著者

星 博幸 山口 尚之 吉田 武義

出版者

一般社団法人 日本地質学会

雑誌

地質学雑誌 (ISSN:00167630)

巻号頁・発行日

vol.129, no.1, pp.551-566, 2023-11-03 (Released:2023-11-03)

参考文献数

81

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One of the tectonic features that characterizes the Miocene geology of Northeast Japan is crustal block rotation during and after the opening of the back-arc Sea of Japan. However, the spatial and temporal characteristics of this block rotation are not well understood. A better understanding of this block rotation would shed light on the Miocene tectonic evolution of Northeast Japan. We carried out geological mapping and geochronological and paleomagnetic analyses to determine the timing of block rotation on the southern margin of the Asahi Mountains, on the back-arc side of Northeast Japan. We present a geological map and chronostratigraphic model of the lower-middle Miocene strata and redefine two lower Miocene formations. K-Ar dates show that andesite dikes and sills were intruded at ~15 Ma. The crust beneath the study area was rotated counterclockwise relative to the Asian continent, although it was broken into blocks with varying degrees of rotation. In the eastern part of the study area, counterclockwise rotation of ~40°-50° relative to the continent occurred between 17.5 and 15 Ma. Rotation probably did not occur during the earlier development of intra-arc rift basins. The timing of the rotation overlaps with that of the formation of the Myozawa Syncline, which is a growth fold that formed during the deposition of shallow marine sediments during the latest early Miocene. Complex block rotation in a possible dextral transtensional regime in the Asahi Mountains was likely confined to a period of 2 Myr between 17 and 15 Ma. This block rotation probably occurred during the late stage of the opening of the back-arc Sea of Japan.