学位論文 Stratigraphy, distribution and widespread correlation of the Middle Pleistocene pyroclastic flow deposits derived from the southern Kyushu caldera region, southwest Japan

西澤, 文勝

pp.1 - 53 , 2017-03-25
内容記述
The purpose of this study is to clarify the stratigraphy and distribution of Middle Pleistocene pyroclastic flow deposits in the southern Kyushu Caldera Region based on petrographic characteristics and to delineate the history of huge caldera-forming eruptions in the caldera region including the eruptive ages estimated from stratigraphic positions. The author analyzed the lithology and samples from the pyroclastic flow deposits in the northern coastal area of Kagoshima Bay in southern Kyushu. The author also identified the petrographic characteristics of the pyroclastic flow deposits and examined their correlation with co-ignimbrite ash fall deposits (CAFDs) derived from large-scale pyroclastic flow. The author comprehensively studied the eruptive ages of tephras based on the radiometric ages and stratigraphic locations of proximal pyroclasts and CAFDs in several areas. Based on the eruptive ages, the author discussed the frequency of large-scale explosive eruptions in the southern Middle Pleistocene Kyushu Caldera Region. Eight pyroclastic flow deposits were identified. In ascending order, they are: Komiyaji (Kmj), Sagise (Sgs), Nabekura (Nb), Shimokado (Smkd), Oda (Oda), Fumoto Tuff, Kobayashi (Kb-Ks), Takeyama (Tkym) and Hegawa (Hgw). The tuff originated from the Aira Caldera, as Middle Pleistocene tephra stratigraphically below Kb-Ks (520-530 ka). In addition, three new widespread tephras were identified based on the re-examination of correlations between the pyroclastic flow deposits and distal ash fall deposits: Shimokado-Ks18 (Smkd-Ks 18), Takeyama-Ks 10 (Tkym-Ks 10), and Hegawa-Ks5 (Hgw-Ks5). Based on previous isotope stratigraphy studies, the eruptive ages of Smkd-Ks18, Tkym-Ks10, and Hgw-Ks5 are 570-580 ka (MIS 15), 480-530 ka (MIS 13), and 430-450 ka (MIS 12), respectively. The apparent volume of each tephra estimated from the distribution area and thickness of the CAFD is approximately 100 km3, assuming that each CAFD originating from the Aira Caldera is distributed concentrically. Therefore, a Volcanic Explosivity Index (VEI) of 7 was assigned to the eruptions. A gigantic eruption leading to a CAFD occurred on average once every -40 ka during the period from 580 to 450 ka, with longer intervals of large-volume eruptions after the eruption of Hgw-Ks5 (430-450 ka). Between the Smkd-Ks 18 to Hgw-Ks5 eruptions, gigantic eruptions occurred at intervals of 40 ka, whereas they took place at an interval of -100 ka after that period, up to the Ata (105 ka) eruption. The volcanic activity of the Middle Pleistocene in the southern Kyushu Caldera Region was considered to be more active between the Sgs and Hgw-Ks5 eruption, approximately 600 to 400 ka.
首都大学東京, 2017-03-25, 博士(理学)
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