Stratigraphic framework of the Green Tuff successions in Japan with reference to the associated geologic events

Abstract

Green Tuff is an informal term that has been widely applied to the volcanic successions distributed mainly along the Japan Sea coast of Honshu, Japan. As Green Tuff underlies the middle Miocene marine sediments and sparsely contain Miocene-type fossil floras, these successions have been generally considered to be Early to Middle Miocene in age. However, recent stratigraphic revisions, based on the facies analysis and more reliable radiometric dating of selected volcano-sedimentary successions, suggest that the Green Tuff successions span from Late Eocene to Middle Miocene in age and can be divided into six time-stratigraphic units: the Upper Eocene to Lower Oligocene (44 to 28 Ma); the Upper Oligocene (28 to 23 Ma); the lower Lower Miocene (23 to 20 Ma); the middle Lower Miocene (20 to 18 Ma); the upper-Lower Miocene (18 to 15.3 Ma); and the lower Middle Miocene (15.3 to 12.3 Ma). These geological units likely reflect the evolution of a continental arc to the present-day island arc through the following stages: 1) onset of continental arc rifting and volcanism accompanied by a local marine transgression; 2) thermal crustal doming accompanied by local rifting and volcanism; 3) onset of spreading in the central doming area, with the propagation of uplifting, rifting, and bimodal volcanism to the surrounding region by successive upwelling of hot asthenosphere, followed by a local marine transgression onto the back-arc region; 4) acceleration of spreading, rifting, and the inflow of a warm ocean current into the back-arc region; 5) rapid rifting along the eastern margin of the Japan Sea by further invasion of hot asthenosphere to the trench side; and 6) onset of collision against, and subduction beneath SW Japan, by the Philippine Sea Plate, alongside migration of the volcanic front to the Pacific coast, and onset of crustal shortening. In this scenario, the last event appears to represent the beginning of the present-day island arc system of Japan.