7100 Subduction zones, island arcs and active continental margins Fig. 7.10 Development of scaly fabric and slaty cleav- formation of scaly fabric slaty cleavage age along thrust planes in an accretionary wedge. incipient advanced ~5 cm

ophiolitic material forms the dominant component, it is called an ophiolitic mélange. Such ophiolitic mélanges are commonly alined along the suture zone between plates aft er a collision. 1 km accretionary wedge Subduction erosion instead of accretion Sedimentary accretionary wedges occur in many subduction zones including large portions of the Gulf of Oman (Makran subduction zone, Fig. 5.16), Sumatra, SW Japan, and in smaller areas of western decollement North America and the Lesser Antilles (Fig. 7.13). oceanic crust Many other subduction zones are characterized by the opposite process, subduction erosion. During subduction erosion, rock material is scraped off sedimentary-tectonic from the bottom of the upper plate and transported mélange sedimentary mélange mud volcano downward with the subducting plate. Such basal erosion of the upper plate is known from the Mari- anas, the Tonga Islands, Costa Rica and Chile. In these cases, an accretionary wedge has not evolved. It is assumed that many modern convergent plate

outer boundaries have accretion at the tip of the upper slope ridge forearc plate and erosion farther down the subduction sediments basin zone. Subduction erosion transfers a considerable

slope basin amount of sediment from the surface to greater depths. In places where subduction erosion occurs, oceanic crust the outer ridge is not a signifi cant feature because

tectonic mélange sedimentary material does not accumulate in an accretionary wedge. Th e forearc basin commonly grades directly into the trench with little or no topographic rise between the two. Subduction erosion is particularly eff ective when the lower plate has a roughly textured surface and is only covered by a thin sedimentary layer (Fig. 7.14). As the lower plate enters the subduction zone and tectonic mélange e.g., e.g., is bent, the upper part of the plate is extended; a fragments ophiolite fragments fragments series of trench-parallel horst and graben structures of sand- (basalt, serpentinite) of radiol- stone arite are subsequently formed. Th e roughness caused by these structures acts like a grater on the basal part limestone of the upper plate (basal erosion, Fig. 7.14), much like a cheese grater removes cheese from the base basalt of the cheese block. Material that is scraped off the upper plate enters the subduction zone where it is radiolarite transported downward into deeper parts of the sub-

10 cm duction zone. Th is process is currently happening in the Mariana Arc. Seamounts on the subducting sedimentary-tectonic mélange from plate create a somewhat diff erent kind of subduc- clayey, sheared matrix the ophiolite of Osa, Costa Rica ~ 1 m to 1 km tion erosion. Single seamounts scrape material from the frontal tip of the accretionary wedge along the Fig. 7.11 Formation of mélange in an accretionary wedge and subduction zone upper plate and carry it into the subduction zone (Cowan, 1985). Inserts show variations across diff erent portions of the mélange. The outcrop photo shows a typical mélange structure. Mélange is characterized by a block- (frontal erosion). A superb example of this setting in-matrix structure. Mud-volcanoes develop by explosive dewatering of sediments occurs off the Pacifi c coast of Costa Rica where under high pressure. seamounts currently buried by the tip of the upper

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