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Submarine topography
Continental slope
Continental slopes are steeper than continental shelves, extending from the outer edges of the continental shelves down to the trench (to a basin in a marginal sea and to a continental rise in a passive margin). Continental slopes range from 1 to 25 degrees (average 4 degrees) and often have submarine canyons. The continental slope is a marginal area between the thick continental crust and the oceanic crust.
Fig.
Forearc basins [
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1) Trench side
The continental slope on the trench side (forearc side) is
characterized by a gentle slope or flat plane, called a deep-sea
terrace or a forearc basin, along the way to the trench. This
morphology is often seen at depths of 2000-2500 m, clearly found on the
continental slopes of the Southwestern Japan Arc and the Ryukyu Arc. In
the Southwestern Japan forearc, shallow oval basins (Hyuga Basin, Tosa
Basin, Muroto Trough, and Kumano Trough) are in a row
parallel to the Nankai Trough. In the Ryukyu forearc, an extensive
deep-sea terrace (Okinawa-Miyako Deep Sea Terrace) lies from Okinawa Island through Taiwan Island.
Basin-like topography is not seen on the continental slope of the
Northeastern Japan forearc, but circular or oval basins buried with
young deposits are found in the slope. The Kuril forearc is also
assumed to have a similar structure.
An outer ridge is a geomorphological or tectonic rise along a margin
between the continental slope and the trench. It is thought that
deep-sea terraces are usually formed by which deposits derived from the
land bury basins on the slope inside the outer ridge parallel to the
arc. The deep-sea terraces along the Nankai Trough possess distinct
outer ridges (see a 3-D topographical map on the website of Japan
Coast Guard).
2) Marginal sea side
The continental slopes in the Sea of Okhotsk and the Sea of Japan on
the west of the Noto Peninsula are gentle slope with slight
undulation. The slopes have relatively stable sedimentary structure,
but some slopes have slump topography. On the other hand, the region
off the continental shelf from the west of Hokkaido through Toyama Bay
is characterized by complex configuration including basins and ridges
trending northeast or north-northeast, parallel to the shoreline. The
tectonic structure of fault, fold and tilting are found in this region.
The eastern margin of the Sea of Japan is thought of the boundary of
plates at which the plate has begun subducting.
3) Submarine canyon
Submarine canyons are commonly formed on the continental slope from the
continental shelf through the trench or the abyssal plane. Sediments
provided from the land flow down to the deep seafloor through these
canyons (turbidity currents). Turbidity currents may develop a
submarine fan at the terminal of the canyon at which the seafloor is
level or gentle slope.
Submarine canyons are well developed around the Japanese Islands. Three
major large-scale submarine canyons are the Kushiro Canyon, the Toyama
Deep-sea Channel, and the Boso Canyon. The Kushiro Canyon greatly
encroached on the continental shelf and deeply eroded the continental
slope. This characteristic is markedly different from other canyons.
The Toyama Deep-Sea Channel is characterized by the length of over 500
km, considerably meandering, a vast submarine fan, and well-developed
submarine natural levees. The Boso Canyon has significantly incised
meander 100 km in length.
(Maps of the
Kushiro Canyon and the
Toyama Deep-Sea Channel on the website of Japan
Coast Guard)
The amount of sediment supplied to deep sea through canyons around
Japan varies according to the manner of plate subduction. For example,
in the Nankai Trough, sediment transported to the trough seemed to
decrease extremely because outer ridges were well developed between the
continental slope and the trough. The oceanic plate descending at a low
angle is developing accretionary prisms along the Nankai Trough.
Trench
Trenches formed by plate subduction are long and narrow depressions along the border between plates. The trench floors are the deepest areas on earth; the depth of the deepest trench is 10920 m in the Mariana Trench. The characteristics of the slope are different in the landward side and the seaward side.
The landward configuration is affected by an angle of the leading edge
of subducting plate and the growth of accretionary prisms; therefore,
the slopes vary in morphology. In the Nankai Trough where the oceanic
plate is subucting with its low-angle leading edge, the slope are gentle
and ridges and troughs parallel to the axis of the Nankai Trough are
developed on the landward slope by which reverse faults divide
accretionary prisms. On the other hand, in the Japan Trench where the
steep leading edge is descending, step-like topography is found. There
are also taluses and gullies indicating active erosion on the steep
cliffs. In the Japan Trench and the Izu-Bonin Trench where trench-fill
sediment is brought underneath the seafloor with the subucting plate,
the landward slopes have slump topography.
The seaward slope is characterized by normal faults which are generally
formed by tension of the surface of the bending plate. Such faults
produce horsts and grabens, often parallel to the axis of the trench.
Normal faults oblique to the axis are also formed where the axis is
curved or tectonically weak zones are present. Good example of these
configurations are found in the Japan Trench (see submarine
topographical maps on the website of Japan Coast Guard [click on maps to
enlarge]; horsts and grabens are on the seaward slope on the maps). A
rise which is called a trench outer buldge is found on the seaward side
of the normal faults in some trenches. In the Northwest Pacific Basin,
there are trench outer buldges 500 m high and 200-500 km wide along the
trench.
On the trench floor, small depressions are often formed discontinuously.
However, when a large amount of sediment is provided from the land to
the trench, depositional topographies, such as a flat basin and a fan,
are formed on the trench floor. These morphologies are found at the
junction of the Japan Trench, the Izu-Bonin Trench, and the Sagami
Trough (triple junction) offshore the Boso Peninsula. Terrigenous and
pelagic sediments thickly accumulated on the floor at this junction, the
thickness of which is 4000 m. Therefore, the bottom of this basin
reaches about 14000 m in depth. There are canyons in the Sagami and
Suruga Troughs and shallow channels in the Nankai Trough.
*The website of Japan Coast Guard provides submarine topographical maps
of plate boundary areas (Go to the page). Click the red frame of an area
you want to see. There is no English page for these maps.
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