9:00 AM - 9:15 AM
[SCG55-12] Understanding the age and crustal structure of the unknown pieces of the Proto-Philippine Sea Plate: Preliminary results of the KH-23-10 cruise
The Izu-Bonin-Mariana (IBM) arc, traditionally regarded as a typical oceanic island arc, began to form in the early Eocene (~52 Ma) with the initiation of subduction of the Pacific Plate beneath the Philippine Sea Plate (Ishizuka et al., 2011). However, fragments of continental crust, consisting mainly of granitic and metamorphic rocks with Jurassic to Cretaceous zircon ages, have been identified in the Daito Ridges, the Izu forearc, and the southern end of the Kyushu-Palau Ridge, which formed the basement of the pre-Eocene proto-Philippine Sea Plate. On the other hand, no region has been conclusively identified as an oceanic crust. These findings suggest that the proto-Philippine Sea Plate was composed of Mesozoic continental crust, and the initial IBM arc may have formed with such crust as the overriding plate.
However, significant gaps remain in our understanding of the proto-Philippine Sea Plate, particularly in the northwestern region, which includes the Huatung Basin and Gagua Ridge.
The Huatung Basin exhibits east-west magnetic lineations, suggesting that it was formed by north-south seafloor spreading. However, the timing of its spreading remains controversial. Deschamps et al. (2000) interpreted the Huatung Basin as formed in the Early Cretaceous based on magnetic anomalies and Ar-Ar dating of gabbroic rocks from the southern part of the basin. In contrast, Doo et al. (2014) argued for an Eocene age based on the matching of magnetic anomaly patterns. Additionally, lithospheric thickness derived from seismic structure data also suggests a relatively young age, postdating the Eocene (Kuo et al., 2009). If the Huatung Basin formed in the Cretaceous, it likely contained oceanic crust as part of the proto-Philippine Sea Plate. Conversely, if it formed in the Eocene, it is more likely to be a back-arc basin that developed contemporaneously with the West Philippine Basin.
The interpretation of the Gagua Ridge also remains debated. Deschamps et al. (1998) proposed that the Gagua Ridge is an uplifted segment of basin crust along a transform fault under compressional stress. In contrast, Qian et al. (2021) reported a Cretaceous age for basaltic andesites with island arc geochemical affinities collected from the ridge crest. Furthermore, these volcanic rocks reported to contain Proterozoic zircons, suggesting that the Gagua Ridge may be a fragment of continental crust detached from the South China Block. Determining whether the Huatung Basin formed in the Cretaceous or the Eocene, and whether the Gagua Ridge consists of uplifted oceanic crust or continental crust, is crucial for understanding the tectonic setting at the onset of IBM arc subduction. Therefore, detailed investigations are needed.
R/V Hakuho Maru KH-23-10 cruise was proposed to elucidate the geological history of the Huatung Basin and Gagua Ridge through geophysical surveys and systematic rock sampling.
In the Huatung Basin, we aimed to compare magnetic anomalies with radiometric ages of basement rocks to reconstruct its spreading history. The basin is also a key region for reconstructing paleoceanographic conditions and understanding the biogeography of the Kuroshio Current. To address these aspects, we planned sediment core sampling and taxonomic studies of benthic organisms. In the Gagua Ridge, we planned systematic rock sampling to understand the crustal structure and the formation age of the ridge.
The KH-23-10 cruise was initially scheduled for 12 days, from December 13 to 24, 2023, with departure and arrival at Naha. However, due to adverse weather conditions caused by a strong winter pressure system, the survey was significantly shortened. Despite this, a total of seven rock dredges were conducted in the Huatung Basin and the Gagua Ridge, successfully collecting basalt from the Huatung Basin and granitoids, felsic volcanic rocks, and other igneous rocks from the Gagua Ridge. Additionally, two piston cores, one multi-corer, and one VMPS sampling were conducted within the basin, along with a geophysical survey covering 260 nautical miles, obtaining seafloor topography, magnetic, and gravity data. This presentation will report an overview of the KH-23-10 cruise and preliminary results from the collected samples.
However, significant gaps remain in our understanding of the proto-Philippine Sea Plate, particularly in the northwestern region, which includes the Huatung Basin and Gagua Ridge.
The Huatung Basin exhibits east-west magnetic lineations, suggesting that it was formed by north-south seafloor spreading. However, the timing of its spreading remains controversial. Deschamps et al. (2000) interpreted the Huatung Basin as formed in the Early Cretaceous based on magnetic anomalies and Ar-Ar dating of gabbroic rocks from the southern part of the basin. In contrast, Doo et al. (2014) argued for an Eocene age based on the matching of magnetic anomaly patterns. Additionally, lithospheric thickness derived from seismic structure data also suggests a relatively young age, postdating the Eocene (Kuo et al., 2009). If the Huatung Basin formed in the Cretaceous, it likely contained oceanic crust as part of the proto-Philippine Sea Plate. Conversely, if it formed in the Eocene, it is more likely to be a back-arc basin that developed contemporaneously with the West Philippine Basin.
The interpretation of the Gagua Ridge also remains debated. Deschamps et al. (1998) proposed that the Gagua Ridge is an uplifted segment of basin crust along a transform fault under compressional stress. In contrast, Qian et al. (2021) reported a Cretaceous age for basaltic andesites with island arc geochemical affinities collected from the ridge crest. Furthermore, these volcanic rocks reported to contain Proterozoic zircons, suggesting that the Gagua Ridge may be a fragment of continental crust detached from the South China Block. Determining whether the Huatung Basin formed in the Cretaceous or the Eocene, and whether the Gagua Ridge consists of uplifted oceanic crust or continental crust, is crucial for understanding the tectonic setting at the onset of IBM arc subduction. Therefore, detailed investigations are needed.
R/V Hakuho Maru KH-23-10 cruise was proposed to elucidate the geological history of the Huatung Basin and Gagua Ridge through geophysical surveys and systematic rock sampling.
In the Huatung Basin, we aimed to compare magnetic anomalies with radiometric ages of basement rocks to reconstruct its spreading history. The basin is also a key region for reconstructing paleoceanographic conditions and understanding the biogeography of the Kuroshio Current. To address these aspects, we planned sediment core sampling and taxonomic studies of benthic organisms. In the Gagua Ridge, we planned systematic rock sampling to understand the crustal structure and the formation age of the ridge.
The KH-23-10 cruise was initially scheduled for 12 days, from December 13 to 24, 2023, with departure and arrival at Naha. However, due to adverse weather conditions caused by a strong winter pressure system, the survey was significantly shortened. Despite this, a total of seven rock dredges were conducted in the Huatung Basin and the Gagua Ridge, successfully collecting basalt from the Huatung Basin and granitoids, felsic volcanic rocks, and other igneous rocks from the Gagua Ridge. Additionally, two piston cores, one multi-corer, and one VMPS sampling were conducted within the basin, along with a geophysical survey covering 260 nautical miles, obtaining seafloor topography, magnetic, and gravity data. This presentation will report an overview of the KH-23-10 cruise and preliminary results from the collected samples.