Variety of subduction and accretion processes in cretaceous to recent plate boundaries around Southwest and central Japan

Abstract

The Shimanto and Miura-Boso terranes in southwest and central Japan are analyzed for the understanding of variation of the subduction-accretion process in convergent plate boundaries. The Shimanto is a Cretaceous to Recent Honshu accretionary prism of 200 km width in the Honshu arc side and is made up of melange, flysch and olistostrome units. These are repeated by many thrust faults and local broad and tight folds to make Schuppen structures. Shallow marine and olistostromal bodies overlie part of the Schuppen structures unconformably or are included within the structures. Normal subduction tectonics prevail for the formation of the accretionary prism but the effects of probable transform tectonics during late Oligocene to early Miocene, are considerable.The Miura-Boso region is a terrane newly accreted from the Izu arc to the Honshu arc. This terrane is made up of Miocene and younger volcaniclastic sedimentary rocks, dismembered ophiolitic materials and some of the rocks from the Shimanto Supergroup. The Miura-Boso terrane is characterized by transpressional tectonics associated with eduction during the sedimentation on the highly oblique subduction zone between the Eurasia (later North America) and Philippine Sea plates.Several cases for the emplacement of the basaltic and other oceanic rocks to the landward side of the accretionary prism are discussed to account for the basic problem of the subduction-accretion tectonics.