[incorporates CLCS/11/Corr.1]

SCIENTIFIC AND TECHNICAL GUIDELINES OF THE COMMISSION ON
THE LIMITS OF THE CONTINENTAL SHELF

Adopted by the Commission on 13 May 1999
at its fifth session

CONTENTS

PREFACE

1.  Introduction

2.  Entitlement to an extended continental shelf and the delineation of its outer limits

2.1  Formulation of the problem:  article 76
2.2  Test of appurtenance
2.3  Delineation of the outer limits of the continental shelf

3.  Geodetic methodologies and the outer limits of the continental shelf

3.1  Formulation of the problem:  paragraphs 1, 4, 5 and 7
3.2  Units, geodetic reference systems and coordinate transformations
3.3  Geodetic definition of baselines
3.4  Outer limits and their confidence zones

4.    The 2,500 metre isobath

4.1   Formulation of the problem:  paragraph 5
4.2   Sources of data and hydrographic measurements
4.3   Bathymetric model
4.4   Selection of points for the delineation of the 100 M limit

5.    Foot of the continental slope determined as the point of maximum change in the gradient at its base

5.1   Formulation of the problem:  paragraph 4
5.2   Sources of data
5.3   Filtering and smoothing
5.4   Delineation of the foot of the continental slope

6.     Foot of the continental slope determined by means of evidence to the contrary to the general rule

6.1   Formulation of the problem:  paragraph 4 (b)
6.2   Geological and geophysical evidence
6.3   Determination of the foot of the continental slope
6.4   Considerations to be given with respect to evidence to the contrary

7.     Ridges

7.1   Formulation of the problem:  paragraphs 3 and 6
7.2   Oceanic ridges and submarine ridges
7.3   Submarine elevations

8.     Delineation of the outer limits of the continental shelf based on sediment thickness

8.1   Formulation of the problem:  paragraph 4 (a) (i)
8.2   Relevant geophysical techniques and data
8.3   Depth conversion and thickness determination
8.4   Sources and magnitudes of error
8.5   Selection of outermost fixed points of 1 per cent sediment thickness

9.     Information on the limits of the extended continental shelf

9.1   Formulation of the problem:  paragraph 8 and Annex II
9.2   Bathymetric and geodetic data
9.3   Geophysical and geological data
9.4   Digital and non-digital data
9.5   Checklist of relevant supporting information and data

10.   References and bibliography

Annex.  List of international organizations

NOTE:

The graphic files in this document have been saved as "thumbnails" to facilitate downloading of the document.  "Click" on the small image to expand it in a new browser window.  It is suggested that you print the figure from that new window.

All figures can be accessed from this list: Fig.2.1, Fig.2.2, Fig.2.3, Fig.2.4, Fig.2.5, Fig.2.6, Fig.2.7, Fig.2.8, Fig.3.1, Fig.6.1, Fig.6.2, Fig.8.1, Fig.8.2 and Fig.8.3

Preface

The preparation of the Scientific and Technical Guidelines of the Commission on the Limits of the Continental Shelf was conducted in two stages.  The first stage consisted of background research conducted along disciplinary and interdisciplinary lines.  The Commission organized six research groups for this purpose, established at its second session in September 1997:

(a)       Hydrography (Srinivasan, Chairman; Albuquerque, Astiz, Awosika, Carrera, Francis and Lamont, with Rio as an alternate);

(b)       Geodesy (Carrera, Chairman; Albuquerque, Astiz, Brekke, Francis, Hamuro, Jaafar, Mdala and Srinivasan, with Rio as an alternate);

(c)       Geology (Park, Chairman; Betah, Brekke, Hamuro, Juracic, Kazmin, Lu, Mdala and Srinivasan, with Carrera as an alternate);

(d)       Geophysics (Croker, Chairman; Awosika, Carrera, Hinz, Lu, Mdala and Park, with Francis as an alternate);

(e)       Foot of the continental slope (Rio, Chairman; Carrera, Francis, Hamuro, Kazmin, Lamont and Srinivasan);

(f)        Outer edge of the continental margin (Brekke, Chairman; Albuquerque, Astiz, Betah, Carrera, Croker, Hamuro, Juracic, Kazmin, Lu, Mdala and Park).

