Page 267 - Global Tectonics

252 CHAPTER 9

Back-arc basin Magmatic arc Forearc Trench
Spreading axis Magmatic Forearc Accretionary Outer
front basin prism trench
high

0
Arc crust 500 C 500 C
1
Lithospheric Subducting lithosphere 1000 C
mantle
Depth (km) 50 Asthenosphere 2
Convecting
Oceanic crust
Asthenosphere
motions
100 Plate motions
Partial melt diapir
Areas of melt
2 generation
Fluid pathways
150
400 300 200 100 0
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).

backarc basin (or marginal basin) behind the island arc. Conversely, the island arc is marked by a large positive
However not all backarc basins are formed by spreading anomaly. Isostatic anomalies over the trench and arc are
above an active subduction zone, as indicated in Fig. 9.3 large and exhibit the same polarity as the free air anom-
(Section 9.10). alies. These large anomalies result from the dynamic
equilibrium imposed on the system by compression, so
that the trench is forced down and the arc held up out
of isostatic equilibrium by the forces driving the
9.3 GRAVITY plates.
ANOMALIES OF

SUBDUCTION ZONES 9.4 STRUCTURE OF
SUBDUCTION ZONES

Figure 9.4 shows a free air gravity anomaly profi le
across the Aleutian arc that is typical of most subduc- FROM EARTHQUAKES

tion zones. The flexural bulge of the downgoing litho-
sphere to seaward of the trench is marked by a positive
gravity anomaly of about 500 g.u. (Talwani & Watts, Subduction zones exhibit intense seismic activity. A
1974). The trench and accretionary prism are typifi ed large number of events occur on a plane that dips on
by a large negative anomaly of some 2000 g.u. ampli- average at an angle of about 45° away from the
tude which results from the displacement of crustal underthrusting oceanic plate (Fig. 9.5). The plane is
materials by sea water and low density sediments. known as a Benioff (or Benioff–Wadati) zone, after