Geoscience Reference
In-Depth Information
Figure 10.7.
The steady-state thermal structure of the upper part of a subduction
zone dipping at 26.6
◦
. The age of the downgoing lithosphere at the start of
subduction is 50 Ma. Solid triangles mark the location of the volcanic line. The
shaded region is the 65-km-thick rigid overriding lithosphere (the base of this
mechanical lithosphere is taken as being at 1000
◦
C). Isotherms are labelled in
◦
C. In
the mantle beneath the lithosphere the adiabatic gradient is 0.3
◦
Ckm
−1
. Subduction
with no shear heating: (a) at 10 cm yr
−1
and (c) at 1 cm yr
−1
. Subduction with shear
heating that increases linearly with depth as 5% of lithostatic pressure down to the
brittle-ductile transition (taken to be at 500
◦
C) below which the shear stress then
decreases exponentially: (b) at 10 cm yr
−1
and (d) at 1 cm yr
−1
. (From Peacock,
Thermal and petrologic structure of subduction zones, Geophysical Monograph 96,
113-33, 1996. Copyright 1998 American Geophysical Union. Reprinted by
permission of American Geophysical Union.)
physical conditions chosen: any models that imply that melting should take place
between the trench and the volcanic line must fail, for melting should first take
place only beneath the volcanic line. The oceanic crust probably starts transform-
ing to eclogite by the time it has subducted to a depth of about 50 km, but not
all of the reactions need be complete until it is much deeper than that because
transformation takes time (though it is hastened by the presence of abundant
fluid).
Many dehydration reactions of the oceanic crust are endothermic (the reactions
require heat), and lack of heat may constrain the transformation. This means that
temperatures in the real slab are lower than those obtained from computer models
that do not include such heat requirements. Some estimates of the heat needed are
5.8
10
4
Jkg
−
1
for the mineral reactions involved in the greenschist-amphibolite
change and 2.5
×
10
5
Jkg
−
1
for the serpentinite-peridotite reactions. The water
released in these reactions rises into the overlying mantle, a process that further
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