The Fukutoku Volcanic Complex: Implications for the northward extension of Mariana rifting and its tectonic controls on arc volcanism

Abstract

Fukutoku-Oka-no-Ba (FOB) volcano is one of the most active submarine volcanoes in the Izu-Bonin-Mariana Arc. Historically, explosive eruptions associated with new island formation have occurred, and pumice rafts and discolored water have been frequently observed. Multiple geochemical studies focusing on the pumice erupted from FOB have been conducted to date, however, volcanic and tectonic features around FOB are poorly understood due to the scarcity of bathymetric and geological data. We compiled previously collected multibeam bathymetry data and major element compositions of dredged volcanic rocks to reveal the nature of magmatic and tectonic activity around FOB. Bathymetry data collected by 6 ship-based surveys between 1999 and 2006 define relatively young volcanic structures consisting of undeformed circular craters, steep-sided edifices and flank edifices on Kita-Fukutoku Bank, Kita-Fukutoku Caldera and Minami-Ioto which surround FOB. All the edifices with signs of relatively young volcanism exist on a common basal platform, together forming a large volcanic complex that is 50 km long and 25 km wide. We have named this feature the Fukutoku Volcanic Complex. Previously unreported N-S trending grabens and faults on the complex were identified from inspection of the bathymetry data. Some faults are continuous of the fault scarps bounding the northern Mariana Trough to the south, suggesting they are surface manifestations of the northward extension of Mariana Trough rifting. Vertical offsets of the grabens and faults decrease northward and the northernmost tip of Mariana rifting can now be observed to extend to 24°28′N on Kita-Fukutoku Bank. The basal platform and some young edifices on the Fukutoku Volcanic Complex are elongated or aligned in the N-S direction suggesting that arc volcanism here is tectonically influenced by Mariana rifting. Compiled major element compositions demonstrate two compositional types of magma with trachytic and basaltic rocks erupted around FOB. Although previous geochemical studies implied the association of basaltic magma with the FOB trachytic eruptions, the distribution of basaltic volcanism on the seafloor had not been well-defined. The presence of basaltic rocks around FOB, whose alkali compositions are similar to those of the northern Mariana Trough, and rift-related tectonic morphology suggest that the intrusion of basaltic magmas may be associated with the northward extension of Mariana rifting. Our detailed regional study provides new insights into the interplay of arc volcanism and extensional tectonics in the earliest stage of the Izu-Bonin-Mariana rift system.