Geoscience Reference
In-Depth Information
Figure 9.42.
An active
convergent plate
boundary is characterized
by a sequence of
geological features.
Extension in the back-arc
region forms a back-arc,
or marginal, basin. (From
The continental crust,
Burchfiel. Copyright
c
1983 by Scientific
American, Inc. All rights
reserved.)
and the line of volcanoes on the overriding plate (Fig. 9.42). The volcanoes
either form an island arc, such as those around the western Pacific, Indonesia and
Caribbean, or are continental, as in the Andes. The horizontal distance between
the trench and the volcanic arc is hundreds of kilometres - some 270 km for the
Japan subduction zone.
Back-arc basins
, otherwise known as
marginal basins
, occur behind the island
arcs of the Western Pacific and Caribbean as well as the Scotia Arc. The crust
in these basins is similar to oceanic crust, and magnetic lineations have been
deciphered over some of them (e.g., the West Philippine Basin and the South Fiji
Basin). The ophiolites which today are found obducted (thrust up) on land may
be crust formed in back-arc basins.
The deep oceanic trench and the outer rise are a consequence of the bending of
the lithospheric plate. Trenches often exceed 8 km in depth, the deepest being the
Mariana Trench (see Section 9.1.1). The deformation of a thin elastic plate under
an external load has been discussed in Section 5.7.1. This theoretical deformation
has been used to explain the bending of the lithosphere at oceanic trenches and to
yield estimates of its elastic parameters (see Figs. 5.14 and 5.15). For a subduction
zone adjacent to a continent, the trench can be almost nonexistent because a high
sedimentation rate may have filled it with sediments. In such cases, deformed
sediments form a substantial
accretionary wedge
and
outer-arc high
. The
volcanic
arc
itself is behind this sedimentary fore-arc basin. In the case of the Makran
subduction zone (part of the boundary between the Arabian and Eurasian plates),
which is an example of a shallow-dipping subduction zone with an extensive
accretionary wedge resulting from a 7 km thickness of undisturbed sediment
on the oceanic plate, the distance from the first deformation of sediment in the
accretionary wedge (the
frontal fold
; see Fig. 10.9)tothe volcanic arc is more
than 400 km.
