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 Risk management
 Prolongation of the Land Mass
 Ridges and submarine elevations
 Accretion and suturing

The Foot of the Continental Slope

The Foot of the Continental Slope

Paragraph 1 of article 76 states that

The continental shelf of a coastal state comprises the sea-bed and subsoil of the submarine areas that extend beyond its territorial sea throughout the natural prolongation of its land territory to the outer edge of the continental margin.

Paragraph 3 of article 76 states that
 

The continental margin comprises the submerged prolongation of the land mass of the coastal State, and consists of the sea-bed and subsoil of the shelf, the slope and the rise. It does not include the deep ocean floor with its oceanic ridges or the subsoil thereof.

This means that if the boundary between the prolongation of the land mass and the deep ocean floor, or the boundary between the rise and the deep ocean floor can be identified directly, then that boundary will establish the extent of the extended continental margin.

In practice, however, the morphological boundary between the rise and the deep ocean floor, or the geological boundary between the submarine prolongation of the land mass and the deep ocean floor, can be difficult to identify. The morphological boundary between the rise and the deep ocean floor is often transitional or very subtle, and in some cases there is no rise present along the margin. Similarly, the geological boundary between continental rocks and rocks of the deep ocean floor can be transitional or masked by sedimentary and volcanic rocks. Even if these boundaries could be identified, not all coastal States will have sufficient resources to acquire the necessary scientific data to do so.

Article 76 recognises the difficulty of finding these boundaries, and provides two formulae to use as proxies to establish the outer edge of the continental margin.

According to paragraph 4(a) of article 76

 For the purposes of this Convention, the coastal State shall establish the outer edge of the continental margin wherever the margin extends beyond 200 nautical miles from the baselines from which the breadth of the territorial sea is measured, by either:

(i) a line delineated in accordance with paragraph 7 by reference to the outermost fixed points at which the thickness of sedimentary rocks is at least 1 per cent of the shortest distance from such point to the foot of the continental slope; or

(ii) a line delineated in accordance with paragraph 7 by reference to fixed points not more than 60 nautical miles from the foot of the continental slope.



These formulas are based on the location of the foot of the continental slope. Paragraph 4(b) of article 76 gives two methods for determining the location of the foot of the continental slope:

In the absence of evidence to the contrary, the foot of the continental slope shall be delineated as the point of maximum change in the gradient at its base.

The Commission on the Limits of the Continental Shelf (CLCS) Guidelines (1999) declare a preference for identifying the foot of the continental slope as the point of maximum change in sea floor gradient as the general rule, with reliance on “evidence to the contrary” as exceptions to the general rule. The Guidelines state that these exceptions can be applied when the foot of the continental slope is not located reliably by the point of maximum change in sea floor gradient, and that they provide an opportunity for coastal States to use geological and geophysical evidence to locate the foot of the continental slope.

The continental slope is the steep part of the margin that lies between the shallow shelf at about 200 metres water depth and the rise or the deep ocean floor. Because continental margins form as the result of diverse tectonic, sedimentary and volcanic processes, there is a large variation in the morphology of the continental slope. It typically has a gradient of a few degrees, but can vary locally from steep escarpments to near-horizontal surfaces across terraces and ponded mid-slope basins.

The rise is a wedge-shaped sedimentary body that lies between the slope and the abyssal plain. Rises are typically characterised by sediment accumulations up to several kilometres thick, with strata generally on-lapping the slope sediments and thinning towards the abyssal plain. They typically have low local relief (less than 40 metres) and very gentle seafloor gradients of 0.1 degrees to 0.6 degrees, dipping oceanward to merge into the flat abyssal plain.


A schematic diagram of a margin that is clearly divided into shelf, slope, rise and abyssal plain

Along morphologically complex margins, establishing the extent of natural prolongation of the land mass requires consideration of crustal structure, sediment deposition patterns, plate tectonic history, and other aspects of the evolution of the continental margin. Analysis of geophysical and geological data may, in some areas, help identify the region of the base of the continental slope where the maximum change of sea floor gradient rule can be applied, or show that the outer edge of the continental margin determined by this rule is not a reliable location of the extent of natural prolongation of the land mass. Areas where “evidence to the contrary” might be used to locate the foot of the continental slope are discussed in more detail later in this document.


