Extensional stress accompanied by Miocene near-trench magmatism in the southern Kii Peninsula, SW Japan

Abstract

Voluminous near-trench magmatism began in the Kumano area, SW Japan, at ca. 15 Ma. The magmatism formed batholiths, cauldrons, and numerous dikes. It has been known that The dikes are oriented perpendicular to the Nankai Trough, and are hosted by young formations that are folded with axes parallel to the trench. Therefore, it has been considered that the magmatism was accompanied by trench-perpendicular compression during the period shortly after backarc opening in the Japan Sea. For this study, we obtained orientation data from 14 felsic and 151 mafic dikes in the early to middle Miocene Kumano Group and Shionomisaki Igneous Complex. Inversion of both datasets yields a normal-faulting stress regime with NE–SW extension. This orientation meets at a high angle with that of the trench. We also obtained zircon fission-track ages of 14.5 ± 1.5 and 13.9 ± 1.4 Ma from two felsic dikes. These ages and existing radiometric and micropaleontological data show that both the groups of dikes were formed at ca. 14.5 Ma. Large- and meso-scale structures indicating trench-perpendicular shortening in igneous rocks suggest that the folding took place after the magmatism. We interpret that the extensional stress resulted from uplift caused by the emplacement of a batholith under the Kumano area. The inferred normal-faulting stress regime raises doubts about popular tectonic models of SW Japan that rely on a transition from extension to compression at ca. 15 Ma.