Page 266 - Global Tectonics

SUBDUCTION ZONES 251

V-shape with the steepest slope, of 8–20°, on the side

opposite the underthrusting ocean floor. The sediment
fill of trenches can vary greatly, from virtually nothing,

as in the Tonga–Kermadec trench, to almost complete,
as in the Lesser Antilles and Alaskan trenches because
of the supply of sediment from adjacent continental
areas. Trench depth is also reduced by the subduction
of aseismic ridges (Section 10.2.2).

9.2 GENERAL

MORPHOLOGY OF

ISLAND ARC SYSTEMS

Figure 9.2 Geometry of an indentation in a sphere of
Island arc systems are formed when oceanic lithosphere inextensible material (redrawn from Bott, 1982, by
is subducted beneath oceanic lithosphere. They are con- permission of Edward Arnold (Publishers Ltd).
sequently 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 The generalized morphology of an island arc system
(Scotia) arcs are formed at the eastern margins of small is shown in Fig. 9.3, although not all components are
oceanic plates isolated by transform faults against the present in every system. Proceeding from the ocean-
general westward trend of movement. ward side of the system, a flexural bulge about 500 m

All of the components of island arc systems are high occurs between 100 and 200 km from the trench.
usually convex to the underthrusting ocean. This con- The forearc region comprises the trench itself, the
vexity may be a consequence of spherical geometry, as accretionary prism, and the forearc basin. The accre-

suggested by Frank (1968). If a flexible spherical shell, tionary prism is constructed of thrust slices of trench
such as a table tennis ball, is indented an angle θ (Fig. fi ll (flysch) sediments and possibly oceanic crust sedi-

9.2), the indentation is a spherical surface with the same ments that have been scraped off the downgoing slab
radius as the shell (R). The edge of the indentation is a by the leading edge of the overriding plate. The forearc
circle whose radius r is given by r = ½Rθ, where θ is in basin is a region of tranquil, fl at-bedded sedimentation
radians. If this theorem is applied to a plate on the between the accretionary prism and island arc. The
Earth’s surface, θ represents the angle of underthrust- island arc is made up of an outer sedimentary arc and
ing of oceanic lithosphere, which averages about 45°. an inner magmatic arc. The sedimentary arc comprises
The radius of curvature of the trench and island arc on coralline and volcaniclastic sediments underlain by vol-
the Earth’s surface is then about 2500 km. This value is canic rocks older than those found in the magmatic arc.
in agreement with some, but not all, island arc systems. This volcanic substrate may represent the initial site of
The general convexity of island arc systems is probably volcanism as the relatively cool oceanic plate began its
a consequence of spherical geometry, and deviations descent. As the “cold” plate extended further into the
result from the oversimplification of this approach, in asthenosphere the position of igneous activity moved

particular the fact that the conservation of surface area backwards to its steady state location now represented
is not required by plate tectonics. Thus, for example, by the magmatic arc. Processes contributing to the for-
the angle of underthrusting at the Mariana arc is almost mation of the island arcs are discussed in Section 9.8
90°, but it has one of the smallest radii of curvature and 9.9. The island arc and remnant arc (backarc ridge),

(Uyeda & Kanamori, 1979). first recognized by Vening Meinesz (1951), enclose a