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Submarine topography
Continental slope
Continental slopes are steeper than continental shelves, extending from the outer edges of continental shelves down to a 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 [
]
1) Trench side
Continental slopes on the trench side (forearc side) are characterized
by gentle slopes or flat plains, known as deep-sea terraces or forearc
basins, on the way to the trench. This morphology is often seen at
depths of 2000 to 2500 m, clearly found on the continental slopes of
the Southwest Japan Arc and the Ryukyu Arc. In the Southwest 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 Northeast 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 at a margin
between a continental slope and a trench. It is thought that deep-sea
terraces are usually formed by which deposits derived from land bury
basins on the slope inside the outer ridge parallel to the trench.
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
Continental slopes in the Sea of Okhotsk and the Sea of Japan on the
west of the Noto Peninsula are gentle slopes 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 resulting from faulting, folding and tilting are
found in this region. The eastern margin of the Sea of Japan is thought
of the boundary between plates at which the plate has begun subducting.
3) Submarine canyon
Submarine canyons are commonly formed from a continental shelf through
a trench or a abyssal plain, eroding a continental slope downward.
Sediments provided from land flow down to the deep seafloor through
these canyons. A submarine fan may be formed on the flat seafloor or
gentle slope at the terminal of a canyon by turbidity currents. The
amount of sediment supplied to the deep sea through canyons around
Japan varies according to the topography of continental slope. For
example, in the Nankai Trough, developed outer ridges between the
continental slope and the trough block the transportation of sediments
to the trough.
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)
Trench
Trenches formed by plate subduction are long and narrow depressions along a border between plates. Trench floors are the deepest areas on earth; the depth of the deepest trench is 10920 m in the Mariana Trench. The characteristics of their slopes are different in the landward side and the seaward side.
The landward topography is affected by an angle of the leading edge of
subducting plate and growth of accretionary prisms. In the Nankai Trough
where the oceanic plate is subducting with its low-angle leading edge,
the gentle landward slope has ridges and troughs parallel to the axis of
the Nankai Trough formed 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, slump topography develops on
the landward slopes because trench-fill sediment is brought underneath
the seafloor with the subducting plate.
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 trench. Normal
faults oblique to the axis are also formed where the axis is curved or
tectonically weak zones are present. Good example of the landforms 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 (marginal swell) may be formed on the seaward side of the
normal faults. In the Northwest Pacific Basin, marginal swell 500 m high
and 200 to 500 km wide is found along the trench.
Submarine landforms including discontinuously chains of small
depressions, flat basins and fans resulting from the provision of a
large amount of sediments from land, and canyons are found in the trench
floor. Small depressions are common in the floor, and depositional
topography are seen where trenches are near land. Flat basin and fans
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 the 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.
References
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