Geoscience Reference
In-Depth Information
appears, it is only as good as the data on which it is based. Other interpretations
of the seismic data may be possible, so this wedge might in reality not be exactly
as shown in Fig. 9.62.
The fine seismic structure of this convergent margin has been imaged by
several deep reflection lines. (For details of shallow-seismic-reflection data on
this margin see Figs. 4.43 and 4.44.) Figure 9.62(a) shows data from a line shot
across Vancouver Island coincident with the refraction line shown in Fig. 9.62(b).
Twovery clear laminated reflections, here marked C and E, were seen on all the
Vancouver Island reflection lines. The C reflection, which dips at 5-8
◦
,isbelieved
to be from the decollement zone (detachment surface) at the base of one of the
accreted terranes (called Wrangellia). The E reflector, which dips at 9-13
◦
,may
represent a zone of porous sediments and volcanic rocks or may mark the location
of trapped water in the crust. Figure 9.62(c) shows the perturbation in S-wave
velocity on a
250-km-long section across this subduction zone. The continental
Moho can be clearly seen east of
∼
122.3
◦
as a boundary between low-velocity
continental crust and high-velocity mantle. However, to the west there is no clear
continental Moho - the very low S-wave velocities in that part of the mantle
wedge are consistent with the mantle being highly hydrated and serpentinized
peridotite.
−
9.6.6 Chemistry of subduction-zone magmas
The igneous rocks above subduction zones include granites, basalts and andesites
as well as some ultramafic rocks (see Section 9.1.1 and Table 9.1). The igneous
rocks of the young Pacific island arcs such as the Tonga and Mariana arcs are pri-
marily basalt and andesite. However, the older island arcs such as the Japan Arc are
characterized by andesite volcanoes as well as diorite intrusions. Figure 9.59(d)
shows a schematic geological cross section through the Andes. Although the
andesite volcanoes provide the surface evidence of the active subduction zone
beneath, the considerable thickening of the crust beneath the Andes is presumed
to reflect the presence of large igneous intrusions.
The subducting plate produces partial melting in a number of ways. The basalts
erupted above subduction zones result from partial melting of the mantle above
the subducting plate. The loss even of small quantities of water from the subduct-
ing plate into the overriding mantle is sufficient to lower the melting temperature
considerably (see Fig. 10.6). However, the magma which produces the andesite
volcanics and the diorite intrusions forms either from partial melting of the sub-
ducted oceanic crust and sediments or, mostly, from melting in the overriding
mantle wedge. The melt then collects beneath the overriding crust, where it frac-
tionates. The subducted oceanic mantle does not undergo partial melting because
it is already depleted-mantle material. Thus, the 'volcanic line' marks the depth
at which material in the subducted plate or overlying mantle first reaches a high
enough temperature for partial melting to occur. At shallow levels, partial melting
is likely to produce basaltic magma; at greater depth, the degree of partial melting
