590
110), Aalto (1989), Moore et al. (1991), von Eynatten et al.
(1993), Liu and Einseie (1996), Hasebe et al. (1997); A
summary of these features is given by Pickering et al.
(1989).
Examples of Arc-Trench Systems
Aneient examples of subduetion complexes have
been described from many regions (e.g., Leggett
1982), but many ofthem are not sufficiently exposed
and therefore not weIl understood. Apart from the
modem aeeretionary wedges mentioned above, we
here discuss two further, relatively young arc-trench
systems. The first one is outstanding beause of its
huge size and some general important trends, the second exarnple demonstrates a number of smaIler-scale
features which can be observed at many other locations.
The Sunda Arc. A partieularly instructive and large,
mainly oceanie arc-trench system is the Sunda arc
between Burma and the Sumba Island to the east of
Java (Fig. 12.23). It represents a 5000 km long
subduction zone of the Indian oceanic plate beneath
the Eurasian plate, which has been interrnittently aetive sinee the Perrnian. The youngest active phase
began in the late Oligocene. During the Neogene, the
northem part of this system was dominated by detritus from the Himalayas which was dispersed by mass
flows and turbidity currents on the huge submarine
Bengal fan, the Nicobar fan, and along the axis of the
Sunda trench.
Whereas sediment supply to the Nicobar fan was cut off in
the mid-Pleistocene as a result of the eastwardly migrating
Ninetyeast Ridge, a major submarine channel still transports sediment today over the long distance from the northern end ofthe trench to the southeast into the region ofthe
Sunda Strait (Moore GF et al. 1982; Velbel 1985). In the
northwest, i.e., the Andaman section, transport of large volumes of material down the trench axis is indicated by a
wide and relatively shallow trench floor (water depth about
3000 m, Fig. 12.23b). West of Sumatra, the trench floor is
still broad, but narrows and deepens to about 5000 m off
Nias Island (Fig. 12.23c). South and southeast of Java, the
trench is forrned by narrow, isolated basins at water depths
of6000 to 6500 m (Fig. 12.23d).
Subduction is oblique in the eastern and particularly in
the northern part of the arc (Fig. 12.23a). This has lead to
significant deforrnations in the forearc region, including the
right-lateral Great Sumatra fault and the opening of the
Sunda Strait (e.g. Baroux et al. 1998). The Banda arc, beginning at the eastern end of the Sunda arc, represents one
of the rare modern examples of continent-arc collision (cf.
Sect. 12.6; e.g. Charlton 1997).
Sediments on the subducting plate are thus primarily
thiek turbidite sequenees in the northwestem and
eentral portions of the system, but only thin (200 to
400 m) pelagie and hemipelagic sediments off Java.
Chapter 12 Basin Evolution and Sediments
Consequently, accretion of thick Neogene fan and
trench sediments created a wide, high outer are ridge
in the northwestem and eentral trench portions, while
the ridge is narrower and less elevated in the southeastem part (Fig. 12.23c,d). Since this outer ridge
forrned, sediments from the volcanic arc, i.e., from
Sumatra and Java, are trapped in aseries of forearc
basins and therefore can no longer reaeh the trench.
Prior to the rnid-Tertiary, volcanic1astic material from
the arc eontributed signifieantly to the trench
sediments and the buildup of the accretionary wedge.
The forearc basins between the arc and the outer
ridge are 150 to 500 km long and 50 to 100 km wide.
In Burma, about 8 km of marine, deltaic, and fluvial
sediments accumulated from the late Cretaceous to
the Plioeene. Off Sumatra, water depths in the basins
are 500 to 2000 m, while off Java the basins are 3500
to 4000 m deep. Each forearc basin is fed by a number of submarine canyons. The sediment fills attain
thicknesses of up to 6000 m. Beeause the basins are
separated from each other by transverse strnetural
highs, the fill of each basin differs in eomposition
from adjacent basins. The sediments in the forearc
basins are derived primarily from the arc terranes and
are dispersed by submarine slope eanyons and small
deep-sea fans. Off north Sumatra, quartzose sediments accumulate, whereas off Java, volcanic1astic
sediments predominate in the forearc basins.
ShaIlow-water carbonates and their detritus also play
a role.
The Shimanto belt in Shikoku, Japan. This is one
of the best known examples of a Cretaceous to Miocene accretionary prism (e.g. Taira et al. 1988; Taira
and Ogawa 1991). It lies landward of the Nankai
trench (Fig. 12.24) and resembles in its structure and
sedimentary facies this modem subduction complex.
The deeollement of the young accretionary prism
takes place above the pelagie and hemipelagic sediments of the downgoing oceanic plate within muddy
sediments of the outer trench fill (Fig. 12.24c,d).
Hence, the sediments of the young accretionary
prism are dorninated by muddy and sandy flysch
from the trench fill. However, at a depth of ab out 5
km below the slope, the decollement zone moves
down to the top of the oceanic ernst and forms duplex structures into which sediments of the oceanic
plate are incorporated. Thus, the older thrust slices
may contain two basic units:
- Melange consisting of basalt slivers, pelagie and
hernipelagic sediments, and fine-grained matrix from
the treneh fill.
- Flysch units from the trench fill (Fig. 12.24d).
Intaet sections of pelagie and hemipelagic sediments
show characteristic vertieal facies successions deposited on pillowed basalt of the subducted oceanic plate
110), Aalto (1989), Moore et al. (1991), von Eynatten et al.