Finding the foot of the continental slope


There are two steps to finding the foot of the continental slope
1. identify the region of the base of the continental slope
2. determine the location of the foot of the continental slope within that region.

Identification of the region of the base of the continental slope may be on the basis of
• morphological evidence
• morphological evidence supported by geological and geophysical evidence
• geological and geophysical evidence.

The complexity of the margin and availability of data will dictate the choice of evidence to use. In areas where the morphology of the continental margin is clearly divided into slope, rise and abyssal plain, and there is no contradicting geological and geophysical evidence, identification of the region of the base of the continental slope and location of the point of maximum change in gradient within this region are based on morphological evidence and are relatively straightforward.

For many reasons, along some margins the outer edge of the continental margin may not be reliably identified solely on the basis of morphological data. The CLCS Guidelines (6.3.10) state that along some margins the region of the base of the continental slope may be located within the continent-ocean transition zone. The location of the foot of the continental slope within that region can be determined by the maximum change in sea floor gradient rule, or, if this is inappropriate, by locating the inner edge of the continent-ocean transition.


Maximum change in the gradient at the base of the continental slope

In areas where the morphology of the continental margin can be clearly subdivided into shelf, slope, rise and abyssal plain, the region of the base of the continental slope is where the lower continental slope meets the rise, or where it meets the abyssal plain in cases where a rise is absent.

The morphology of the seabed where the continental slope merges with the rise may be an abrupt boundary where gently-dipping rise sediments onlap a smooth, relatively steep slope, or it may be a complex transition where local relief on the ocean floor meets an irregular lower slope. Regional gradients can be used to narrow the search for the region of the base of the continental slope. Regional gradients less than 1 degree are generally considered representative of the rise and abyssal plain, and regional gradients greater than 2 degrees are generally considered representative of the slope. There are many exceptions to these values, however, and other evidence, such as the erosion effects of deep sea currents, underwater slides, margin collapse, local volcanic activity and deep-sea canyons, must be considered before finally establishing the region of the base of the continental slope.

The width of the region of the base of the continental slope is typically 4 – 10 kilometres, but can vary according to the complexity of the margin.

 
A schematic showing identification of the region of the base of the continental slope from the margin morphology
Having established the region of the base of the continental slope, the point of maximum change in gradient is determined by computing the second derivative of the bathymetry within that region. The foot of the continental slope has the maximum value of the second derivative of the bathymetry.


A schematic showing the determination of the foot of the continental slope using the maximum change in gradient rule


Evidence to the contrary and the Continent-Ocean Transition (COT)


Analysis of geophysical and geological data can help identify the region of the base of the slope where the maximum change of sea floor gradient rule can be applied. If these data show that the maximum change of sea floor gradient rule does not reliably locate the edge of the continental margin within that region, then they might be used as “evidence to the contrary” in terms of article 76.

Situations in which the maximum change in sea floor gradient rule might not reliably locate the foot of the continental slope include

• continental margins that have a very smooth transition from slope to rise, and no single point represents a significant maximum change in gradient
• continental margins with irregular seafloor morphology, and a point of maximum change in gradient at its base that does not reliably locate the foot of the continental slope.

For non-volcanic rifted and sheared continental margins, the CLCS Guidelines (6.3.10) state that

 If the foot of the continental slope is very difficult to define on the basis of bathymetric data, the Commission might consider the continental-oceanic transitional (COT) … as the place to determine the outer edge of the continental margin. Since the transitional zone can extend over several tens of kilometres, the Commission may consider the landward limit of the transitional zone as an equivalent of the foot of the continental slope in the context of paragraph 4, provided that the submitted geophysical and geological data conclusively demonstrate that the submerged land mass of the coastal State extends to this point.


Convergent plate boundaries

For active convergent plate boundaries, the Commission Guidelines (6.3)(a) identify either the “seaward edge of the accretionary wedge” or “the foot of the upper plate and … the foot of the inner trench wall” as the seaward extent of the continental margin. The use of these locations is applicable where deep ocean sea floor is being subducted, but is not relevant where the continental landmass occurs on both sides and the plate boundary does not disrupt continental prolongation. In those cases the outer edge of the continental margin lies at the outboard edge of the continental blocks.


