Geoscience Reference
In-Depth Information
Figure 9.53.
Cross
sections of a subduction
zone, illustrating the
deformation that occurs
between major
earthquakes (
interseismic
deformation) and during
an earthquake (
coseismic
deformation). Between
earthquakes the seaward
edge of the plate is
dragged downwards
while just inland the
flexural bulge means that
there is uplift. During the
earthquake, the locked
zone is suddenly released,
causing the seaward edge
to rebound and the
uplifted bulge to subside.
(From Hyndman and
Wang, Tectonic
constraints on the zone of
major thrust earthquake
failure: the Cascadian
subduction zone,
J.
Geophys. Res.
,
98
,
2039-60, 1993. Copyright
1993 American
Geophysical Union.
Reprinted by permission
of American Geophysical
Union.)
Uplift
Between earthquakes
Shortening
During earthquakes
Uplift
Subsidence
Extension
traditionally been used as evidence in support of the hypothesis that there is a
physical barrier at 650-700 km depth and an upper-mantle convection system that
is separate from the lower mantle (Section 8.2.3). A mechanism that may account
for intermediate earthquakes is the sudden release of strain on pre-existing faults
lubricated by the dehydration of hydrous minerals, but any applicability of this
mechanism to deep earthquakes below 300 km remains unproven. Another mech-
anism is the transformational faulting of metastable olivine in a wedge-shaped
region in the interior of the subducting plate (Fig. 9.44). These mechanisms imply
that earthquakes should occur within a narrow part of the subducted plate.
High-pressure laboratory experiments on olivine
3
indicate that deep earth-
quakes may be associated with the change of olivine to spinel in the subducting
plate. Figure 9.44 shows that the equilibrium position of this phase change in the
subducting plate is elevated above its position in the mantle. However, the kinetics
of the reaction suggests that a wedge of olivine several tens of kilometres in thick-
ness can persist in the centre of the slab well below the equilibrium depth, even
down to 670 km. This situation arises because the kinetics of the olivine-spinel
change mean that, in a fast subducting cold slab, the rate at which the change takes
place is slower than the actual subduction rate. (The reaction rate is slow because
it is temperature-dependent and decreases as temperature decreases.) This mech-
anism, which may be the origin of deep earthquakes, is termed 'anticrack' or
3
Peridotite, the upper-mantle rock type, is mostly olivine, with about 10% pyroxene.
