Mitsubishi Corporation Exploration Company, Japan

The Miocene and Plio-Pleistocene accretionary prism formations in both the Nankai and Sagami Troughs on the northern boundary of the Philippine Sea Plate subduction and their on-land Miura-Boso prism analog south of Tokyo represent incipient stages of accretionary orogenic belt construction. Recent IODP expeditions and our extensive submersible studies in the Nankai accretionary prism have shown that both frontal thrust propagation and out-of-sequence thrust faulting contribute to the lateral and vertical growth of accretionary prisms. Deformation of unlithified sediments into semi-lithified during slow and rapid stress generation produces various states of deformation in the accretionary prism strata. Deep-burial of trench sediments and oceanic rocks via underplating at depths in accretionary prisms leads to the formation of regional metamorphic rocks of high-pressure type. These metamorphic rocks locally become exhumed as a result of lateral transformation of core complexes either by doming or forearc sliver fault movement. Such modern examples are shown in the western and eastern sides of the Izu Arc collision zone. Another case for the emplacement of metamorphic rocks in accretionary prisms and forearc settings occurs in association with serpentinite diapirism in the very frontal part of trench slope and along serpentinite schist-bearing transform faults in the forearc area, best exemplified in the Izu-Bonin Trench slope edge and on the Ohmachi Seamount close to the present volcanic front, respectively. All these processes involved in accretionary prism growth and the incorporation of ophiolitic and high-grade metamorphic rocks into forearc settings, as well documented in the Tertiary to Recent evolution of the Japanese arc-forearc complexes, are significant components of accretionary orogenic belt evolution.