[1ch417-22] R7[レギュラー］新生代の地質事変記録

[R7-O-5] 本州北部，西津軽地域の地殻回転運動

*星 博幸¹、久野 元晴² (1. 愛知教育大学、2. 愛知教育大学（卒業生）)

キーワード：東北日本、地殻回転、日本海拡大、テクトニクス、中新世、古地磁気、青森県、西津軽

Northeast Japan rotated counterclockwise during the early Miocene major opening stage of the Japan Sea, but the timing, amount, and mode of the rotation remain poorly constrained. These issues should be resolved by an interdisciplinary geological study in areas where the well-dated rock sequences formed in the period of the Japan Sea opening are present. Here we present geological and paleomagnetic data from northern Honshu and show that counterclockwise rotation occurred after the formation of early Miocene volcanic rocks. After mapping geological structures in the field, we collected samples for paleomagnetic analysis at more than 60 sites in two areas (Fukaura and Kodomari) in the western Tsugaru region of Aomori Prefecture. In the Fukaura area, andesites and rhyolites of the Odose Formation (ca. 20–18 Ma: Hoshi et al. 2003) have a remanent magnetization direction counterclockwise deflected from the early Miocene expected direction, although several site-mean directions of lava flows are not corrected for bedding tilt due to possible paleotopographic slopes. A similar magnetization direction is recorded in the various types of early Miocene volcanic rocks of the Gongenzaki Formation (ca. 23–20 Ma: Danhara et al. 2005) and Fuyube Formation (ca. 17–16 Ma: Hoshi et al. 2013, 2016) in the Kodomari area. It is noteworthy that a rhyolite lava unit of the Fuyube Formation preserves a westerly magnetization direction, suggesting counterclockwise rotation after 17 Ma. Future research should aim to determine magnetization directions and U–Pb zircon dates of middle Miocene and younger formations in western Tsugaru.

Ref.
Danhara, T. et al., 2005, J. Geol. Soc. Japan 111, 476–487; Hoshi, H. et al., 2003, J. Japan. Assoc. Petrol. Tech. 68, 191–199; Hoshi, H. et al., 2013, J. Japan. Assoc. Petrol. Tech. 78, 414–418; Hoshi, H. et al., 2016, J. Geol. Soc. Japan 122, 163–170.