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The accretionary prism of the Nankai Trough off Shikoku Island is divided into four tectonic zones, based chiefly upon the characteristics of thrust faults and the velocity distribution within the wedge. They are, from trough axis to Tosa Terrace: the protothrust zone, imbricate thrust zone, multiple décollement zone, and earthquake thrust zone. The stacking velocity of the nondeformed trench-fill turbidites is 1.7 km/s. After accretion it gradually increases to 4.0 km/s or higher in the earthquake thrust zone. This value represents the critical geophysical boundary between the elastic deformation in the earthquake thrust zone and the aseismic deformation in the multiple décollement, imbricate thrust, and protothrust zones. Within the outer margin of the Tosa Terrace, the earthquake thrusts extend to the surface from this boundary, raising the outer margin at a rate of 70 to 200 cm every 120 yr. Thus, sediment accretion in the Nankai wedge is controlled by development of these thrusts. Two subzones within the imbricate thrust zone and two types of décollements are recognized within the zones of aseismic deformation. The western subzone of the imbricate thrust zone is represented by imbricate thrusts with vertical displacement up to 1000 m. The eastern subzone is characterized by a duplex thrust or an imbricate thrust without vertical displacement, which probably caused great historical earthquakes because of the effective horizontal movement of the thrust. Primary décollements develop in the protothrust and imbricate thrust zones, and multiple décollements develop only in the multiple décollement zone. The primary décollement is made by an overburden of trench-fill sediments that creates high pore-water pressure within the Shikoku Basin hemipelagite with porosity between 30 and 50%. The multiple décollement may be formed by the secondary dewatering, which is related to dissolution or diagenesis of sediments in the 20% porosity range. INTRODUCTION This report describes the development of the accretionary prism along the Nankai Trough off Shikoku Island, based chiefly upon the characteristics of thrust faults and the velocity distribution seen in already published multichannel seismic lines (Aoki et al., 1982; Nasu et al., 1982; Tamano et al., 1983) and single-channel air-gun data provided by the* Hydrographic Office of Japan. Additional multichannel lines taken by the Ocean Research Institute are discussed by Kagami and others (1985). Original concepts published in Japanese (Kagami et al., 1983) concerning the development of the accretionary prism are modified slightly here. The terminology for features within this setting is rather confusing, but this report follows the vocabulary of Moore and others (1982) and Boyer and Elliott (1982). Much of the emphasis attached to the protothrust zone resulted from discussions with D. Karig on board the Glomar Challenger during the Leg 87 cruise. Convergent plate boundaries can represent sites of crustal growth and are characterized by great earthquakes, intense deformation of sediments, and extraordinary topographic relief. The operative process could be similar to those of mountain belts (Bally and Snelson, 1980), although the process of accretionary tectonics at convergent boundaries is a much younger and shorter process. Kagami, H., Karig, D. E., Coulbourn, W. X, et al., Into. Repls. DSDP, 87: Washington (U.S. Govt. Printing Office). * Address: Ocean Research Institute, University of Tokyo 1-15-1 Minamidai, Nakano, Tokyo 164, Japan. DESCRIPTION OF SINGLE-CHANNEL AIR-GUN DATA The observed area off Shikoku can be divided into four areas according to variations of the accretionary prism. The Eastern Area between Tosabae and Shionomisaki Peninsula As shown in a single-channel air-gun profile of Line 38 (Figs. 1 and 2), the protothrust zone is not well developed in the floor of the trough. The first imbricate thrust block has a height of 200 to 500 m and is relatively broad (8 km). A major thrust bounds this imbricate zone on the north. The topography seen between 3 and 4 s two-way traveltime is complicated by many peaks (Fig. 2). This setting is where the Nankaido earthquake thrust occurred in 1946. The Minami Tosabae, a broad sediment-covered ridge, lies at about 2 s two-way traveltime. A sediment-filled trough separates Minami Tosabae and Tosabae, which is located further northward (Line 45-56 in Fig. 3). The northern border of the filled trough is thought to be another major seismic thrust, the result of the 1854 Ansei earthquake. A piston-core sample taken from the trough floor near Shionomisaki Canyon recovered sandy turbidites (Kagami, 1975). Transitional Area off Muroto Peninsula Line N55-1 crosses the middle of the boundary area between Shionomisaki and the transitional area off Muroto Peninsula where the strike of surface and subsurface

The Accretionary Prism of the Nankai Trough off Shikoku, Southwestern Japan