Submarine Lava-fountain Eruptions in the Back-arc Basin during Opening of the Japan Sea

Abstract:

In northeast Japan, basaltic rock generated during the middle Miocene opening of the Japan Sea, is widely distributed. Most of them are found on the back-arc side of the northeast Japan. They are thought to be related to back-arc rifting during the opening of the Japan Sea. Although the magma genesis of these basalts have been studied, there are few studies have investigated on submarine volcanic edifices and a detailed description of the type of volcanism related to opening of the Japan Sea. The present study aimed to reconstruct details of the subaqueous volcanism that formed the back-arc basin basalts (BABB) during this event, and to discuss the relationship between volcanism and tectonics, using detailed facies analyses based on field investigation. The study area of the southern Dewa Hills contains well-exposed basalt related to opening of the Japan Sea. Five types of basaltic rock facies are recognized: (1) coherent basalt facies, (2) massive basalt dyke facies, (3) jigsaw-fitting monomictic basaltic breccia facies, (4) massive or stratified monomictic basaltic breccia facies, and (5) massive or stratified monomictic basaltic lapillistone–tuff facies. The basaltic rocks consist mainly hyaloclastite with minor amount of lavas. Based on facies distributions, we infer that the paleo-volcanic edifice was at least 10 km across and 1.5 km high, and that volcanism occurred mainly at the center of the study area. Given that the rocks contain many fluidal clasts, submarine lava-fountaining is inferred to have been the dominant eruption style. An ENE–WSW alignment of many dikes and feeder dykes of these basaltic rocks intrude the study area indicate that the palaeo-stress field during the eruption stage was a NNW–SSE tension and the reconstructed submarine volcanoes probably formed by fissure eruptions. The basaltic rocks are interpreted as the products of back-arc volcanism that resulted from asthenospheric upwelling and crustal extension related to opening of the Japan Sea, which drove strong tectonic subsidence and active lava-fountain volcanism. Zircon U-Pb ages of the rhyolite that erupted soon after the end of basaltic volcanism show 10.8 ± 0.2 Ma. Basaltic volcanism related to opening of the Japan Sea is continued until about 11 Ma in the back-arc side of Japanese archipelago.