Crustal structure beneath the Jiamusi Block and the Nadanhada Terrane from seismic and magnetotelluric studies: Implications for the subduction of the Pacific Plate

Abstract

The Nadanhada terrane is considered to accrete to the eastern end of the Jiamusi Block during the Pacific Plate subduction. Constraints on the deep structure of the Nadanhada terrane and Jiamusi Block are essential to study the evolution, transformation and current activity of the eastern Asian continental margin. To reconstruct the accretion processes, two magnetotelluric sounding sections and a seismic reflection section were conducted in the eastern part of Jiamusi Block, the middle, and the southern end of the Nadanhada terrane respectively. Geophysical data revealed resistant stable structures and west-dipping discontinuous reflections with a thickness of around 15 km in the upper crust of Jiamusi Block. In contrast, the lower crust of the Jiamusi block exhibited high conductivity and short sub-horizontal reflections that suggest upwelling of mantle material under Pacific Plate subduction. Moreover, the low resistivity zone observed connecting the shallow crust with the upper mantle in the Tongjiang-Yuejinshan fault was interpreted as the boundary line between the Jiamusi block and Nadanhada terrane. The Nadanhada terrane can be subdivided into the Yuejinshan and Raohe complexes based on their distinct geophysical features. The Yuejinshan Complex is characterized by irregular bodies with high resistivity and short arc reflections with a thickness of about 10 km. In contrast, the Raohe Complex displays high-resistivity blocks and west-dipping thrust nappe strong reflections with a thickness of about 5 km. Additionally, the high-conductivity layer located between the Raohe Complex and the underlying basement is believed to be the detachment zone. Since the Cretaceous, the continuous subduction of the Pacific Plate has caused magmatism that has led to the high conductivity crust. The observation of low-resistivity channels and arcuate reflections within the crust supports the notion of upwelling of mantle-derived materials brought about by the subduction of the Pacific Plate in the Cenozoic era.