FOOT OF THE SLOPE DETERMINED BY MEANS OF

FOOT OF THE SLOPE DETERMINED BY MEANS OF EVIDENCE TO THE CONTRARY (CHAPTER 6) Harald Brekke

Foot of the continental slope • ”In the absence of evidence to the contrary, the foot of the continental slope shall be determined as the point of maximum change of gradient at its base” (Paragraph 4 (b)) • The Convention does not prescribe any specific methods to define the foot of the slope when evidence to the contrary is invoked

Application of evidence to the contrary • A provision designed to allow coastal States to use the best geological and geophysical evidence available to them to locate the foot of the continental slope when the geomorphological evidence given by the maximum change in the gradient does not or cannot locate reliably the foot of the continental slope (See 6. 3. 1)

Application of evidence to the contrary (Continued) • The determination of the foot of the continental slope is achieved as a general rule by means of the point of maximum change in the gradient at its base (See 6. 1. 1) • The determination of the foot of the slope by means of evidence to the contrary is regarded by the CLCS as an exception to the general rule (See 6. 1. 2) • If such evidence to the contrary is presented, the CLCS will request that it be accompanied with the results of applying the general rule (See 6. 4. 1)

The continental slope • Some continental margins consist of three elements – the shelf, the slope and the rise – whereas others show no rise • The continental slope forms a portion of the continental margin and extends from the shelf edge to the top of the rise, or to the top of the deep ocean floor where no rise exists (See 6. 2. 1)

Base of the continental slope • The base of the slope is a zone where the lower part of the slope merges into the top of the continental rise or into the top of the deep ocean floor, in the case where no rise exists (See 6. 2. 2) • The foot and the base of the continental slope are inseparable, and commonly lie close to the outer edge of the continent, that is, near the place where the crust changes from continental to oceanic (See 6. 2. 3)

Relevant cases • Where the curvature of the seabed along the base of the continental slope is constant, so that the maximum change in gradient encompasses not only a point, but also a region (See 6. 3. 2. ) • Where the seabed topography reveals a number of local maxima in the change of the gradient at the base of the continental slope, so that it is possible that its maximum maximorum may not be indicative of the location of its foot (See 6. 3. 3)

Continental margins -plate tectonic categories • Convergent continental margins - at active plate boundaries • Divergent (“rifted”) continental margins - at inactive plate boundaries • Transform (“sheared”) continental margins - at both active and inactive plate boundaries

Convergent continental margins PORTRAIT Fig 6. 1. A, B og C om konvergente kontinentalmarginer CLCS/11, Fig. 6. 1

Eksempel på Accretionary convergent margin Evt et kart over hele Stillehavet som viser alle de tre typene, pluss at man får vist forskjellen på Asiaplategresen med øybue-back-arcsystemer og Amerikaplategrnesen som er rensket for slike. Convergent continental margins

Divergent continental margins Fig. 6. 1. D og E CLCS/11, Fig. 6. 1

Divergent continental margins PORTRAIT Divergent margins – Atlantic margins examples

Transform continental margin CLCS/11, Fig. 6. 1

Gulf of Alaska transform continental margin Eks på sheared margin

Combination of different types of plate boundaries

Foot of the continental slope • Any point selected as the foot of the slope must be located within the base of the continental slope described and documented as such (See 6. 2. 3) • Any point selected as the foot of the slope must be located inside the geological continental margin (See 6. 3. 5)

Convergent continental margins • The outer edge of the continental margin is defined by either: – The seaward edge of the accretionary prism – The foot of the upper plate – The foot of the inner trench wall • Data required: – Modern multi-channel seismic data – Modern bathymetric data (See 6. 3. 6, 6. 3. 7)

Rifted (non-volcanic) and sheared continental margins • The outer edge of the continental margin is defined by the transition between continental and oceanic crust • The landward limit of this transition zone may be considered as an equivalent to the foot of the slope • Data required: – Modern multi-channel reflection seismic data – Modern refraction/wide angle reflection data – Magnetic and gravimetric data (See 6. 3. 8 -10)

Rifted volcanic continental margins • The outer edge of the continental margin is defined by the transition zone, where thickness of the igneous continental crust decreases to normal oceanic crust thickness, i. e. less than 15 kilometres • The landward limit of this transition zone may be considered as an equivalent to the foot of the slope • Data required: – Modern multi-channel reflection seismic data – Modern refraction/wide angle reflection data (See 6. 3. 11)