12.5 Subduction Complexes
seismic lines that, due to the deep positton of the
decollement zone, most of the incoming sediment was incorporated into the accretionary prism during the Pleistocene.
These results should not be extrapolated to other sections of the North American convergent Pacific margin.
The Oregon-Washington margin, for example, is affected
by strike-slip, transport and rotation of forearc blocks, and
change from seaward to landward vergence of structures
developing at the front of the accretionary prism (e.g.
MacKay 1995: Wells et al. 1998).
The dip angle of the subducting oceanic plate normally
increases landward or arcward (Fig. 12.22c,d). The
Cascadia example demonstrates that the relatively steep dip
(~18°) of the subducting plate beneath the Vancouver island margin prevents the buildup of a highly elevated
ridge. In contrast, the Olympic Mountains farther to the
south (Fig. 12.22d) owe their existence to a more gently
dipping subduction zone ( ,,;11°), as weil as to a particularly
high sediment supply at the accretionary wedge front.
Aleutian trench. The thickness of undeformed sediment
being underthrust with the subducting oceanic plate varies
considerably. Along the central Aleutian trench it is 1 km
(McCarthy and Scholl 1985). Sediment accumulation in
this trench is also rapid, and the convergence rate reaches 8
cmla. Evaluating a new seismic section across the eastern
portion of the arc-trench system, von Huene et al. (1998)
have found a total sediment input ofabout 130 km 3/kmIMa
(average for the last 3 Ma to the subduction zone. One
third of this mass is underthrust at the deformation front
and two thirds are underplated. Here, the young trench fill
consists of sandy and muddy turbidites, overlying a Tertiary deep-sea fan. The dewatering of the accretionary
wedge is briefly mentioned in Sect. 13.8.3.
Northern Barbados ridge. This active accretionary
wedge in the region of the Lesser AntilIes has a convergence rate of 2 cmla and displays only an indistinct depression instead of a deep-sea trench. The zone of decollement
at the deformation front is shallow (about 300 m below the
sea floor, Fig. 12.22e). Here, Neogene hemipelagic sediments are detached from thicker (about 600 m), little deformed Oligocene to Campanian underthrust strata. The
rate of sediment accretion amounts to about 24 km 31km1Ma
(von Huene and Scholl 1991).
Japan trench and Nankai trench. At the northern Japan trench, the sediment beneath the accretionary wedge is
1 to 1.5 km in thickness (Cadet and Kobayashi et al. 1987).
Here, the surface of the oceanic basement is rough due to
faulting (cf. Fig. 12.20a). Further south in the Nankai
trench, 400 to 500 m thick oceanic sediments are
underthrust (Karig 1986), but deeper below the accretionary prism the decollement zone moves down to the
top of the oceanic crust (cf. Fig. 12.24d; Taira et al. 1988).
For a subduction rate of 3.2 cmla, a mass balance has
shown that total accretion amounts to 44 km 31km1Ma, of
which 70% are consumed by frontal accretion and 30% by
underplating (Gaedicke 1995). The sediments of frontal
accretion consist of about 13 km 3 /km!Ma of trench and
slope material and 17 km 3 /km!Ma of incoming oceanic sediment.
587
Conclusions for the Sediment Budget
Von Huene and Scholl (1991) have summarized data
from all present-day subduction zones, in total about
43 000 km in length. They found that ~57% of this
length is characterized by accretionary prisrns and
~43% by non-accretion or very limited accretion. At
accreting margins with large prisms (ab out 20% of
all margins), roughly 70% of the incoming trench
floor section is used for frontal accretion seaward of
a resistive rock buttress (backstop ). Prisrns of small
to medium size reflect scenarios where only 20% of
the incoming sediment goes into frontal accretion.
The remaining sediment subducts beneath the buttress (underplating, uplift of the overlying rocks) or
reaches greater depth. It should be noted that rnass
balances of sediment accretion are valid only for individual cross sections through subduction zones.
The amounts of accretion can greatly vary along
strike.
The seaward advance, thickness, and elevation of
an accretionary wedge are controlled mainly by the
rate of incoming sediment (in terms of volume) and
the dip of the underthrusting oceanic crnst (Davis et
al. 1983). Gently dipping subduction and a low convergence rate favor frontal accretion. Detachment
zones preferentially evolve at some depth below the
trench fills where low sediment permeabilities have
led to excess pore pressure and decreasing shear
strength (cf. Sect. 13.1).
Specific Structures and other Phenomena
in Accretionary Wedges
The internal structure of modern accretionary
wedges is poorly known. Better opportunities for the
study of specific phenomena offer uplifted older
accretionary wedges creating nappes in orogenie
belts. In this section, the formation of melange, liquefaction phenomena, mud diapirs and mud volcanoes
is addressed.
The best studied modern examples of accretionary wedges
include the Barbados Ridge in the Caribbean Sea, the
Cascadia accretionary prism off Oregon and Washington,
the Aleutian arc-trench system, the Nankai trough off Japan, and the Peru-Chile trench (see above). These exampies were explored by various seismic methods, detailed
topographic mapping (side-scan sonar, swath mapping),
deep-see drilling, submersibles, and special devices for
recording venting fluids, measuring pore pressure and
chemical properties in drill holes, etc.
