Page 274 - Global Tectonics

SUBDUCTION ZONES 259

progressively greater from west to east. The angle of
underthrusting is very shallow in this region. The prob-
able cause of this seismic gap and shallow underthrust-
ing is the presence of copious quantities of terrigenous
sediments within the trench that become increasingly
abundant towards that section of the trench adjacent to
Alaska. The unconsolidated nature of these sediments
probably prevents any build-up of the strain energy
necessary to initiate earthquakes, and their high positive
Figure 9.14 A model of stress distributions in the buoyancy may force the subducting plate to descend at
descending lithosphere. Solid circles, extensional stress an anomalously shallow angle.
down dip; open circles, compressional stress down dip In reviewing the data for numerous subduction
(redrawn from Isacks & Molnar, 1969, with permission zones, Fukao et al. (2001) noted that subducted slabs
from Nature 223, 1121–4. Copyright © 1969 Macmillan are either defl ected horizontally within or just beneath
Publishers Ltd).
the transition zone, or penetrate the 660 km disconti-
nuity and descend into the lower mantle (Plate 9.2
between pp. 244 and 245). Beneath Chile, the Aleu-
into either down-dip compression or extension. Isacks tians, southern Kurile, and Izu-Bonin the slabs appear
& Molnar (1969) have suggested that the distribution to flatten out within the transition zone, whereas

of stress type in the seismic zone may result from the beneath the Aegean, central Japan, Indonesia, and
degree of resistance experienced by the plate during Central America they penetrate deep into the lower
its descent, and Spence (1987) has described this resis- mantle. The slab beneath Tonga both fl attens out
tance in terms of the net effect of ridge push and slab within the transition zone and extends into the lower
pull forces (Section 12.6). In Fig. 9.14a the plate is mantle (van der Hilst, 1995) (Plate 9.2e between pp.
sinking through the asthenosphere because of its neg- 244 and 245). There is no relationship between the age
ative buoyancy and is thrown into down-dip tension as of a subducting slab and penetration into the lower
its descent is unimpeded. In Fig. 9.14b the bottom of mantle. Some researchers maintain that in places there
the plate approaches the mesosphere, which resists is evidence for the slabs descending throughout the
descent and throws the leading tip into compression. lower mantle to the core–mantle boundary (Section
As the plate sinks further (Fig. 9.14c), the mesosphere 12.8.2); others consider that there is little evidence for
prevents further descent and supports the lower margin slab penetration beneath 1700 km depth (Kárason &
of the plate so that the majority of the seismic zone van der Hilst, 2000). The possible implications of these
experiences compression. In Fig. 9.14d a section of the tomographic results for convection in the mantle are
downgoing slab has decoupled so that the upper considered in Section 12.9.
portion of the plate is thrown into tension and the
lower portion into compression. A global summary of
the stress directions determined from focal mechanism
solutions (Isacks & Molnar, 1971) is shown in 9.5 THERMAL
Fig. 9.15.
vide a possible explanation for the seismic gaps observed STRUCTURE OF THE
The stress distributions shown in Fig. 9.14b,d pro-
along the middle parts of the Benioff zone at certain
trenches, such as the Peru–Chile Trench (Figs 9.15), DOWNGOING SLAB
where it is known that the slab is continuous (James &
Snoke, 1990). A further type of seismic gap appears to
be present in some island arcs at shallow depths. Figure The strength and high negative buoyancy of subducting
9.16 shows sections through the Benioff zone at the oceanic lithosphere and its capacity for sudden failure
Aleutian–Alaska arc (Jacob et al., 1977). There is a prom- in the generation of earthquakes are consequences of
inent gap in seismicity between the trench and a point its relatively low temperature with respect to normal
about halfway towards the volcanic arc that becomes mantle material at these depths. The subducting