Paleozoic to Mesozoic micro-block tectonics in the eastern Central Asian Orogenic Belt: Insights from magnetic and gravity anomalies

Abstract

The NE China region involves a complex accretionary belt formed by multiple stages of assembly of micro-blocks from the Paleozoic to Mesozoic, the tectonics of which provides important clues on the evolution of the East Asian continental margin. With a view to evaluating the micro-block evolution in NE China, we used gravity and magnetic methods to analyze the geophysical characteristics of the micro-blocks and their boundaries. Combining with regional geology, paleomagnetism and paleontology, we reconstructed the micro-block assembly history in the eastern Central Asian Orogenic Belt. Five Paleozoic to Mesozoic micro-blocks are recognized in NE China: the Erguna, Xing'an, Songnen, Jiamusi and Nadanhada, and their boundaries show significant differences in gravity and magnetic fields. The boundary between the Erguna Block (EB) and the Xing'an Block (XB) shows as a continuous tectonic line in the gravity field, whereas the one between the XB and the Songnen Block (SB) exhibits a rather discrete but strong magnetic anomaly belt. The boundary between the SB and the Jiamusi Block (JB) is characterized by both distinct gravity and magnetic anomalies. The Nadanhada Micro-block (NMB) appears as an independent micro-block in the fourth-order detail field of the Bouguer gravity anomaly, where it shows as a short axis or circinate structure in this anomaly field. The Paleozoic to Mesozoic tectonic evolution of these micro-blocks in NE China is closely related to five sutures in the eastern Central Asian Orogenic Belt. The Xinlin-Xiguitu Suture was first formed at ~500 Ma, leading to the assembly of the EB and the XB, the Heihe-Hegenshan Suture was formed at 320–290 Ma, marking the amalgamation of the XB and the SB, the Mudanjiang-Yilan Suture was formed at ~200 Ma, marking the collision of the JB and the SB. The Paleo-Asian Ocean closed during 250–230 Ma in a scissor style. The subduction of the Paleo-Pacific Ocean was initiated at ~200 Ma. Based on the deep structural architecture imaged from gravity and magnetic anomalies, we propose an evolutionary model of the micro-blocks in the NE China, which provides important insights into the tectonic evolution of the NE China region during the Paleozoic to Mesozoic.