4D volcanic geology of Hachijo-jima islet, Izu-Bonin arc
Abstract This study provides geological, geochemical, and chronological insight into the evolution of the Hachijo-jima 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 4-dimensional) structure of the dissected Mihara-yama volcano, the older of the two volcanoes on Hachijo-jima. 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 un-dissected 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.