structure of the QLOB reveals a fossil bidirectional subduction of the NQO plate in the Proto-Tethyan tectonic domain. According to deep electrical structure, the original width of the subducted North Qilian Ocean plate was over 600 km. The southward subducted NQO plate probably reached the north margin of the Qaidam Block (the present Qaidam Basin) in the south, with a gentle subduction angle underneath the Central and South Qilian Block. Then, the melting-broken plate (or slab) might have migrated downward to the depth. In the north, the subducted NQO plate probably reached the southern margin of the Yin'e Basin of the present position, with a steep subduction angle. The dumpling melting-broken NQO plate was separated from the crustal retention plate in the north, through the northward extrusion of the upper mantle. The authors hold that the folding deformation of the subducted ocean plate was probably caused by the compressional tectonics in the Mesozoic and the Cenozoic, especially during the Late Cimmerian orogeny of Early Cretaceous and the India-Asian continental collision in Neogene. The shallow part of the electrical structure proves that the Yumushan thrustnappe structure was developed northward to the North Qilian Fault, with the Yumushan as a klippe in the north. The concealed Early Cretaceous basin underneath the klippe should be a good area for oil and gas prospecting. According to the normal faulting developed in the Early Cretaceous, the authors constrain the main stage thrusting of the North Qilian Fault in the Early Cretaceous, which might provide the evidence for the uplift and expansion of the northern margin of the Tibetan Plateau in the Early Cretaceous. Based on combined data from previous researches and this study, the authors put forward a conceptual tectonic evolution model for the Qilian Orogenic Belt.