Middle Miocene rift magmatism of basaltic rocks in the Japan arc

Abstract

Intense and widespread Middle Miocene basaltic magmatism occurred within and adjacent to the Akita-Yamagata oil field, located on the west flank of the NE Japan arc. This magmatism is distributed over a 200 km x 30 to 40 km area. Its estimated volume is 6,000 krn3, much larger than the 1,500 km3 estimated for the Quaternary volcanics in the NE Japan arc. It consists of extrusive and intrusive basaltic rocks that exhibit major and trace element compositions commonly associated with back-arc basin basalt. This composition suggests a mantle source with slight metasomatism by subduction components. The type of basaltic magmatism changed from extrusive to intrusive from early to late Middle Miocene. In early Middle Miocene, these rocks were extruded in an extensional tectonic environment, as feeder dykes trending NNE-SSW to NE-SW. The lava associated to this magmatism occurs intercalated with bathyal sediments and formed numerous submarine volcanoes. During the late Middle Miocene, basaltic rocks were mostly intruded into the sediments as dolerite sills. In the area surrounding the rift zone, widespread early Middle Miocene magmatism occurred in an extensional regime. This magmatism was bimodal, producing both acidic and basaltic rocks. The lava associated was subaerial and subaqueous (shallow water environment). We suggest that the rift zone is a failed continental rift possibly developed during the Middle Miocene along the periphery of the then actively spreading Japan Sea.