Characteristics of Magnetic Anomalies in the Northern South China Sea and Their Implications for Pre‐Cenozoic Tectonics

In order to study the pre-Cenozoic tectonics of the northern South China Sea (SCS), we have calculated the reduction to the pole (RTP) anomalies, apparent magnetization and pseudo-gravity anomalies using the latest shipboard magnetic data in the north of the South China Sea. The density and magnetic susceptibility along three OBS profiles were determined by inversion calculation. The result shows that the high magnetic anomaly belt (HMAB) of the Dongsha Uplift (DSU) is the continuation of the volcanic belt in the coastal area of Zhejiang and Fujian Provinces, and is dislocated by the NW trending NW fault F10 which may be the old transformation boundary. The NW trending fault F2 is the south boundary of the HMAB, and limits the north boundary of underplating. The fault F3 is a week zone before the crustal extension and thinning, and deep thermal states are different on both sides, which may reveal the location of the Late Mesozoic subduction. The magnetization of magnetic layer reduces in the magnetic quiet zone between faults F2 and F3. We ascribe the reducing magnetization to the hydrothermal alteration while high temperature mantle materials being underplated. However, on the south side of F3, the magnetization increases and magnetic source blocks of the Magnetic Quiet Zone (MQZ) are fragmented. The cause of weak positive magnetic anomalies on the surface is due to the suppression between positive and negative magnetic anomalies under oblique magnetization. The unique characteristics of the magnetic layer in this area may indicate the existence of “ancient oceanic crust”.