The Geodynamic Process of the Convergence and Transformation of the Songnen and Jiamusi Blocks in the Northeast China： Revealed by Magnetotelluric Sounding

The Songnen and Jiamusi blocks are located in the eastern Central Asian Orogenic Belt, whose convergence and transformation were affected by the Mudanjiang ocean closure and the Pacific plate subduction. In order to reveal the effects of these events in the Mesozoic, we arranged a magnetotelluric profile with 460 km across the Lesser Khingan range in the Songnen and Sanjiang basins in the Jiamusi block. Two-dimensional resistivity model of the lithosphere is obtained based on nonlinear conjugate gradient algorithm after data analysis. Inversion model show that the Lesser Khingan range lithosphere is 80-100 km thick with high resistivity, related to granites crystallized by multi-stage magmatism. The collage zone of the two blocks consists of the west dipping conductive Mudanjiang fault, the resistive anomaly on the east side of the fault and the resistive Mudanjiang oceanic plate remnant. The structures indicate the westward subduction of the Mudanjiang ocean and the "hard collision" between the two blocks. The lithosphere of the Sanjiang basin is thin relatively, about 50-60 km. The conductive structures in the Suibin and Qianjin depressions correspond to the sediment and basement of the basin. The resistive structure in the Fujin uplift represents the mafic intrusive rock formed by crystallization of the mantle materials. There is a conductive belt under the basin, extending downward from the bottom of the lithosphere, indicating the upwelling molten mantle materials. Pacific plate subduction has transformed the Songnen and Jiamusi blocks. The dehydration of the plate reduced the solidus temperature of the mantle material, and upwelling molten material led to the thermal (chemical) erosion of the lithosphere. The extensional setting, formed by the rolled-back of the subduction plate in the Cretaceous, resulted in the occurrence of large-scale magmatic events in the Lesser Khingan range.