Paleozoic suture and Mesozoic tectonic evolution of the lithosphere between the northern section of the Xing'an Block and the Songnen Block: Evidence from three-dimensional magnetotelluric detection

Abstract

The Xing'an and Songnen Blocks are microcontinents in the eastern Central Asian Orogenic Belt (CAOB). The Heihe–Hegenshan suture has been interpreted to represent the boundary between the blocks. However, the lack of ophiolite exposure and deformation towards the northern extent of the suture mean that its precise nature and location remain controversial. To better understand the position and structure of the suture, magnetotelluric data were acquired at 104 stations along five profiles in the northern Songnen and Xing'an Blocks. A three-dimensional nonlinear conjugate gradient algorithm was used to image the resistivity structure of the lithosphere. Our resistivity models indicate that the lithospheres of both the Xing'an and Songnen Blocks are characterized by overall high resistivity and have heterogeneous electrical structures. We define a highly conductive anomaly (C2) to the east of the Duobaoshan Island Arc that divides lithosphere of high resistivity and controls the distribution of Carboniferous magmatic rocks related to collision. We suggest that C2 is the northern extension of the Heihe–Hegenshan suture. The highly conductive anomaly (C1) is located in the Xing'an Block on the west side of the island arc. While the exposed lithology on the surface is Jurassic intrusions, usually with high resistivity. We suggest that C1 may be an early structure, which was transformed by the closure of the Mongolia–Okhotsk Ocean. No large-scale Mesozoic intrusive bodies occur above C3, a highly conductive anomalous region in the Songnen Block. And the seismic reflection characteristics have changed significantly in the region C3. Therefore, we interpret C3 as a shear zone in the crust related to a change in the subduction direction of the paleo-Pacific plate. The resistivity model provides new geophysical evidence for determining the suture and the evolutionary traces of the Mongolia-Okhotsk Ocean and the Paleo-Pacific Ocean.