On-land evidence for subduction of the proto-South China Sea beneath northern Borneo and tectonic reconstruction of Southeast Asia during the Early Permian-Oligocene

The development of the Mesozoic proto-South China Sea was closely related to the tectonic evolution of the Tethys and Paleo-Pacific domains and may have been a key driving mechanism for the opening of the South China Sea. Despite its importance, direct geologic evidence for the proto-South China Sea remains limited, and its development is debated between two primary models: intra-oceanic subduction and oceanic-continental subduction. Here, we present petrographic, geochronologic, and geochemical data from tuffs and Eocene-Pliocene sedimentary records in Sabah, northern Borneo. The results reveal that these tuffs, formed ca. 28 Ma, possess geochemical characteristics indicative of oceanic arc origins. Combined with changes in sediment provenance since the Oligocene and various subduction timelines around Borneo, we suggest that these tuffs resulted from the subduction of the proto-South China Sea. This oceanic arc, along with Eocene mid-ocean-ridge-like mafic rocks in Sabah, indicates that a younger oceanic basin had formed within the proto-South China Sea. This aligns with observations in Palawan, Philippines, illustrating the transition from an oceanic spreading center to a subduction zone and indicating the proto-South China Sea extended from eastern Sabah to Palawan. In addition, inherited zircons (ca. 280 Ma) in the tuffs exhibit geochemical signatures typical of continental arcs. Combined with the Permian-Cretaceous arc-related igneous rocks in Southeast Asia, this suggests that southern Sabah was part of the Paleo-Tethys Ocean before integrating into the Paleo-Pacific domain since the Triassic. By shedding light on these processes, our research provides critical insights into the existence, extent, and evolution of the proto-South China Sea and reconstructs the multiphase transitions between different tectonic domains in Southeast Asia.