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The three-dimensional (3-D) electrical structure of the upper-mantle was used to examine the deep origins of and relationship among the Cenozoic volcanoes located in Northeast China (NEC). High-quality, long-period magnetotelluric (LMT) full-impedance tensor data were collected in NEC and subjected to 3-D Gauss-Newton inversion in order to construct a resistivity model. The resulting model reveals the presence of multiple localized low-resistivity anomalies (LRAs) within the high resistivity lithosphere beneath NEC. These LRAs partially coincide with Cenozoic volcanoes on the surface. Three LRAs that form a larger, annular LRA were observed in the deep upper mantle beneath the Songliao Basin, whereas vein-like LRAs were found in the asthenosphere that connect the lithosphere and deep upper mantle. Petrophysical analyses suggest that the LRAs may have been caused by fluid-induced melting. Based on our electrical model, we propose that, following dehydration of the subducted Western Pacific slab into the mantle transition zone (MTZ) beneath NEC, the released water migrated upward and caused partial melting at the top of the MTZ beneath the Songliao Basin. Under the effect of buoyancy, the melted mantle formed a thermal upwelling that caused melting of asthenosphere before diapiring at the base of the dry lithosphere. The magma then penetrated structural boundaries (such as thinner, weaker, or activated suture zones) and finally reached the Earth’s surface. This melting and upwelling of hot mantle materials may have resulted in large-scale volcanism in the region throughout the Cenozoic, including the eruption of Changbai Mountain and Halaha Volcanoes. Our results suggest that the Cenozoic NEC volcanoes may all share a similar mode of genesis, and probably originated from the annular LRA in the deep upper mantle.

The creation and evolution of the Songliao Basin is closely related to the closure of the Paleo-Asian Ocean and the Mongolia-Okhotsk Ocean, and the subduction of the Paleo-Pacific Ocean. In an atte...

The Central Asian Orogenic Belt (CAOB) is the largest accretionary orogenic belt in the world. The Songnen and Jiamusi blocks, which are located along the eastern segment of the CAOB, form a complex structure influenced by the Mudanjiang ocean closure and the Paleo-Pacific plate subduction. Constraints on the deep structure of the Songnen and Jiamusi blocks are critical for studying the evolution, transformation and current activity of the CAOB. This study presents an analysis and interpretation of a 450-km-long magnetotelluric profile that imaged the deep lithospheric structure of these two blocks. The best-fit three-dimensional resistivity model, based on the nonlinear conjugate gradient algorithm, highlights the key lithospheric structures. The results show that the lithosphere beneath the Lesser Xing'an Range in the Songnen block is characterized by a high-resistivity structure, which is mainly related to the granite formed in multiple stages. The crust forming the Sanjiang basin in the Jiamusi block possesses a high–low–high resistivity structure in the transverse direction, corresponding well to the tectonic units. The low-resistivity structure of the Fujin uplift extends to the mantle, which represents mantle material that intruded the crust. There is a low-resistivity structure beneath the suture zone of the two blocks that is homologous with the large-scale low-resistivity anomaly beneath the basin and is related to the subducted Paleo-Pacific plate. Strong dehydration at the inflection point of the plate reduces the solidus temperature of the mantle material, resulting in melt and upwelling of mantle material. The Jiamusi block can be divided into southern and northern parts, possessing different structures. The southern part represents an original part of either the Gondwana continent or Tarim craton. The Sanjiang basin, in the northern part, has been heavily influenced by the subducted Paleo-Pacific plate, and is a region of strong activity.

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 Longzhong block).

The electrical resistivity tomography (ERT) has the characteristics of low cost, high efficiency, rich information, non-destructive exploration, and large detection depth. At present, it is being widely used in archaeological exploration. It can provide a more accurate result whether it is the preliminary survey delineation or the later detailed exploration. Chengsijiazi ancient city, located at the bank of Tao’er river, was built during the Liao and Jin dynasties. Except for the western wall destroyed by the river, the rest of the city walls were well preserved. The main target body of this survey is a part of wall foundation of an ancient building in the ancient city ruins. The ERT data are acquired along 55 survey lines and the horizontal differential field line method is introduced to interpret the inversion results of the measurement data. The horizontal differential field line method could more accurately describe the structure and boundary characteristics of the target body. The ancient building wall foundation is characterized by obvious high-gradient dense field line, so that we can accurately delineates the position of the wall base, which was excavated by archaeologists and has been verified.

The Qilian Shan is located along the northeastern margin of the Tibetan Plateau, which is a key area for studying the growth and expansion of the Tibetan Plateau. There have been several geodynamic models developed to explain the deformation pattern of the Tibetan Plateau extending to the northeast, but they remain controversial. The magnetotelluric (MT) method can study subsurface electrical conductivity and rheological properties. A dense broadband and long period magnetotelluric (MT) profile across the middle Qilian Shan has been completed. The MT profile starts from the northeastern margin of the Qaidam Basin, crosses the Qilian Shan and Hexi Corridor, and finally reaches the southwestern margin of the Alashan Block. Based on data analysis results, a three-dimensional (3-D) resistivity model of the crust and upper mantle has finally been obtained. The MT results show that a relatively low resistivity zone exists at the bottom of the upper crust of the Qilian Shan, features as an intracrustal decollement, which indicates the decoupling deformation between the upper and lower crust. The isolated conductive bodies in the middle-lower crust of the Qilian Shan may have formed during the primary stage of plateau growth, suggesting that there may be no existence of the middle-lower crustal flow beneath the Qilian Shan. The Alashan Block lithospheric mantle may have partly underthrust southward the Northeastern Tibetan Plateau. The Tibetan Plateau may have grown and expanded northeastward across the Hexi Corridor to the Heli Shan Fault.