Crustal rotation in Tsugaru in Aomori Prefecture, northern Japan

Abstract

The northeastern Japan arc 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 geoscientific approach 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 Japan and show that the 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 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 Tsugaru.