(1993), Liu and Einseie (1996), Hasebe et al. (1997); A
summary of these features is given by Pickering et al.
(1989).
Examples of Arc-Trench Systems
Aneient examples of subduetion complexes have
been described from many regions (e.g., Leggett
1982), but many ofthem are not sufficiently exposed
and therefore not weIl understood. Apart from the
modem aeeretionary wedges mentioned above, we
here discuss two further, relatively young arc-trench
systems. The first one is outstanding beause of its
huge size and some general important trends, the second exarnple demonstrates a number of smaIler-scale
features which can be observed at many other locations.
The Sunda Arc. A partieularly instructive and large,
mainly oceanie arc-trench system is the Sunda arc
between Burma and the Sumba Island to the east of
Java (Fig. 12.23). It represents a 5000 km long
subduction zone of the Indian oceanic plate beneath
the Eurasian plate, which has been interrnittently aetive sinee the Perrnian. The youngest active phase
began in the late Oligocene. During the Neogene, the
northem part of this system was dominated by detritus from the Himalayas which was dispersed by mass
flows and turbidity currents on the huge submarine
Bengal fan, the Nicobar fan, and along the axis of the
Sunda trench.
Whereas sediment supply to the Nicobar fan was cut off in
the mid-Pleistocene as a result of the eastwardly migrating
Ninetyeast Ridge, a major submarine channel still transports sediment today over the long distance from the northern end ofthe trench to the southeast into the region ofthe
Sunda Strait (Moore GF et al. 1982; Velbel 1985). In the
northwest, i.e., the Andaman section, transport of large volumes of material down the trench axis is indicated by a
wide and relatively shallow trench floor (water depth about
3000 m, Fig. 12.23b). West of Sumatra, the trench floor is
still broad, but narrows and deepens to about 5000 m off
Nias Island (Fig. 12.23c). South and southeast of Java, the
trench is forrned by narrow, isolated basins at water depths
of6000 to 6500 m (Fig. 12.23d).
Subduction is oblique in the eastern and particularly in
the northern part of the arc (Fig. 12.23a). This has lead to
significant deforrnations in the forearc region, including the
right-lateral Great Sumatra fault and the opening of the
Sunda Strait (e.g. Baroux et al. 1998). The Banda arc, beginning at the eastern end of the Sunda arc, represents one
of the rare modern examples of continent-arc collision (cf.
Sect. 12.6; e.g. Charlton 1997).
Sediments on the subducting plate are thus primarily
thiek turbidite sequenees in the northwestem and
eentral portions of the system, but only thin (200 to
400 m) pelagie and hemipelagic sediments off Java.
Chapter 12 Basin Evolution and Sediments
Consequently, accretion of thick Neogene fan and
trench sediments created a wide, high outer are ridge
in the northwestem and eentral trench portions, while
the ridge is narrower and less elevated in the southeastem part (Fig. 12.23c,d). Since this outer ridge
forrned, sediments from the volcanic arc, i.e., from
Sumatra and Java, are trapped in aseries of forearc
basins and therefore can no longer reaeh the trench.
Prior to the rnid-Tertiary, volcanic1astic material from
the arc eontributed signifieantly to the trench
sediments and the buildup of the accretionary wedge.
The forearc basins between the arc and the outer
ridge are 150 to 500 km long and 50 to 100 km wide.
In Burma, about 8 km of marine, deltaic, and fluvial
sediments accumulated from the late Cretaceous to
the Plioeene. Off Sumatra, water depths in the basins
are 500 to 2000 m, while off Java the basins are 3500
to 4000 m deep. Each forearc basin is fed by a number of submarine canyons. The sediment fills attain
thicknesses of up to 6000 m. Beeause the basins are
separated from each other by transverse strnetural
highs, the fill of each basin differs in eomposition
from adjacent basins. The sediments in the forearc
basins are derived primarily from the arc terranes and
are dispersed by submarine slope eanyons and small
deep-sea fans. Off north Sumatra, quartzose sediments accumulate, whereas off Java, volcanic1astic
sediments predominate in the forearc basins.
ShaIlow-water carbonates and their detritus also play
a role.
The Shimanto belt in Shikoku, Japan. This is one
of the best known examples of a Cretaceous to Miocene accretionary prism (e.g. Taira et al. 1988; Taira
and Ogawa 1991). It lies landward of the Nankai
trench (Fig. 12.24) and resembles in its structure and
sedimentary facies this modem subduction complex.
The deeollement of the young accretionary prism
takes place above the pelagie and hemipelagic sediments of the downgoing oceanic plate within muddy
sediments of the outer trench fill (Fig. 12.24c,d).
Hence, the sediments of the young accretionary
prism are dorninated by muddy and sandy flysch
from the trench fill. However, at a depth of ab out 5
km below the slope, the decollement zone moves
down to the top of the oceanic ernst and forms duplex structures into which sediments of the oceanic
plate are incorporated. Thus, the older thrust slices
may contain two basic units:
- Melange consisting of basalt slivers, pelagie and
hernipelagic sediments, and fine-grained matrix from
the treneh fill.
- Flysch units from the trench fill (Fig. 12.24d).
Intaet sections of pelagie and hemipelagic sediments
show characteristic vertieal facies successions deposited on pillowed basalt of the subducted oceanic plate