Geoscience Reference
In-Depth Information
Figure 10.11.
Three stages in the development of a subduction zone when there
is a great thickness of sediment on the oceanic plate. Stage 1: a trench is visible.
Stage 2: the large amount of sediment scraped off the subducting oceanic plate has
choked the trench. The accreted sediments all belong to the overriding plate, and
thrusting takes place beneath them. The Makran subduction zone is now at this
stage. Stage 3: the subducted plate sinks into the asthenosphere, resulting in the
gradual extinction of the volcanoes and, in the accretionary wedge, extension and
(eventually) new volcanoes. (From Jackson and McKenzie (1984).)
develops first, followed by a major thrust fault, which raises the fold some 1200 m
above the abyssal plain. Reflectors can be traced from the abyssal plain into the
frontal fold but not beyond because deformation and faulting are too extreme
further into the wedge. Detailed wide-angle seismic-velocity measurements show
that dewatering of the sediments, and hence compaction, occurs in the frontal fold.
The sediments are then sufficiently strong to support the major thrust fault. The
continuous process of forming this accretionary wedge results in the southward
advance of the coastline by 1 cm yr
−
1
.Bythis process, a considerable volume of
material is being added to the Eurasian plate every year. Figure 10.11 illustrates
the possible stages in the development of a thick accretionary wedge. Sometime
in the future, the situation shown in Fig. 10.11 may be appropriate for the Makran
subduction zone: the subducted Arabian plate may fall into the asthenosphere,
resulting in extension and a new volcanic region in the present accretionary
wedge.
Figure 10.12(a) shows the detail of the style of deformation in the accretionary
wedge in a more 'normal' subduction zone, the Sunda Arc, where the Indian
plate is being obliquely subducted beneath Sumatra at
7cmyr. The plate dips
at about 3
◦
close to the deformation front, but the dip increases with depth. The
rugged top of the oceanic crust can be clearly seen beneath the accretionary
wedge. The seismic-velocity and density models determined from detailed
∼
−→
Figure 10.12.
(a) A seismic-reflection line across the Sumatra Trench. The active
part of the accretionary wedge extends from the detachment front to the slope break
that marks the backstop structure. Intense faulting within the accretionary prism
makes imaging the detail of the structure very difficult. (b) The gravity anomaly.
(c) The best-fitting density model. (d) Recorded (left) and synthetic (right) wide-angle
seismic data from a strike line located at 230 km on the cross section (b). On the
velocity-depth structure, the depth extent of the subducted plate is shaded gray.
(From Kopp
et al
.(2001).)
