Stratigraphy and styles of caldera-forming eruption and subsidence of the Akakura caldera in the South Kurikoma geothermal area, Northeast Japan.

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  • 栗駒南部地熱地域，赤倉カルデラの層序と火砕流噴出・陥没様式
  • クリコマ ナンブ チネツ チイキ アカクラ カルデラ ノ ソウ ジョ ト カサイリュウ フンシュツ カンボツ ヨウシキ

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The South Kurikoma geothermal area has a caldera cluster within 25 by 15 km area. They are the Mukaimachi caldera, Akakura caldera, Nakayamadaira collapse basin, and are covered with Pliocene to Pleistocene felsic pyroclastic rocks including welded tuff. The caldera cluster coincides with a broad negative gravity anomaly outlined by steep marginal gradients and it surrounds the Lower to Middle Miocene basements, showing a regional tumescence.The Akakura caldera is about 10 km north-south by 9 km east-west of topographic wall and has a well-preserved resurgent dome, corresponding to a local positive gravity anomaly zone. This zone is characterized by radially outward dips of the caldera fill and a half-circular reverse fault. The Akakura caldera fill, >1, 400 m thick, has been divided into earlier "caldera-forming" and later "post-caldera" deposits. The caldera-forming deposits consist of two pyroclastic facies ; (1) felsic lapilli tuff facies interpreted as caldera-forming ash flows whose eruption caused the collapse of caldera. Analysis of accidental fragments within the felsic lapilli tuff facies indicated that the voluminous ash flows ejected from vents along structural boundary. K-Ar age data indicate that the caldera-forming eruption took place during Pliocene or Pleistocene. (2) tuff breccia facies, intecalated with the felsic lapilli tuff facies, is interpreted as debris-avalanche deposits derived from oversteepened caldera wall scarp during and after cruption of ash flows. The post-caldera deposits which ponded with the collapse basin are composed of breccia, volcanic sandstone, pumice tuff, fine tuff, and mudstone. These were mainly deposited by sediment gravity flows in subaqueous environments with dacitic volcanism.The Akakura caldera is similar to the Valles-type caldera because of pre-caldera tumescence, the pattern of gravity anomaly, caldera fill facies, vents along structural boundary inferred from lithic analysis, and resurgent domc.

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