Introduction to the Landforms and Geology of Japan

GLGArcs

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.

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).


View Forearc basins in a larger map

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 the accretionary prism. 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 the accretionary prism. 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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