12.5 Deep-Sea Trenches
Most ancient subduction complexes, whose internal
structure has been exposed on land by deep erosion
following uplift, are much more complex than may
be inferred from the simple models described above.
Besides packets of bedded sequences, in which stratification is preserved in spite of faulting and folding,
other, once deeply buried portions of the accretionary
wedge display pervasive metamorphic fabrics which
indicate low temperature and high pressure conditions. Zones of intensive shearing creating a scaly
fabric are common.
Trench FiUs, Accretion and Subduction
of Sediment (Quantitative Aspects)
Residence Time and Vo/ume olTrench Fills Accreted
The sediments in a deep-sea trench have little chance
of being preserved in their original position, because
they are continuously scraped off by the overriding
accretionary wedge and incorporated into the
subduction complex. Principally, one can distinguish
between two or three possibilities (end members) for
the fate of sediments at subduction zones:
(1) The sediments of the trench and incoming oceanic plate are predominantly or entirely incorporated
into the accretionary wedge (frontal accretion). This
process is favored by thick trench sediments, a slow
subduction rate, and a low angle of subduction.
(2) All sediments are subducted and entirely or partially used for underplating (cf. Fig. 11.35). This
mechanism seems to prevail in settings with thinner
trench fillings, relatively rapid and steep-angle
subduction.
(3) In addition to sediment, material of the oceanic
crost is accreted in the upper plate. This is facilitated
by a "rough" (e.g. faulted) surface of the subducting
oceanic crost. Then the total volume of accreted and
subducted material can exceed the volume of trench
and incoming pelagic sediment.
Here, particularly case (1) is considered. In a first
approximation, the residence time of sediments in a
deep-sea trench is a function of the convergence rate,
eR, and the width, W, ofthe trench.
Given, for example, an axial sediment source, the sediment
wedge would be widest and thickest near the source and
diminish distally (Fig. 12.19a). The maximum residence
time of a sediment layer in the trench is W/CR. Sediment
deposited in a 20 km wide trench subject to a convergence
rate of 5 cmla would have a maximum residence time of
400 ka. The following calculation also takes into account
the dip angle of subduction (Thomburg and Kulm 1987).
In the meantime, more sophisticated models have been
worked out (e.g. von Huene and Scholl 1991; Gaedicke
1995; Mountney and Westbrook 1996; von Huene et al.
1998).
583
The general processes operating along subduction zones,
including frontal accretion and underplating of sediment,
have been addressed by many authors (e.g. Le Pichon et al.
1993; Lallemand et al. 1994) and references listed below.
The relationship between new sediment accumulation
in the trench and sediment loss to frontal accretion is
demonstrated by the models in Fig. 12.21a,b. If the
sedimentation rate in the trench and the convergence
rate of the subduction process are kept constant,
steady-state conditions at the front of the accretionary wedge can be achieved. This signifies that the
sedimentation rate exactly balances the rate of sediment attrition caused by the seaward migration of the
deformation front.
Sediment accretion here includes sediment lost to
subduction; changes in the porosity of the accreted material
are neglected. In spite of these !imitations, this approach
demonstrates the order of magnitude in this type of sediment transfer.
The sedimentation rate, SR, can also be expressed as a
function of the convergence rate, CR, and the angle of dip,
ß, ofthe subducting oceanic plate:
SR = CR' tanß,
(12.1)
as can be seen from simple geometric considerations (Fig.
12.21b). In this case, SR is independent from the width, W,
ofthe trench and the thickness, T, ofthe trench sediments.
Examples given by Thomburg and Kulm (1987) for the
Chile Trench yield the following results for a dip angle of
the subducting plate of 3° and a convergence rate of 10
cmla:
a) Sediment-starved trench off central Chile (width W=1O
km, thickness T=O.5 km).
b) Sediment-filled trench off south Chile (width W=30 km,
thickness T=1.5 km).
For both examples, SR is found to be about 0.5 cmla (5000
mlMa), which appears to be extremely high, particularly
for the relatively sediment-starved central Chile trench.
Hence, it is assumed that, in addition to trench material,
incoming sediment from the subducting oceanic plate is
being incorporated into the accretionary wedge (Fig.
12.21c). This assumption has been confirmed for several
active subduction zones by seismic records and ocean drilling.
The vo/ume 01 sediment derived from newly deposited trench deposits and incorporated into an
accretionary wedge per unit time depends on the
width of the trench fill (the dip angle of subduction
and the convergence rate being the same as above).
This means that trenches oversupplied with sediment
provide more sediment for accretion (and
subduction) than narrow trench fills, even when their
sedimentation rate is equal.