Tectonic Deformation
The drilled cross sections of accretionary prisrns,
such as the Barbados Ridge, can display two genera-
seismic lines that, due to the deep positton of the
decollement zone, most of the incoming sediment was incorporated into the accretionary prism during the Pleistocene.
These results should not be extrapolated to other sections of the North American convergent Pacific margin.
The Oregon-Washington margin, for example, is affected
by strike-slip, transport and rotation of forearc blocks, and
change from seaward to landward vergence of structures
developing at the front of the accretionary prism (e.g.
MacKay 1995: Wells et al. 1998).
The dip angle of the subducting oceanic plate normally
increases landward or arcward (Fig. 12.22c,d). The
Cascadia example demonstrates that the relatively steep dip
(~18°) of the subducting plate beneath the Vancouver island margin prevents the buildup of a highly elevated
ridge. In contrast, the Olympic Mountains farther to the
south (Fig. 12.22d) owe their existence to a more gently
dipping subduction zone ( ,,;11°), as weil as to a particularly
high sediment supply at the accretionary wedge front.
Aleutian trench. The thickness of undeformed sediment
being underthrust with the subducting oceanic plate varies
considerably. Along the central Aleutian trench it is 1 km
(McCarthy and Scholl 1985). Sediment accumulation in
this trench is also rapid, and the convergence rate reaches 8
cmla. Evaluating a new seismic section across the eastern
portion of the arc-trench system, von Huene et al. (1998)
have found a total sediment input ofabout 130 km 3/kmIMa
(average for the last 3 Ma to the subduction zone. One
third of this mass is underthrust at the deformation front
and two thirds are underplated. Here, the young trench fill
consists of sandy and muddy turbidites, overlying a Tertiary deep-sea fan. The dewatering of the accretionary
wedge is briefly mentioned in Sect. 13.8.3.
Northern Barbados ridge. This active accretionary
wedge in the region of the Lesser AntilIes has a convergence rate of 2 cmla and displays only an indistinct depression instead of a deep-sea trench. The zone of decollement
at the deformation front is shallow (about 300 m below the
sea floor, Fig. 12.22e). Here, Neogene hemipelagic sediments are detached from thicker (about 600 m), little deformed Oligocene to Campanian underthrust strata. The
rate of sediment accretion amounts to about 24 km 31km1Ma
(von Huene and Scholl 1991).
Japan trench and Nankai trench. At the northern Japan trench, the sediment beneath the accretionary wedge is
1 to 1.5 km in thickness (Cadet and Kobayashi et al. 1987).
Here, the surface of the oceanic basement is rough due to
faulting (cf. Fig. 12.20a). Further south in the Nankai
trench, 400 to 500 m thick oceanic sediments are
underthrust (Karig 1986), but deeper below the accretionary prism the decollement zone moves down to the
top of the oceanic crust (cf. Fig. 12.24d; Taira et al. 1988).
For a subduction rate of 3.2 cmla, a mass balance has
shown that total accretion amounts to 44 km 31km1Ma, of
which 70% are consumed by frontal accretion and 30% by
underplating (Gaedicke 1995). The sediments of frontal
accretion consist of about 13 km 3 /km!Ma of trench and
slope material and 17 km 3 /km!Ma of incoming oceanic sediment.
587
Conclusions for the Sediment Budget
Von Huene and Scholl (1991) have summarized data
from all present-day subduction zones, in total about
43 000 km in length. They found that ~57% of this
length is characterized by accretionary prisrns and
~43% by non-accretion or very limited accretion. At
accreting margins with large prisms (ab out 20% of
all margins), roughly 70% of the incoming trench
floor section is used for frontal accretion seaward of
a resistive rock buttress (backstop ). Prisrns of small
to medium size reflect scenarios where only 20% of
the incoming sediment goes into frontal accretion.
The remaining sediment subducts beneath the buttress (underplating, uplift of the overlying rocks) or
reaches greater depth. It should be noted that rnass
balances of sediment accretion are valid only for individual cross sections through subduction zones.
The amounts of accretion can greatly vary along
strike.
The seaward advance, thickness, and elevation of
an accretionary wedge are controlled mainly by the
rate of incoming sediment (in terms of volume) and
the dip of the underthrusting oceanic crnst (Davis et
al. 1983). Gently dipping subduction and a low convergence rate favor frontal accretion. Detachment
zones preferentially evolve at some depth below the
trench fills where low sediment permeabilities have
led to excess pore pressure and decreasing shear
strength (cf. Sect. 13.1).
Specific Structures and other Phenomena
in Accretionary Wedges
The internal structure of modern accretionary
wedges is poorly known. Better opportunities for the
study of specific phenomena offer uplifted older
accretionary wedges creating nappes in orogenie
belts. In this section, the formation of melange, liquefaction phenomena, mud diapirs and mud volcanoes
is addressed.
The best studied modern examples of accretionary wedges
include the Barbados Ridge in the Caribbean Sea, the
Cascadia accretionary prism off Oregon and Washington,
the Aleutian arc-trench system, the Nankai trough off Japan, and the Peru-Chile trench (see above). These exampies were explored by various seismic methods, detailed
topographic mapping (side-scan sonar, swath mapping),
deep-see drilling, submersibles, and special devices for
recording venting fluids, measuring pore pressure and
chemical properties in drill holes, etc.
Tectonic Deformation
The drilled cross sections of accretionary prisrns,
such as the Barbados Ridge, can display two genera-