Post-caldera geology of Gora Region in Hakone Volcano Group, Japan

Abstract

This study examined the geological development of the Gora region in the NW quadrant of the caldera of the Hakone Volcano Group in Japan, covering the period since the first caldera-forming stage (< 240ka). Previous studies on borehole samples in this region have documented subsidence during MIS7 and MIS4, probably as a result of large pumice flow eruptions; these ages are supported by the pollen assemblage in tuffaceous sand-mud of the caldera fill. A new borehole drilled in 2009 by the Japan Meteorological Agency (JMA-V29) yields a continuous succession from the second calderaforming stage during MIS4 to the present surface. The lake that formed in this area during MIS4 is considered to have been saline to some extent, as indicated by brackish diatom species in lacustrine deposits. The source of salt was probably volcanic fluid, which also feeds the present-day NaCl-rich hot springs in this area. The lake was then filled with pumice, which is a secondary deposit of Plinian eruptions after the caldera-forming stage (tephra group named HkCC). Analysis of the lacustrine deposit indicates that the ancient lake level of MIS4 was >470 m above sea level, which is higher than the present-day terrace formed at the caldera outlet. The pumice is over-lain by the Hayakawa Debris Flow (HDF), which forms the basal part of the Younger Central Cones (YCC) of this region. A block-and-ash flow deposit overlies the HDF. The juvenile andesite clasts in the block-and-ash flow are almost identical to other YCC lavas, although with a much higher Rb content (>10 ppm). The uppermost part of the section comprises a talus deposit that is ~28 m thick and that formed at ca. 20 ka.