A Miocene island-arc volcanic seamount: the Takashibiyama Formation, Shimane Peninsula, SW Japan

Abstract

The Miocene volcanic complex of the Takashibiyama Formation consists largely of subalkali, subaqueous basalt to andesite lavas and andesite to dacite subaqueous volcaniclastic flow deposits. Most of subaqueous lavas are moderately to intensely brecciated with rugged rough surfaces and ramp structures similar to subaerial block lava. Volcaniclastic flow deposits commonly include basalt to andesite lava fragments and/or pyroclastic materials, and are similar in internal features to debris flow and turbidite deposits. Subaqueous block lavas and minor pillow lavas form volcanic piles above the vents, being flanked or covered with volcaniclastic flow deposits. Each volcanic pile is several kilometers wide and several hundred meters thick, and overlaps one after another as the major eruptive center migrates from time to time. Thus, the volcanic complex comprises three or more superimposed volcanic piles which are intruded by basalt to andesite dikes and sills, and are underlain by mafic to intermediate large hypabyssal to plutonic bodies. Hydrothermal alteration occurs in places but especially concentrates in the lower part of the volcanic complex. After the cessation of opening of the Japan Sea, the volcanic complex started to erupt in a middle bathyal environment on the Japan Sea side, and became emergent as it grew to a thickness over 1500 m during 4 m.y. under a N-S trending horizontal compressional stress. It is a volcanic seamount probably in a typical island-arc setting.