
Citation: | HE X B, HE M L, WANG X Y, et al. Subduction Initiation and Mafic Intrusions: Rethinking the South China Sea-Luzon Connection[J]. CT Theory and Applications, 2025, 34(5): 1-4. DOI: 10.15953/j.ctta.2025.176. |
The conventional view suggests that the subduction of the South China Sea plate beneath Luzon occurred due to the oceanic lithosphere’s high density, facilitating subduction initiation. However, before the South China Sea opened, a continental margin likely existed, meaning that Luzon was directly adjacent to the continental margin rather than the oceanic basin. This would make subduction initiation more challenging. Here, we propose a new model suggesting that during the formation of the South China Sea, extensive mafic magmatic underplating occurred along its continental margin. The high-density magmatic additions may have increased the overall density of the continental margin, potentially exceeding that of Luzon, thereby enabling subduction to proceed.
The dynamics and mechanisms of subduction initiation have long been a central topic in geological research[1]. In intra-oceanic settings, subduction—whether induced or spontaneous—is commonly understood to occur when an older, denser oceanic plate sinks beneath a younger, less dense plate[2]. However, the South China Sea plate (~33~16 Ma), despite being relatively young, is subducting beneath the older western Philippine Sea plate (~59~33 Ma) along the Manila Trench[3] (Fig.1). This atypical subduction scenario challenges conventional models and necessitates a reevaluation of the underlying processes governing intra-oceanic subduction initiation.
A previous study[6] attributed this atypical subduction initiation to the presence of microcontinents, which may act as triggers for the process. Specifically, the existence of microcontinental blocks within the Luzon arc is supported by the presence of ancient zircon in modern intra-oceanic arcs. This model is compelling as it facilitates subduction initiation by introducing these exotic components. The concept is plausible, given that microcontinental blocks exhibit continental characteristics, making them inherently less dense than the subducting South China Sea plate.
Unfortunately, multiple studies suggest that the South China Sea likely has an eastern continental margin[7-9]. If true, this implies that Luzon was directly adjacent to the continental margin rather than an oceanic basin. Such a configuration complicates subduction initiation, as a continental margin subducting beneath a microcontinent presents a unique challenge, given that both share continental characteristics.
In this study, we attribute subduction initiation to lower-crustal mafic intrusions within the continental margin, which facilitate the progression of subduction. The following section will further elaborate on this process.
High-velocity, high-density anomalies in the continental lower crust have been widely detected across the northern[10], southern[11-12], and northeastern[13-15] margins of the South China Sea. These anomalies have been attributed to either magmatic activity associated with paleo-Pacific subduction or post-rift magmatic modification[14]. They play a crucial role in subducting plate deformation and hydration processes, particularly in interactions with the Manila Trench[16]. Existing observations indicate that high-density anomalies are widely distributed along the continental margins of the South China Sea.
Therefore, it is reasonable to infer that similar anomalies were also present along the eastern margin, though most have since been subducted into the deep mantle.
The key factor in subduction initiation is the density contrast between two colliding lithospheres[1]. Given that the high-velocity anomaly in the lower crust suggests increased density, we propose that it plays a critical role in facilitating subduction initiation. Without intrusive materials, the density difference between the continental margin and Luzon may be insufficient to drive subduction. However, after mafic intrusions, the continental margin becomes denser than Luzon, enabling subduction to proceed. An evolutionary model illustrating subduction initiation along the Manila Trench is presented in Fig.2.
Key components of this conceptual framework include magmatic intrusions driven by Paleo-Pacific subduction, post-spreading magmatic activity, and the transformation of mafic intrusions into eclogites following collision-induced crustal thickening, ultimately leading to continental margin break-off.
This study revisits the conventional understanding of South China Sea subduction beneath Luzon by introducing a new model. Traditionally, subduction initiation was thought to be driven by the high density of oceanic lithosphere. However, before the South China Sea opened, a continental margin likely existed, complicating direct subduction.
Our findings suggest that extensive mafic magmatic underplating during South China Sea expansion increased the density of the continental margin, potentially surpassing that of Luzon. This density contrast could have facilitated subduction, offering new insights into the geodynamic evolution of the region. In the future, plate reconstruction studies combined with numerical modeling will provide deeper insights into the role of lower-crustal mafic intrusions in intra-oceanic subduction initiation.
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