FORMATION AND FILLING PROCESSES OF A FOREARC BASIN IN RESPONSE TO MIGRATION OF A TRIPLE JUNCTION: THE UPPER CRETACEOUS IZUMI GROUP, SOUTHWESTERN JAPAN

Ridge-trench interaction causes large effects on magmatism in the volcanic front and sedimentation in the forearc basin along the subduction zone. Strata in forearc basins adjacent to active volcanic fronts preserved various histories related with such interaction. The Izumi Group in southwestern Japan is a possible example of this type of basins during Late Cretaceous time. The group consists mainly of felsic volcano-plutonic detritus, which overlies Early to Late Cretaceous plutono-metamorphic complex (the Ryoke complex). We studied a drilled core obtained from the basal part of the Izumi Group by analyzing sedimentary facies and U-Pb dating of detrital zircon grains in a tuff bed in order to reconstruct the depositional environments and to constrain the age of deposition. On the basis of the lithofacies associations, the core was subdivided into six units from base to top, as follows: mudstone-dominated unit nonconformably deposited on the Ryoke granodiorite; tuffaceous mudstone-dominated unit; tuff unit; tuffaceous sandstone–mudstone unit; sandstone–mudstone unit; and sandstone-dominated unit. This succession suggests that the depositional system changed from non-volcanic muddy slope or basin floor, to volcaniclastic sandy submarine fan. Based on a review of published radiometric age data of the surrounding region of the Ryoke complex and the Sanyo Belt which was an active volcanic front during deposition of the Izumi Group, the U-Pb age (82.7±0.5 Ma) of zircon grains in the tuff unit corresponds to those of felsic volcanic and pyroclastic rocks in the Sanyo Belt. The eastward younging (parallel to the arc) of the depositional ages in the Izumi Group is the same as the eastward younging trend of the volcanic rocks in the Sanyo Belt. Migration of a triple junction caused by oblique subduction could be one of the major reasons of their eastward diachroneity along the axis of the forearc.