Clockwise rotation of SW Japan and timing of Izanagi–Pacific ridge subduction revealed by arc migration

Abstract

Igneous rocks associated with the Cretaceous to Paleogene volcanic arc in SW Japan show ages that young from west to east in a direction parallel to the Median Tectonic Line suggesting corresponding translation of a heat source traditionally interpreted in terms of oblique subduction of a spreading ridge. However, recent oceanic plate reconstructions suggest ridge subduction may be younger than the main arc activity. Age compilations of 1227 points of felsic to intermediate Cretaceous and Cenozoic igneous rocks from the Japan arc show arc magmatism that can be separated into an early active period 130–60 Ma (stage 1), a subsequent period of quiescence 60–46 Ma (stage 2), which is followed by a resumption of igneous activity from 46 Ma onward (stage 3). In southwest Japan, the orientations of the magmatic arcs of stages 1 and 3 show and angular discordance of about 20°. The lack of active arc magmatism and the occurrence patterns of adakitic and high-Mg andesitic magmas indicate that ridge subduction occurred during stage 2. The arc age distribution pattern of stage 1 is explained by the slab shallowing related to a younging of the subducting slab as the ridge approaches. Furthermore, the obliquity of the arcs formed at stages 1 and 3 is explained by a 20° clockwise rotation of the inner zone of southwest Japan during the ridge-subduction phase. Oceanic plate reconstructions show counterclockwise rotation in the subduction direction after the ridge subduction phase, and coupling of the subducting oceanic plate with the upper plate would support microplate rotation in the inner zone. The new proposed tectonic reconstructions provide a framework to related Paleogene subduction of an active spreading ridge along the east Asia margin not only to the distribution of granitic bodies but also to rift-related basin formation on the eastern margin of the Eurasian continent and to rotation of crustal blocks indicated by paleomagnetic data of Cretaceous terranes.