## 4 Results and Discussion
### The Origin of the Mesozoic Volcanic Arc  
The tectonic evolution of the SCS region has long been a focus of research, with many scholars proposing that plate subduction plays a critical role in the geological activities of this area. [PERSON] et al. (2005) [PERSON] et al. (2005), based on multiple deep seismic reflection data (profile DPSR2002, marked as profile 2 in Figure 1), revealed the potential existence of a subduction zone beneath the Baiyun Depression. The boundary between the Dongsha Rise and the Baiyun Depression is considered to be the junction of the subducting plate and the Moho discontinuity. The study found that the igneous bodies are located above the Moho at the leading edge of the subducting plate, while the uplift of the high-velocity body in the lower crust suggests the upwelling of mantle-derived material. This can be interpreted as the upwelling of mafic material induced by the subducting plate, forming high-temperature magma chambers that gradually evolve into a volcanic arc. Furthermore, to the north of the high-magnetic anomaly belt, the distribution of large areas of higher magnetism material and scattered high-magnetism anomalous bodies may be the result of subsequent upwelling, diffusion, and localized intrusion of mafic mantle-derived material within the plate. In addition, the drilling data indicate that during the Late Jurassic to Early Cretaceous periods, the sedimentary environment in the Chaoshan Depression transitioned rapidly from deep-sea facies to coastal facies [35]. This suggests that the northern SCS experienced a significant uplift of the continental crust during the Late Jurassic period, corresponding to the subduction of the plate. Overall, the characteristics of the deep magnetic source closely  
Figure 11: Magnetic inversion results along profiles A–D. ZHU I D: Zhu I depression; PHR: Penghu Rise; TXNB: Taixinan Basin; DSR: Dongsha Rise; CSD: Chaoshan Depression. White lines represent the 0 m isobath. match the multiple deep reflection seismic and OBS seismic data, indicating that the high-magnetic anomalies are consistent with regional deep structures. Petrophysical data indicate that the high-magnetism materials are primarily igneous rocks, and drilling data indicate Mesozoic subduction in the region. According to these findings, it is concluded that the high-magnetic anomaly belt in the northern SCS is caused by a Mesozoic volcanic arc. This volcanic arc extends in an NE direction, with the main depth between 20 and 28 km, stretching from the Dongsha Rise to Taiwan Island. Drilling data from [PERSON] et al. (2022) [35] reveal the rapid uplift of the crust in the northern SCS during the Mesozoic period. Petrochemical composition analysis by [PERSON] et al. (2016) [12] identified Mesozoic granites associated with subduction-related environments. Additionally, multiple deep seismic reflection data from [PERSON] et al. (2005) [16], [PERSON] et al. (2015) [9], [PERSON] et al. (2025) [17], and others reveal relics of subduction. Based on these results, we propose that these igneous bodies represent the volcanic arc roots formed by the upwelling of mantle-derived material following subduction, acting as magma chambers at the time, with the upper portions erupting to form numerous seamounts [34]. The volcanic arc near the Dongsha Rise has a larger scale, which is inferred to be the initiation point of the subduction zone. Mesozoic sedimentary layers in the northern SCS represent the forearc basin associated with the Mesozoic subduction of the Paleo-Pacific plate, as shown by the seismic reflection data and the results of the drill hole LF35-1-1 [35]. The high-magnetism belt in the lower crust indicates the volcanic arc connecting the forearc basin on the northern margin of the SCS, which contributes to the formation of the Paleo-Pacific Plate subduction system.