Encyclopedia of Marine Geosciences

Living Edition
| Editors: Jan Harff, Martin Meschede, Sven Petersen, Jörn Thiede

Accretionary Wedge

  • Martin MeschedeEmail author
Living reference work entry
DOI: https://doi.org/10.1007/978-94-007-6644-0_101-1

Keywords

Subduction Zone Sedimentary Layer Accretionary Wedge Seismogenic Zone Accretionary Complex 
These keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.

Synonyms

Definition

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).

Formation

Accretionary wedges essentially develop as compressional fold-and-thrust belts which are composed primarily of oceanic-plate deposits and, in many cases, continentally derived trench-floor sediment from a nearby continental plate (Fig. 1). Sometimes scraped-off parts of the subducted oceanic lithosphere are added to the accretionary complex, which then form ophiolitic rocks in the succession of the accreted material.
Fig. 1

3D sketch of an accretionary wedge with fold and thrust structures and their morphological expression in the forearc region with basins and trenches filled with sediments (Modified after Fisher, 1996)

Accretionary wedges or prisms typically have wedge-shaped cross sections. They are characterized by one of the most complex internal structures of any tectonic element known on Earth caused by imbricate thrusting and folding of the incoming material. Some parts of accretionary wedges are composed of numerous thin rock layers that are repeated by thrust faults (stacking by duplexing); other parts of wedges or even entire wedges are characterized by large-folded and partly brecciated packages of rocks. Frequently they comprise tectonic mélanges that are composed of mixtures of blocks and thrust slices of many rock types (e.g., basalt, sandstone, limestone, chert, graywacke, and others) that are incorporated in a matrix of fine rock material such as shale or serpentinite. The faults and folds, in general, verge toward the subducting plate (Fig. 2). However, anticlinal structures that include some landward-verging reverse faults, the so-called backthrusts, are produced, which create tectonic ridges that are expressed as positive morphological structures at subduction zones (Fig. 1). Deformation and sedimentation occur concurrently and incrementally throughout the evolution of the system. The ongoing process causes the most devastating earthquakes on Earth, in some cases of magnitudes more than 9.0 (moment magnitude scale). The earthquakes are concentrated in the seismogenic zone, which in general is situated in the accretionary wedge.
Fig. 2

Schematic section of an accretionary wedge showing its internal structure with frontal and basal accretion and internal deformation caused by contraction and extension (Modified from Cawood et al., 2009). The transparent arrows indicate particle paths within the accretionary wedge during the accretion process

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.

Occurrences

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.

The Sunda Arc provides an instructive example demonstrating how the shape of the accretionary wedge depends on the amount of sediments transported into the subduction zone (Fig. 3). Fed by an enormous monsoon-controlled supply of sedimentary material, the Ganges and Brahmaputra rivers accumulated the huge several-kilometer-thick submarine Bengal fan that extends to the southernmost point of Sumatra. The sedimentary fan rests on the Indo-Australian Plate and is transported toward the NNE where it is being subducted in the Sunda Arc. Therefore, the deep sea trench along the northwestern part of this island-arc system is mostly masked by the high sedimentation rate, while the outer ridge, built by the tectonic stacking of fan-supplied sediments, is emerged several hundred meters above sea level. It forms islands such as the Andaman and Nicobar Islands and the Mentawai Ridge in front of Sumatra (Figs. 3 and 4). Farther southwest away from the influence of the Bengal fan and adjacent to Java, the trench is distinctive with depths of almost 7500 m. Accordingly, the outer ridge is not well developed and lies mostly below a water depth of 2000 m.
Fig. 3

Map showing contrasting plate-tectonic conditions along the Sunda Arc (Modified from Frisch et al., 2011). In front of Sumatra, sediment of the thick Bengal fan are scraped off and incorporated into the accretionary wedge. This causes the outer ridge to emerge from the sea at this location (Mentawai Ridge; see Fig. 4). In front of Java, the deep sea trench and the outer ridge are significantly deeper. In front of Australia, the continental crust of the Sahul Shelf is being subducted beneath the Sunda Arc; this causes a particularly strong uplift of the outer ridge (Timor Ridge) and marks the initial stage of orogenesis

Fig. 4

Example for an accretionary wedge with ridges uplifted above the sea level (islands of Mentawai and Siberut) and numerous slope basins at the subduction zone off the island of Sumatra (Modified from Frisch et al., 2011)

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).

Cross-References

References

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Copyright information

© Springer Science+Business Media Dordrecht 2015

Authors and Affiliations

  1. 1.Institute of Geography and GeologyErnst-Moritz-Arndt UniversityGreifswaldGermany