12.5 Deep-Sea Trenches
sitional and erosional phases. In the downcurrent
direction, the basin fill can become "sheeted" (Fig.
l2.19d), as in distal fluvial plains, where fine-grained
materials predominate. Contour currents can rework
and redeposit some of these materials.
- Lateral migration of oceanic sediment on top of the
subducting plate to the site ofthe trench. These sediments may consist of pelagic calcareous andJor siliceous oozes, red c1ay, and hemipelagic silts and
c1ays. In regions adjacent to the deltas of major
rivers, turbidite successions may rest on the incoming
oceanic plate.
- Pelagic and hemipelagic sediments directly settling
from the overlying water body to the trench floor.
However, this sediment source is comparative1y insignificant, because of the low sedimentation rate of
pelagic and hemipelagic sediments in a basin whose
sediments are permanently consumed by subduction.
Trench sections with high sediment input (over-supplied systems) become fi1led and have a wide cross
section (Fig. 12.19a,b). Ifthe deposits of a submarine
fan spill over the outer trench margin, trench sediments are overlain by the sediments of a deep-sea fan
association (Seet. 5.4.2). In eontrast, sedimentstarved trenches form narrow, ponded basins (Fig.
12.19a,f).
Seamounts, oeeanie ridges and faulted erust approaehing the trench may ereate major irregularities
in the treneh fill and the ensuing subduetion process
(Fig. 12.20).
Trench Fills and Sediment Accretion
(Qualitative Aspects)
The considerations in the previous section and the
assumption that a kind of equilibrium between sediment supply and sediment loss to aecretion is established in are-treneh systems (see below), lead to the
qualitative models shown in Fig. 12.19b-g:
(1) Oversupplied trenches. Sediment fills tend to be
sheared off along a deeollement within the young
treneh fil!. Failure by thrusting may be faeilitated by
inereased pore pressures of the rapidly aceumulated,
underconsolidated sediments. As a result, the sequenee within a slice of the aeeretionary wedge is
built up entirely of treneh sediments. These mainly
represent the axial ehannel system and fan deposits
from the foot of the landward treneh slope (Fig.
l2.l9b,e). In this ease, the sediments within a slice of
the subduction eomplex have ab out the same age, but
may eontain some isolated, displaced older material
transported by mass wasting from the landward slope
into the trench.
(2) Trenches with low to medium sediment supply,
moderate convergence rate (3-5 em/a). These
581
trenches usually develop medium-size sediment
wedges with diseontinuous turbidite basins. The
subduetion deeollement commonly develops below
the young treneh fill in pelagie or hemipelagie strata
of the ineoming oceanic plate (Fig. 12.19d,e). In
eases where the pelagie sediment cover on the oceanic erust is scraped off, the idealized stratigraphie
sequence within a little-disturbed slice of the
accretionary wedge coarsens upward from older pelagic to hemipelagic to younger trench fill sediments
(Fig. 12.1ge). This sequenee may represent a comparatively long time period.
(3) Sediment-starved trenches which receive little
material from both the landward side and the incoming subducting plate are narrow, diseontinuous, and
displaya partially almost empty axis. Modem exampIes ofthis type inc1ude trenehes associated with volcanie island arcs in the western and southwestern
Pacifie. Starved trenehes need a very long time span
to build up an aceretionary wedge of some size. Consequently, the deformation front progrades slowly,
and the landward slope tends to steepen. In this ease,
thrust faults may propagate along weak zones in the
upper oeeanic erust and scrape off slices of basaltic
erust, pelagie sediments and isolated bodies of
ponded treneh fill (Fig. l2.19f,g). Such settings allow the study of basal deep-water sediments on top
of aeereted oceanic erust in the subduction complex.
(4) Tectonic erosion. When the subdueting oeeanic
plate haf> a marked relief and the total sediment supply into the trench is low, sediment from the upper
plate (accretionary wedge) ean be scraped off and
subducted as, for example, off southern Peru and
northern Chile or in the northern portion of the Japan
treneh (Fig. 12.20) and along several arc-treneh systems in the western Pacifie. This proeess is referred
to as "tectonic erosion" which plays a major role in
global sediment mass balanees (cf. Seet. 11.7). It ereates stratigraphie gaps, reduces the volume of the
accretionary wedge, and may finally lead to the co 1lapse and retreat of this complex.
Off northwestern Peni, the edge of the continenta1 crnst
may be as elose as 20-40 km to the trench axis (Fig.
12.20b). Here, the subducting oceanic p1ate has scraped off
the underside and edge of the adjacent continenta1 crnst. It
has been suggested that intense tectonic erosion is followed
by episodes of strong explosive volcanism and arc uplift
(LaBemand 1996). In a transect of the Midd1e America
trench off Costa Rica, deep-sea drilling has shown that the
entire incoming sediment section is being underp1ated (Si1ver et al. 1997).
The bui1dup and composition of accretionary wedges as
weB as tectonic erosion have been topics of great interest
(see, e.g., Moberly et al. 1982; Lash 1985; von Huene
1986; Cadet, Kobayashi, et al. 1987; von Huene and
Lallemand 1990; von Huene and Scholl 1991; and many
others).
