Geoscience Reference
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to shallow heat sources, namely, magma beneath the volcanoes. (Examples from
the Japan and Cascadia subduction zones are shown in Figs. 9.58(e) and 9.61(a)).
Subduction zones have been modelled by many researchers, and, although
there are differences between their starting models and the resulting isotherms
and heat flow, the results are broadly the same for all and could probably already
have been predicted by a reader of Chapter 7.Figure 7.32 shows the thermal
re-equilibration of a thrust fault. A subduction zone is merely a type of thrust
that keeps moving as new cold plate is continually subducted. Figure 9.44 shows
two conductive thermal models of the temperature structure in and around a
subducting oceanic plate. The 95-km-thick plate is heated by the surrounding
mantle by conduction. Since no allowance is made for shear, latent or radiogenic
heating of the slab, the temperatures shown are low estimates. Note that these
model subducting slabs retain low temperatures compared with adjacent mantle,
even at depths of more than 700 km, which result in high seismic velocities and
densities for the slabs. A slab subducting at 45 at 5 cmyr 1 would take only
20 Ma to reach 700 km depth; a slab subducting at 10 cm yr 1 would reach that
depth in 10 Ma. The thermal parameter , equal to the product of the vertical rate of
descent of the plate into the mantle and the age of the subducting lithosphere, is a
useful parameter. The temperature at any depth varies smoothly with the thermal
parameter: the thermal parameter is effectively proportional to the maximum
depth of the 'V' in the temperature contours (isotherms). A low thermal parameter,
representing slow subduction of a young (hot) plate (Fig. 9.44(a)), means that
agiven isotherm would be at a shallow depth (the plate heats up quickly). A
high value for the thermal parameter, representing fast subduction of old (cold)
plate (Fig. 9.44(b)), means that isotherms are deeper (the plate is much slower to
heat up).
Many factors affect the fine details of subduction-zone temperature structure.
Estimates of the magnitude of frictional heating shear stresses have ranged up to
100 MPa, though values in the range 10-40 MPa are reasonable on the basis of
values of the heat flow in the trench-volcanic-arc region. Of central importance to
the shallow thermal structure is the role of water, which transports heat along the
thrust fault and through the overriding plate. In addition, the melting behaviour
of the subducting plate and the overriding mantle depends very strongly on the
amount of water present. Heat is advected by rising magma beneath the volcanic
arc and the back-arc basin. A small-scale convection system tends to operate
beneath the back-arc basin, giving rise to the ocean-type crust and magnetic
anomalies there (which are discussed further in Section 10.2.1).
The main changes which occur in the subducting plate are the shallow reaction
of the oceanic crust to eclogite and the changes deeper in the mantle of olivine
to a spinel structure and then to post-spinel structures (see Section 8.1.5). These
changes result in increases in the density of the subducting slab. Figure 9.45 shows
equilibrium pressure-temperature curves for the olivine-spinel and spinel-post-
spinel structures. The change of olivine to spinel is exothermic (heat-releasing)
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