## General Morphology Of Island Arc Systems

Island arc systems are formed when oceanic lithosphere is subducted beneath oceanic lithosphere. They are consequently typical of the margins of shrinking oceans such as the Pacific, where the majority of island arcs are located. They also occur in the western Atlantic, where the Lesser Antilles (Caribbean) and South Sandwich (Scotia) arcs are formed at the eastern margins of small oceanic plates isolated by transform faults against the general westward trend of movement.

All of the components of island arc systems are usually convex to the underthrusting ocean. This convexity may be a consequence of spherical geometry, as suggested by Frank (1968). If a flexible spherical shell, such as a table tennis ball, is indented an angle 0 (Fig. 9.2), the indentation is a spherical surface with the same radius as the shell (R). The edge of the indentation is a circle whose radius r is given by r = ^R0, where 0 is in radians. If this theorem is applied to a plate on the Earth's surface, 0 represents the angle of underthrusting of oceanic lithosphere, which averages about 45°. The radius of curvature of the trench and island arc on the Earth's surface is then about 2500 km. This value is in agreement with some, but not all, island arc systems. The general convexity of island arc systems is probably a consequence of spherical geometry, and deviations result from the oversimplification of this approach, in particular the fact that the conservation of surface area is not required by plate tectonics. Thus, for example, the angle of underthrusting at the Mariana arc is almost 90°, but it has one of the smallest radii of curvature (Uyeda & Kanamori, 1979).

Figure 9.2 Geometry of an indentation in a sphere of inextensible material (redrawn from Bott, 1982, by permission of Edward Arnold (Publishers Ltd).

The generalized morphology of an island arc system is shown in Fig. 9.3, although not all components are present in every system. Proceeding from the ocean-ward side of the system, a flexural bulge about 500 m high occurs between 100 and 200 km from the trench. The forearc region comprises the trench itself, the accretionary prism, and the forearc basin. The accre-tionary prism is constructed of thrust slices of trench fill (flysch) sediments and possibly oceanic crust sediments that have been scraped off the downgoing slab by the leading edge of the overriding plate. The forearc basin is a region of tranquil, flat-bedded sedimentation between the accretionary prism and island arc. The island arc is made up of an outer sedimentary arc and an inner magmatic arc. The sedimentary arc comprises coralline and volcaniclastic sediments underlain by volcanic rocks older than those found in the magmatic arc. This volcanic substrate may represent the initial site of volcanism as the relatively cool oceanic plate began its descent. As the "cold" plate extended further into the asthenosphere the position of igneous activity moved backwards to its steady state location now represented by the magmatic arc. Processes contributing to the formation of the island arcs are discussed in Section 9.8 and 9.9. The island arc and remnant arc (backarc ridge), first recognized by Vening Meinesz (1951), enclose a

100-

100-

200 100 Distance from trench (km)

Figure 9.3 Schematic section through an island arc system (modified from Stern, 2002, by permission of the American Geophysical Union. Copyright © 2002 American Geophysical Union).

200 100 Distance from trench (km)

Plate motions y Partial melt diapir

Areas of melt generation

^ Fluid pathways 0

Figure 9.3 Schematic section through an island arc system (modified from Stern, 2002, by permission of the American Geophysical Union. Copyright © 2002 American Geophysical Union).

backarc basin (or marginal basin) behind the island arc. However not all backarc basins are formed by spreading above an active subduction zone, as indicated in Fig. 9.3 (Section 9.10).

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### Responses

• pearl labingi
How is a "sedimentary arc" formed island arc?
8 years ago