Page 273 - Global Tectonics

258 CHAPTER 9

Figure 9.13 Schematic focal mechanism solution distribution on a section perpendicular to an island arc. Inset shows
alternative intermediate depth mechanism (redrawn from Isacks et al., 1969, with permission from the Geological
Society of America).

by brittle failure. The process is termed dehydration by rapid shearing of the crystal lattice along planes on
embrittlement. which minute spinel crystals have grown (Green, 1994).
Peacock (2001), using a detailed thermal model for At normal mantle temperatures this phase change
the subduction zone beneath northeast Japan, has occurs at a depth of approximately 400 km (Sections
shown that the lower seismic zone (Fig. 9.12) migrates 2.8.5, 9.5). However, the anomalously low temperatures
across the isotherms, from approximately 800 to 400°C, in the core of a downgoing slab enable olivine to exist
as the focal depths increase from 70 to 180 km. If these metastably to greater depths, potentially until it reaches
temperatures and implied pressures are plotted on a a temperature of about 700°C (Wiens et al., 1993). In
P–T diagram, the pressure/temperature values and old, rapidly subducting slabs this may, exceptionally, be
negative slope are very analogous to those for the dehy- at a depth of approximately 670 km, explaining the ter-
dration reaction serpentine to forsterite + enstatite + mination of subduction zone seismicity at this depth. It
water. This strongly suggests that these earthquakes are is also probable that a similar transformation from
the result of the dehydration of serpentinized mantle enstatite to ilmenite contributes to subduction zone
within the downgoing oceanic plate. This explanation seismicity in this depth range (Hogrefe et al., 1994). The
assumes that the oceanic mantle is serpentinized to a phase changes that occur in the slab at a depth of
depth of several tens of kilometers, whereas hydrother- approximately 700 km (Sections 2.8.5, 9.5) are thought

mal circulation and alteration at mid-ocean ridges is to produce fine-grained materials that behave in
thought to be restricted to the crust. However, the a superplastic manner and thus cannot generate
normal faulting associated with the outer rise and earthquakes (Ito & Sato, 1991).
bending of the oceanic lithosphere oceanward of the The deep events of regions “c” and “d” (Fig. 9.8)
trench may well permit ingress of seawater and hydra- are characterized by principal stress directions that are
tion of the lithosphere to depths of tens of kilometers either parallel or orthogonal to the dip of the descend-
(Peacock, 2001). ing plate (Isacks et al., 1969) (Fig. 9.13). Consequently,
Below 300 km (zone “d” in Fig. 9.8) the earthquake the nodal planes determined by focal mechanism solu-
mechanism is believed to be a result of the sudden tions do not correspond to the dip of the Benioff zone
phase change from olivine to spinel structure, produc- or a plane perpendicular to it. The principal stress
ing transformational or anticrack faulting. This takes place directions show that the descending plate is thrown