The San Andreas Fault in California marks the boundary between the Pacific and North American tectonic plates. It passes out to sea in Bodega Bay north of San Francisco and the Gulf of California in the south. Because continental landmass is on both sides of the plate boundary the fault does not disrupt continental prolongation. (photo from NASA).

Geological and geophysical supporting evidence
 
In morphologically complex margins, where there are several points with maximum change in sea floor gradient, the region of the base of the continental slope might be identified by determining the location of the COT from geological and geophysical data. Even though in many instances the extent of COT may be difficult to identify it is clear from paragraphs 1 and 3 of article 76 that the edge of the continental margin lies within the COT, so the inner and outer edges of the COT can be used to constrain the region of the base of the continental slope. If a point of maximum change in sea floor gradient can be identified in this region, then it establishes the location of the foot of the continental slope. If such a point cannot be identified, then “evidence to the contrary” can be used to establish the location of the foot of the continental slope.
 


A schematic showing the use of geological and geophysical data as supporting evidence to distinguish the region of the base of the continental slope and use of the maximum change in gradient rule to identify the foot of the continental slope
 

The region of the Western Lau Terrace, north of New Zealand is an example of the use of geological and geophysical evidence to identify the region of the base of the continental slope. This part of the New Zealand continental margin is characterised by terraces and sediment-filled basins formed by tilted basement blocks that step down to the deep ocean floor of the South Fiji Basin. The COT lies at the western edge of the Western Lau Terrace. On these profiles the foot of the continental slope is the point with the maximum change in sea floor gradient within the transition zone.

Western Lau Terrace - Geological and geophysical supporting evidence
 

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Seismic sections and a location map (looking south) of the Lau Terrace and Western Lau Terrace. Basement fault blocks are shown by red dashed lines

Evidence to the contrary

If the maximum change in sea floor gradient does not reliably locate the foot of the continental slope, then that position can be located on the basis of geological and geophysical evidence. As mentioned above, on some margins foot of the continental slope positions located at the inner edge of the continent-ocean transition zone may be used to define the outer edge of the continental margin according to the formulae of article 76 paragraph 4(a) (CLCS Guidelines 6.3.10).

In order to use geological and geophysical evidence to locate the foot of the continental slope, the data must clearly demonstrate that the natural prolongation of the landmass extends to that point

Geophysical evidence, such as seismic reflection data, gravity and magnetic data, and geological evidence from dredges or drill-holes, might establish the crustal structure, plate tectonic and geological history, and sediment depositional patterns of a region. Any of this information could help define the natural prolongation of the landmass and the outer edge of the continental margin.

A schematic diagram of a rifted continental margin where geological and geophysical evidence might establish the COT. The CLCS Guidelines state that the landward limit of the transition zone may be used as the foot of the continental slope from which the outer limit of the continental shelf can be established by the formulae of article 76 paragraph 4(a)

Hikurangi Plateau – evidence to the contrary

The Hikurangi Plateau, east of New Zealand, is a large igneous province that sutured to the New Zealand continent in the Cretaceous and now forms the base of much of the continent. It is an example of a continental margin where geological and geophysical evidence may be used to establish the natural prolongation of the landmass and the foot of the continental slope.

Seismic lines across the northern margin of the Chatham Rise and the Hikurangi Plateau show a prominent break in the bathymetric slope along the base of the Chatham Rise. These seismic data, along with gravity models, show that this break does not correspond to the extent of the natural prolongation of the landmass, and therefore is not the foot of the continental slope.

The plateau extends up to 700 kilometres north of the Chatham Rise. The northeastern boundary of the plateau changes along strike from a one kilometre high seafloor escarpment in the west (profile A below) to a buried one kilometre high escarpment in the east (profiles B and C below). The foot of the continental slope is located at the point of the maximum change in seafloor gradient on profile A. On profiles B and C the foot of the continental slope is located at the inner edge of the COT between the Hikurangi Plateau and ocean crust of the Pacific Basin. This boundary is near the outer limit of the natural prolongation of the land mass, about 100 km beyond the point of maximum change in seafloor gradient on profile C.
 
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Seismic sections and location diagram (looking to the southwest) over the Chatham Rise and Hikurangi Plateau. The seismic profiles show basement (pink) and seabed (yellow line). A close-up of profile C at the bottom of the figure shows a maximum change in seafloor gradient at the southern profile limit and an interpretation of the continent-ocean transition at northern edge of the plateau