Integrated Geophysical Constraints on the Lithosphere–Asthenosphere Structure and Tectonic Dynamics of the Jiamusi Block

ABSTRACT The Jiamusi Block in Northeast Asia provides a critical window for investigating the subduction of the Pacific Plate beneath the Western Pacific Orogenic Belt during the Mesozoic and Cenozoic. To constrain the structure and evolution of the lithosphere–asthenosphere system, this study integrates regional gravity and magnetic surveys with two NW–SE‐trending magnetotelluric (MT) profiles of lengths 250 and 150 km across the block. Two‐dimensional nonlinear conjugate gradient inversion derived the electrical structure to a depth of 200 km. The inversion models reveal distinct crustal and mantle low‐resistivity bodies (labelled C1–C8) that spatially correlate with abrupt positive gravity and aeromagnetic anomalies. These low‐resistivity features are interpreted as signatures of mantle‐derived fluids and partial melts, indicative of asthenospheric upwelling and lithospheric thinning. We attribute these processes to the westward subduction of the Pacific Plate, which induced significant crust–mantle interaction and fluid metasomatism. Furthermore, a comparative analysis reveals that the lithospheric thickness of the Jiamusi Block (70–95 km) is intermediate between that of the Songnen Block (65–80 km) and the Xing'an Block (100–120 km). This variation suggests that lithospheric thickness in the region is primarily controlled by the geometry of the subducting slab's mantle wedge and the intensity of the associated fluid metasomatism.