Sedimentary processes in deep-sea trenches have been
described, e.g., by Thornburg and Kulm (1987), Le Pichon
et al. (1987), Okada (1988,1989), Macdonald (1993).
sitional and erosional phases. In the downcurrent
direction, the basin fill can become "sheeted" (Fig.
l2.19d), as in distal fluvial plains, where fine-grained
materials predominate. Contour currents can rework
and redeposit some of these materials.
- Lateral migration of oceanic sediment on top of the
subducting plate to the site ofthe trench. These sediments may consist of pelagic calcareous andJor siliceous oozes, red c1ay, and hemipelagic silts and
c1ays. In regions adjacent to the deltas of major
rivers, turbidite successions may rest on the incoming
oceanic plate.
- Pelagic and hemipelagic sediments directly settling
from the overlying water body to the trench floor.
However, this sediment source is comparative1y insignificant, because of the low sedimentation rate of
pelagic and hemipelagic sediments in a basin whose
sediments are permanently consumed by subduction.
Trench sections with high sediment input (over-supplied systems) become fi1led and have a wide cross
section (Fig. 12.19a,b). Ifthe deposits of a submarine
fan spill over the outer trench margin, trench sediments are overlain by the sediments of a deep-sea fan
association (Seet. 5.4.2). In eontrast, sedimentstarved trenches form narrow, ponded basins (Fig.
12.19a,f).
Seamounts, oeeanie ridges and faulted erust approaehing the trench may ereate major irregularities
in the treneh fill and the ensuing subduetion process
(Fig. 12.20).
Trench Fills and Sediment Accretion
(Qualitative Aspects)
The considerations in the previous section and the
assumption that a kind of equilibrium between sediment supply and sediment loss to aecretion is established in are-treneh systems (see below), lead to the
qualitative models shown in Fig. 12.19b-g:
(1) Oversupplied trenches. Sediment fills tend to be
sheared off along a deeollement within the young
treneh fil!. Failure by thrusting may be faeilitated by
inereased pore pressures of the rapidly aceumulated,
underconsolidated sediments. As a result, the sequenee within a slice of the aeeretionary wedge is
built up entirely of treneh sediments. These mainly
represent the axial ehannel system and fan deposits
from the foot of the landward treneh slope (Fig.
l2.l9b,e). In this ease, the sediments within a slice of
the subduction eomplex have ab out the same age, but
may eontain some isolated, displaced older material
transported by mass wasting from the landward slope
into the trench.
(2) Trenches with low to medium sediment supply,
moderate convergence rate (3-5 em/a). These
581
trenches usually develop medium-size sediment
wedges with diseontinuous turbidite basins. The
subduetion deeollement commonly develops below
the young treneh fill in pelagie or hemipelagie strata
of the ineoming oceanic plate (Fig. 12.19d,e). In
eases where the pelagie sediment cover on the oceanic erust is scraped off, the idealized stratigraphie
sequence within a little-disturbed slice of the
accretionary wedge coarsens upward from older pelagic to hemipelagic to younger trench fill sediments
(Fig. 12.1ge). This sequenee may represent a comparatively long time period.
(3) Sediment-starved trenches which receive little
material from both the landward side and the incoming subducting plate are narrow, diseontinuous, and
displaya partially almost empty axis. Modem exampIes ofthis type inc1ude trenehes associated with volcanie island arcs in the western and southwestern
Pacifie. Starved trenehes need a very long time span
to build up an aceretionary wedge of some size. Consequently, the deformation front progrades slowly,
and the landward slope tends to steepen. In this ease,
thrust faults may propagate along weak zones in the
upper oeeanic erust and scrape off slices of basaltic
erust, pelagie sediments and isolated bodies of
ponded treneh fill (Fig. l2.19f,g). Such settings allow the study of basal deep-water sediments on top
of aeereted oceanic erust in the subduction complex.
(4) Tectonic erosion. When the subdueting oeeanic
plate haf> a marked relief and the total sediment supply into the trench is low, sediment from the upper
plate (accretionary wedge) ean be scraped off and
subducted as, for example, off southern Peru and
northern Chile or in the northern portion of the Japan
treneh (Fig. 12.20) and along several arc-treneh systems in the western Pacifie. This proeess is referred
to as "tectonic erosion" which plays a major role in
global sediment mass balanees (cf. Seet. 11.7). It ereates stratigraphie gaps, reduces the volume of the
accretionary wedge, and may finally lead to the co 1lapse and retreat of this complex.
Off northwestern Peni, the edge of the continenta1 crnst
may be as elose as 20-40 km to the trench axis (Fig.
12.20b). Here, the subducting oceanic p1ate has scraped off
the underside and edge of the adjacent continenta1 crnst. It
has been suggested that intense tectonic erosion is followed
by episodes of strong explosive volcanism and arc uplift
(LaBemand 1996). In a transect of the Midd1e America
trench off Costa Rica, deep-sea drilling has shown that the
entire incoming sediment section is being underp1ated (Si1ver et al. 1997).
The bui1dup and composition of accretionary wedges as
weB as tectonic erosion have been topics of great interest
(see, e.g., Moberly et al. 1982; Lash 1985; von Huene
1986; Cadet, Kobayashi, et al. 1987; von Huene and
Lallemand 1990; von Huene and Scholl 1991; and many
others).
Sedimentary processes in deep-sea trenches have been
described, e.g., by Thornburg and Kulm (1987), Le Pichon
et al. (1987), Okada (1988,1989), Macdonald (1993).