This study provides geological, geochemical, and chronological insight into the evolution of the Hachijojima volcanoes of the Izu Bonin arc. The regional Ata-Torihama tephra (Ata-Th; 0.24 Ma) and Kikai-Tozurahara tephra (K-Tz; 0.095 Ma) from Kyushu are intercalated within the voluminous proximal volcanic products. Our study combines detailed geologic mapping, tephrochronology, U-Pb zircon dating, and published drill core data from NEDO (1993) to evaluate the time transgressive 3-dimensional (thus 4dimensional) structure of the dissected Mihara-yama volcano, the older of the two volcanoes on Hachijojima. The volcanic succession comprises terrestrial volcanic basement, marine volcaniclastic rocks, and terrestrial tuff intercalated with tholeiitic basalt and the regional calc-alkaline tephra layers. The undissected Hachijo-Fuji stratovolcano (tholeiitic basalt) overlies marine volcaniclastics abutting the northwestern paleo sea cliffs of Mihara-yama. Newly-described folding and normal faulting of the marine and overlying terrestrial volcaniclastic rocks suggests NW-SE shortening that may be associated with the collision of the Izu-Bonin arc with the Honshu arc. The proto Hachijo-jima volcano emerged above the sea > 0.24 Ma, and this date can be applied as a molecular biological calibration date for organisms on this island.
HighlightThe erosionally-dissected Mihara-yama volcano was built on 2.1 Ma altered volcanic basement. Deposition of volcanic strata over this basement buttress unconformity began in submarine conditions at < 1.8 Ma, followed by subaerial pyroclastic ows and a tholeiitic-lava ow from 0.24 Ma onward.Emergence of the Mihara-yama volcano above sea level was > 0.24 Ma.The Ata-Torihama tephra (Ata-Th; 0.24 Ma; U-Pb age < 0.46 Ma) and Kikai-Tozurahara tephra (K-Tz; 0.095 Ma; U-Pb age < 0.24 Ma) from giant calderas of Ata and Kikai of Kyushu (calc-alkaline) are intercalated in the terrestrial scoriaceous-pumice tuffs derived from the Mihara-yama volcano (tholeiite). Previous correlations to the Aira-Tn tephra (Aira-Tanzawa; AT; 0.0275 Ma) are not viable.Adjacent to the northwestern paleo sea cliff of this old volcano, a shallow marine basin was formed at < 0.26 Ma. Subaerial scoria and tuff cones were also formed as small-scale lateral volcanoes.Tholeiitic lava ows built the subaerial Hachijo-Fuji stratovolcano over the shallow marine basin, and two major volcanoes were connected to form a single Hachijo-jima island.The island deformed by NNW-SSE shortening, re ecting the Izu-Bonin collision toward the central Japan arc.