Geoscience Reference
In-Depth Information
Figure 9.46.
The shallow geometry of subduction zones as defined by earthquake
foci. Abbreviations of subduction-zone names: NH, New Hebrides; CA, Central
America; ALT, Aleutian; ALK, Alaska; M, Mariana; IB, Izu-Bonin; KER, Kermadec; NZ,
New Zealand; T, Tonga; KK, Kurile-Kamchatka; NC, North Chile; CC, Central Chile;
SC, South Chile; P, Peru. Solid triangles marks the volcanic lines; vertical or
horizontal lines mark the location or extent of the oceanic trench. Some sections
have, for clarity, been offset from the others. (After Isacks and Barazangi (1977).)
focal mechanisms of these intermediate-depth earthquakes being due to brittle
rupture shows that melting within the slab is not taking place at these depths. The
upper and lower boundaries of an 80-km-thick model subducting plate are shown
superimposed on the foci. This subducting plate has P- and S-wave velocities
which are 6% higher than those in the surrounding mantle. The tomographic
inversion of the seismic travel times of P- and S-waves yields a detailed image of
the perturbations both in P- and in S-wave velocity beneath Japan. Low-velocity
zones in the crust are present beneath active volcanoes - many of those in the
upper crust may be caused by the presence of water (on the basis of low values
of
v
p
/
v
s
), whereas those in the lower crust apparently result from the presence
of partial melt (on the basis of higher values of
v
p
/
v
s
). Within the mantle wedge
the low-velocity regions are consistent with the presence of partial melts. Thus
the process imaged here can be summarized as follows: water lost by the sub-
ducting plate at depths of 140-150 km moves into the mantle wedge where it
causes partial melting; these partial melts move upwards and oceanwards, appar-
ently following the flow-lines of the induced convection in the mantle wedge,
i.e., along the fabric of the shear structures; melts rise and intrude the lower crust
beneath active volcanoes; water expelled from cooling lower-crustal melt bodies
rises into the upper crust; and some rising melts are erupted along the volcanic
arc. (See Section 10.2.1 for further detailed discussion.)