The volume of accreted trench sediment, V" per unit time
and length of the trench is
Most ancient subduction complexes, whose internal
structure has been exposed on land by deep erosion
following uplift, are much more complex than may
be inferred from the simple models described above.
Besides packets of bedded sequences, in which stratification is preserved in spite of faulting and folding,
other, once deeply buried portions of the accretionary
wedge display pervasive metamorphic fabrics which
indicate low temperature and high pressure conditions. Zones of intensive shearing creating a scaly
fabric are common.
Trench FiUs, Accretion and Subduction
of Sediment (Quantitative Aspects)
Residence Time and Vo/ume olTrench Fills Accreted
The sediments in a deep-sea trench have little chance
of being preserved in their original position, because
they are continuously scraped off by the overriding
accretionary wedge and incorporated into the
subduction complex. Principally, one can distinguish
between two or three possibilities (end members) for
the fate of sediments at subduction zones:
(1) The sediments of the trench and incoming oceanic plate are predominantly or entirely incorporated
into the accretionary wedge (frontal accretion). This
process is favored by thick trench sediments, a slow
subduction rate, and a low angle of subduction.
(2) All sediments are subducted and entirely or partially used for underplating (cf. Fig. 11.35). This
mechanism seems to prevail in settings with thinner
trench fillings, relatively rapid and steep-angle
subduction.
(3) In addition to sediment, material of the oceanic
crost is accreted in the upper plate. This is facilitated
by a "rough" (e.g. faulted) surface of the subducting
oceanic crost. Then the total volume of accreted and
subducted material can exceed the volume of trench
and incoming pelagic sediment.
Here, particularly case (1) is considered. In a first
approximation, the residence time of sediments in a
deep-sea trench is a function of the convergence rate,
eR, and the width, W, ofthe trench.
Given, for example, an axial sediment source, the sediment
wedge would be widest and thickest near the source and
diminish distally (Fig. 12.19a). The maximum residence
time of a sediment layer in the trench is W/CR. Sediment
deposited in a 20 km wide trench subject to a convergence
rate of 5 cmla would have a maximum residence time of
400 ka. The following calculation also takes into account
the dip angle of subduction (Thomburg and Kulm 1987).
In the meantime, more sophisticated models have been
worked out (e.g. von Huene and Scholl 1991; Gaedicke
1995; Mountney and Westbrook 1996; von Huene et al.
1998).
583
The general processes operating along subduction zones,
including frontal accretion and underplating of sediment,
have been addressed by many authors (e.g. Le Pichon et al.
1993; Lallemand et al. 1994) and references listed below.
The relationship between new sediment accumulation
in the trench and sediment loss to frontal accretion is
demonstrated by the models in Fig. 12.21a,b. If the
sedimentation rate in the trench and the convergence
rate of the subduction process are kept constant,
steady-state conditions at the front of the accretionary wedge can be achieved. This signifies that the
sedimentation rate exactly balances the rate of sediment attrition caused by the seaward migration of the
deformation front.
Sediment accretion here includes sediment lost to
subduction; changes in the porosity of the accreted material
are neglected. In spite of these !imitations, this approach
demonstrates the order of magnitude in this type of sediment transfer.
The sedimentation rate, SR, can also be expressed as a
function of the convergence rate, CR, and the angle of dip,
ß, ofthe subducting oceanic plate:
SR = CR' tanß,
(12.1)
as can be seen from simple geometric considerations (Fig.
12.21b). In this case, SR is independent from the width, W,
ofthe trench and the thickness, T, ofthe trench sediments.
Examples given by Thomburg and Kulm (1987) for the
Chile Trench yield the following results for a dip angle of
the subducting plate of 3° and a convergence rate of 10
cmla:
a) Sediment-starved trench off central Chile (width W=1O
km, thickness T=O.5 km).
b) Sediment-filled trench off south Chile (width W=30 km,
thickness T=1.5 km).
For both examples, SR is found to be about 0.5 cmla (5000
mlMa), which appears to be extremely high, particularly
for the relatively sediment-starved central Chile trench.
Hence, it is assumed that, in addition to trench material,
incoming sediment from the subducting oceanic plate is
being incorporated into the accretionary wedge (Fig.
12.21c). This assumption has been confirmed for several
active subduction zones by seismic records and ocean drilling.
The vo/ume 01 sediment derived from newly deposited trench deposits and incorporated into an
accretionary wedge per unit time depends on the
width of the trench fill (the dip angle of subduction
and the convergence rate being the same as above).
This means that trenches oversupplied with sediment
provide more sediment for accretion (and
subduction) than narrow trench fills, even when their
sedimentation rate is equal.
The volume of accreted trench sediment, V" per unit time
and length of the trench is