Geoscience Reference
In-Depth Information
within the mantle, irrespective of the type of overriding plate. The complication
introduced by the continental crust is that it allows the possibility of further
chemical complexity and variation in the erupted and intruded melts.
There are some particular conditions under which the subducting hydrous
basaltic slab can melt - generally, if it is very young (less than 5 Ma) and hot, it
can melt at depths of 60-80 km. The melts that result are termed adakites, being
dacitic in composition. However, adakites are not confined to volcanism over
very young subducting plates - any process that causes the temperature of the
subducting plate to be higher than normal will mean that melting of that plate and
formation of adakites may be possible. Particular instances where this may occur
include a very slow subduction rate or a torn subducting plate. This may explain
the presence of adakites in the Aleutian/Kamchatka corner region (Fig. 2.2). In
addition, they are also found over older subducted lithosphere, apparently when
the subduction zone has temporarily had a very shallow dip - perhaps when some
buoyant crust, such as an oceanic plateau, enters the subduction zone - temporary
'flat subduction'.
10.2.2 Sediments at subduction zones
A schematic cross section of a subduction zone was shown in Fig. 9.42,which
illustrates the characteristic geological features. In reality, a subduction zone does
not necessarily have all these features. The accretionary wedge is the region of
folding, and then of faulting and thrusting of the sediments on the subducting
oceanic plate. Then comes the outer-arc high. The fore-arc basin is an active
sedimentary basin, being filled largely by material (detritus) eroded from the
adjacent arc. It may be underlain by oceanic crust marking the position of the old
passive continental margin before subduction began. The volcanic arc and the
back-arc region were discussed in Sections 10.2.1 and 9.6.
One subduction zone with a well-developed accretionary wedge is the Makran
subduction zone in the Gulf of Oman off Iran and Pakistan. There the oceanic
part of the Arabian Plate is being subducted beneath the Eurasian Plate. This
900-km-long subduction zone is unusual in a number of ways:
1. the dip of the subducting Arabian Plate is very low;
2. there is no clear expression of an oceanic trench;
3. the background seismicity is very low; and
4. there is a prominent accretionary wedge, much of which is at present exposed on land
in Iran and Pakistan.
Figure 10.10 shows a seismic-reflection profile across part of the offshore portion
of this thick accretionary wedge. The undeformed abyssal plain sediments on the
Arabian plate are 6-7 km thick. The deformation of these sediments as they
are pushed against the accretionary wedge is clearly visible. A gentle frontal fold
