島弧衝突に関連した高遠-守屋地域（長野県）の地殻回転

抄録

Miocene rocks in central Honshu should provide paleomagnetic data used to analyze crustal rotation associated with the collision of the Izu and Japan arcs. Here we present paleostress, U–Pb, and paleomagnetic data from Miocene igneous rocks of the Takato-Moriya area in the eastern part of SW Japan. We mapped numerous dolerite dikes, and a paleostress analysis suggests two different sets of extensional stress conditions with a NE–SW least principal stress axis. The dikes intrude the extrusives of the Moriya volcanic rocks (MVRs) (Makimoto et al. 1996) and the late Cretaceous granites and metamorphic rocks of the Ryoke belt. We obtained a U–Pb zircon date of 15.8 ± 0.2 Ma for an upper stratigraphic site of the MVRs, which is compatible with published fission-track zircon dates for baked granites adjacent to the dolerite dikes (ca. 16 Ma: Hoshi et al. 2015). Thus, the igneous rocks of the study area were formed immediately after the clockwise rotation of SW Japan associated with the Japan Sea opening (Hoshi 2018). We collected igneous rocks for magnetic measurements from more than 30 sites and obtained 29 site-mean magnetization directions. All the directions for the dolerites have reverse polarity, while the directions for the MVRs have both normal and reverse polarities. Overall, the directions are counterclockwise deflected from the expected direction, indicating counterclockwise rotation in the study area relative to the main part of SW Japan. The counterclockwise rotation can be associated with the collision of the Izu arc with the Japan arc. Comparison of the paleomagnetic direction for the study area with those for the main part of SW Japan implies the onset of the formation of curvature of the Median Tectonic Line in central Honshu before 16 Ma.

Ref.

Hoshi (2018) J. Geol. Soc. Japan 124, 675–691; Hoshi et al. (2015) Island Arc 24, 221–231; Makimoto et al. (1996) Geology of the Takato District, with Geological Sheet Map at 1:50000, Geol. Surv. Japan.