Characteristics of the asthenosphere structure beneath the eastern segment of the Central Asia orogenic belt inferred from a long-period magnetotelluric survey

The eastern section of the Central Asian orogenic belt experienced multiple functions of the Paleo-Asian Ocean, the Mongolia-Okhotsk Ocean, and the Paleo-Pacific tectonic system. The multi-phase tectonic activities have not only reshaped the topography and lithosphere structure, but also changed the geometry of the asthenosphere beneath this belt. To reveal the characteristics of the asthenosphere under the complex background system, we acquired an electromagnetic sounding profile about 1800 km long in the eastern part of the Central Asia orogenic belt. Non-linear conjugate gradient inversion was performed to the data to establish a two-dimensional electrical structure model. The results show that there are many low resistivity anomalies inside the lithosphere beneath this region, which, on the surface, mostly correspond to the Quaternary volcanic groups or ancient suture zones. It implies that these low resistance anomalies are related to asthenosphere uplifting invasion. The asthenosphere exhibits high conductivity and is slightly thin in the south with a resistivity value of 10 to 30 Omega m and thicker in the north with a resistivity value of 10 to 0.1 Omega m. This electrical structure evidences the heterogeneity of the thickness, and partial melting of the asthenosphere on the north and south sides in the eastern Central Asian orogenic belt. The existing tomographic results show that the thickness of the asthenosphere in this orogenic zone is thicker in the east and thinner in the west. Combining with regional geology, we propose that like the lithosphere, the thickness and partial melting of the asthenosphere also changed in the process of collision orogenesis. The asthenosphere in the eastern Central Asia Orogenic belt has undergone three stages of tectonic events: the Paleo-Asian oceanic tectonic system, the Mongolian-Ihotsk tectonic system, and the Pacific tectonic system. Therefore, we estimate that the north-south difference of the asthenosphere beneath the eastern Central Asian orogenic belt was likely caused by that the closure of the Paleo-Asian Ocean was earlier than the Okhotsk Ocean. And The east-west variation was related with the influence of the Pacific tectonic system.