Page 277 - Global Tectonics

262 CHAPTER 9

Aleutians Tonga - Kermadec of seismic zones should be proportional to the product
Kurile Japan of convergence rate and age. That this is generally so is
New Zealand x Central America illustrated by Fig. 9.17, and although there is consider-

able scatter the data appear to fit the relationship length
South America + Lesser Antiles −1
(km) = rate (mm a ) × age (Ma)/10.
1400
1200
Length of seismic zone (km) 1000 9.6 VARIATIONS IN

SUBDUCTION ZONE
800
CHARACTERISTICS
600
400
+ x The age and convergence rate of the subducting oceanic
200 lithosphere affect not only the thermal structure of the
downgoing slab, and the length of the seismic zone, but
a number of other characteristics of subduction zones.
2000 4000 6000 8000 10000 12000
It can be seen from Fig. 9.15 that, although the dip of
Rate x age (km)
the Benioff zone is often approximately 45°, as typically
Figure 9.17 Relationship between length of Benioff illustrated, there is a great variation in dips, from 90°
zone and the product of convergence rate and age. beneath the Marianas to 10° beneath Peru. It appears
Approximate uncertainties given by error bars in upper that the dip is largely determined by a combination of
left corner (redrawn from Molnar et al., 1979, with the negative buoyancy of the subducting slab, causing

permission from Blackwell Publishing). it to sink, and the forces exerted on it by flow in the
asthenosphere, induced by the underthrusting litho-
sphere, which tend to uplift the slab. A higher rate of
Different solutions for the temperature distribution underthrusting produces a greater degree of uplift.
have been derived by various workers, depending on the Young oceanic lithosphere is relatively thin and hot;
assumptions made concerning the relative contribu- consequently it is more buoyant than older oceanic
tions of the above phenomena. Two models derived by lithosphere. One would predict, therefore, that young
Peacock & Wang (1999) and representing relatively cool subducting lithosphere, underthrusting at a high rate,
and warm subducting lithosphere are shown in Plate 9.3 will give rise to the shallowest dips, as in the case of
(between pp. 244 and 245). Although differing in detail, Peru and Chile. It seems probable that the absolute
all such models indicate that the downgoing slab main- motion of the overriding plate is also a contributing
tains its thermal identity to great depths and that, excep- factor in determining the dip of the Benioff zone (Cross
tionally, temperature contrasts up to 1000°C may exist & Pilger, 1982).
between the core of the slab and normal mantle at a Subduction zones with shallow dips have a stronger
depth of 700 km. coupling with the overriding plate (Uyeda & Kanamori,
As noted in Section 9.4, the length of the Benioff 1979), giving rise to larger magnitude earthquakes in
zone depends on the depth to which the subducting region “b” of Fig. 9.8. Shallow dips also restrict the fl ow
oceanic lithosphere maintains a relatively cold central of asthenosphere in the mantle wedge above the sub-
core. Molnar et al. (1979) deduced that the downward duction zone, in extreme cases suppressing all supra

deflection of isotherms, and hence the length of the subduction zone magmatism (Section 10.2.2), and in all
seismic zone, is proportional to both the rate of subduc- cases giving rise to backarc compression rather than
tion and the square of the thickness of the lithosphere. extension. Thus, Uyeda & Kanamori (1979) recognized
Lithosphere thickness is proportional to the square root two end-member types of subduction zone, which they
of its age (Turcotte & Schubert, 2002) so that the length referred to as Chilean and Mariana types (Fig. 9.18).