Geoscience Reference
In-Depth Information
Figure 9.54.
(a) The
measured present-day
uplift of coastal regions
over the
northern-Cascadia (black
circles) and
southwestern-Japan
subduction zones (white
circles). Solid lines show
uplift predicted by a
numerical thermal model
of locked subduction
zones. Position of
coastline: Cascadia, solid
line; Japan, dashed line.
(b) Cross sections of the
temperature models show
the extent of the locked
portion of the subduction
zones. The behaviour of
deformation on the
subduction zone is
temperature-controlled:
locked zones (150-350
◦
C),
thick solid line; transition
(350-450
◦
C), thin solid
line; and stable sliding
(
>
450
◦
C), dashed line.
(From Hyndman and
Wang, The rupture zone
of Cascadia great
earthquakes from current
deformation and the
thermal regime,
J.
Geophys. Res.
,
100
,
22 133-54, 1995.
Copyright 1995 American
Geophysical Union.
Reprinted by permission
of American Geophysical
Union.)
10
SW Japan
N Cascadia
5
0
coast
-5
o
150 C
0
o
350 C
o
20
450 C
SW Japan
N Cascadia
40
60
0
50
100
150
200
250
Distance from base of continental slope (km)
'transformational' faulting. When under stress the metastable olivine in the cen-
tral wedge transforms to spinel in crack-like inclusions that form perpendicular
to the direction of maximum stress. With sufficient cracks in a small region of
the wedge the microspinel crystals become superplastic and a catastrophic slip
occurs - an earthquake. Theoretically, transformational faulting should occur for
reactions for which both the latent heat and the change in volume are negative,
in other words, for exothermic rections with an associated increase in density.
The olivine-spinel change satisfies these critera, as does the other major mantle
mineral change, clinoenstatite
4
-ilmenite. With the maximum compressive stress
oriented parallel to the slab, predicted earthquake mechanisms are consistent
with those observed. The focal mechanism for these deep earthquakes, slip on a
fault, is thus very similar to mechanisms for shallow earthquakes, although the
mechanical cause is completely different. Earthquakes are not associated with
spinel-perovskite and spinel-magnesiowustite or ilmenite-perovskite reactions
at 670 km because these are endothermic, absorbing, rather than releasing, heat.
Any olivine or clinoenstatite still remaining in the metastable wedge at 670 km
depth will also decompose to perovskite without producing earthquakes because
the latent heat for these reactions is positive. Since there are no further phase
changes in the lower mantle, there is no opportunity for any deeper earthquakes
to occur. Thus the cessation of seismicity at the base of the upper mantle seems
to be a direct consequence of mantle mineralogy.
4
Clinoenstatite is a pyroxene mineral.
