KeywordsSubduction Zone Sedimentary Layer Accretionary Wedge Seismogenic Zone Accretionary Complex
Accretion defines a process at a convergent plate margin above a subduction zone where material of the subducting lower plate is scraped off and transferred to the overriding upper plate. The offscraped material is accumulated in a wedge-shaped stack of sedimentary layers sometimes containing also offscraped material from the oceanic crust of the subducting plate. It is located directly at the boundary between the two converging plates. This region is called the forearc region of the convergent plate boundary (see entry “Morphology Across a Convergent Plate Boundary” and Fig. 3 therein, this volume).
A detachment surface, or décollement, separates the upper part of the accreted section (i.e., zone of offscraping) from material that is underthrust beyond the base of the slope. Above the décollement, scraped-off sediment is transferred to the accretionary prism, and this prism displays a rugged and irregular seafloor morphology governed by numerous tectonic ridges that form by folding and fault dislocation (Fig. 1). As the subducting plate transports its sedimentary fill from the trench toward the arc, some portion of the sediment is subducted and transported within the subduction channel down to great depths where it becomes an important factor in the feeding of subduction-related magmas. The remaining portion, or in some cases the entire sedimentary layer and parts of the oceanic crustal basement, can be scraped off forming the accretionary wedge on the upper plate.
An accretionary wedge grows from below. The scraped-off sedimentary layers are stacked and continuously uplifted by renewed underplating from below, a process that results in morphological elevation of the outer ridge. The more material that is scraped off, the higher the elevation of the outer ridge. The process of accretion has been modeled in sandbox experiments so the evolution of the accretionary wedge is well understood (e.g., Gutscher et al., 1996; Dominguez et al., 2000). The underplating process resembles large-scale nappe thrusts in mountain ranges. Previously juxtaposed layers of sediment are stacked during the shortening process, and at each overthrust, older sediments are placed on top of younger ones. The process and sequence of events can also be viewed from the opposite perspective – younger units are forced below older ones by underthrusting.
About half of the convergent plate boundaries on Earth are dominated by the process of accretion in an accretionary wedge (von Huene and Scholl, 1991), which is the opposite process characterized by subduction erosion (see entry “Subduction Erosion” and Fig. 1 therein, this volume). Numerous examples of studies exist that examine accretionary wedges from around the world (e.g., Scholl et al., 1980, Silver and Reed, 1988, Westbrook et al., 1988; Kukowski et al., 2001; Gulick et al., 2004, and others). Large accretionary wedges are represented in southwest Japan, Sumatra, large portions of the Gulf of Oman (Makran subduction zone), in the Lesser Antilles, along the Aleutians, and in smaller areas of western North and South America.
Eight overthrust planes that display repetitions of the sedimentary layers have been drilled at the Vanuatu accretionary wedge in the Southwest Pacific (Ocean Drilling Program, ODP Leg 134; Meschede and Pelletier, 1994). Because the sedimentary layer at the subducting plate has a thickness of slightly more than 100 m, the much larger thickness of the accretionary wedge is a result of intense tectonic stacking. In contrast, south of Japan, where the Philippine Sea Plate subducts beneath the Eurasian Plate, a thick sedimentary layer of more than 1000 m is entering the Nankai subduction zone (Gulick et al., 2004). Here, the décollement zone remains in the sedimentary layer and does not cut through the underlying oceanic crust as is the case in Vanuatu. Approximately the lower third of the sedimentary layer is being subducted and does not contribute to the growth of the accretionary wedge.
Along Sumatra, the slope of the accretionary wedge falling from the outer ridge to the deep sea trench typically displays distinctive subdivisions. Here active thrusts produce elongated flat areas and depressions – the so-called slope basins (Fig. 4). They are common along the Mentawai Ridge where some are rising above sea level. During their complex history, these basins served as sediment traps recording the uplift history of the outer ridge. Analyses of the microfauna indicate uplift from deep to shallow water conditions. The youngest sediments contain reefs having formed in very shallow water before uplifting above sea level (Moore et al., 1980).
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