The second stage consisted of the preparation of draft Guidelines, which began at the third session of the Commission, held at United Nations Headquarters in New York from 4 to 15 May 1998.  An Editorial Committee was established at the session and Galo Carrera was elected as its Chairman.  The Editorial Committee considered and adopted the document structure for the Guidelines proposed by its Chairman.

The Editorial Committee was organized into 13 working groups, whose Chairmen reported to the Chairman of the Editorial Committee, as follows:

(1)       Introduction (Carrera, Chairman; Editorial Committee);

(2)       Entitlement to and delineation of the outer limits of the continental shelf (Carrera, Chairman; Albuquerque, Brekke, Hamuro, Hinz, Lamont and Rio);

(3)       Geodetic methodologies and the outer limits of the continental shelf (Carrera, Chairman; Albuquerque, Astiz, Francis, Hamuro, Jaafar, Mdala, Rio and Srinivasan);

(4)       The 2,500 metre isobath (Lamont, Chairman; Albuquerque, Astiz, Awosika, Carrera, Francis, Hinz, Kazmin, Rio and Srinivasan);

(5)       Foot of the continental slope determined as the point of maximum change in the gradient at its base (Rio, Chairman; Albuquerque, Astiz, Carrera, Croker, Francis, Hamuro, Kazmin and Lamont);

(6)       Foot of the continental slope determined by means of evidence to the contrary (Hinz, Chairman; Betah, Brekke, Carrera, Jaafar, Juracic, Kazmin and Park);

(7)       Ridges (Hamuro, Chairman; Brekke, Hinz, Juracic, Kazmin, Lu and Park);

(8)       Delineation of the outer limits of the continental shelf based on sediment thickness (Brekke, Chairman; Awosika, Croker, Juracic and Park);

(9)       Information on the outer limits of the extended continental shelf (Albuquerque, Chairman; Brekke, Carrera, Hamuro, Hinz, Lamont and Rio);

(10)       References and bibliography (Carrera, Chairman; Editorial Committee);

(11)       List of international organizations (Carrera, Chairman; Editorial Committee);

(12)       Flowcharts, tables and illustrations summarizing the procedure for establishing the outer limits of the continental shelf (Jaafar,  Chairman; Carrera, Chan Chim Yuk, Juracic, Lamont, Rio);

(13)       Oversight (Awosika, Chairman; Astiz, Beltagy, Betah, Chan Chim Yuk and Hamuro).

The Editorial Committee assigned to the first 12 working groups the task of preparing 10 chapters and 2 annexes.  The Oversight Working Group was entrusted with two assignments:  it was first asked to identify the totality of issues raised in the studies prepared by the Division for Ocean Affairs and the Law of the Sea on the basis of discussions held during two meetings of groups of experts in 1993 and 1995.  Secondly, it was requested to determine whether those issues were addressed in the Guidelines.  The 12 drafting groups produced a preliminary outline draft of the Guidelines, which was discussed during the last plenary meeting of the Editorial Committee held during the third session of the Commission.

All working groups conducted their main drafting efforts during the inter-session period of 1998.  On 20 July 1998, the revised version of the draft Guidelines was submitted to the Chairman of the Editorial Committee, who proceeded to edit them for consistency in content and style.

The Editorial Committee reconvened at the fourth session of the Commission, held at United Nations Headquarters from 31 August to 4 September 1998.  The draft of the Guidelines edited by the Chairman of the Editorial Committee was discussed at various plenary meetings of the Editorial Committee, where amendments and clarifications were introduced in an iterative revision process.  The oversight group then proceeded to prepare and submit an interim report based on the final draft produced by the Editorial Committee at that session.

The Chairman of the Editorial Committee submitted the final draft Guidelines to the Commission at large for consideration during the last meeting of its fourth session.  The Commission, in turn, considered them and agreed to adopt them provisionally.  The Commission also agreed to make them available to States on 4 September 1998 as a document in the "L" (limited distribution) series.

The Commission worked during the inter-session period 1998-1999 with a view to considering the recommendations made in the interim report prepared by the Oversight Working Group at its 4th meeting.  The members of the Commission also considered other issues on which consensus was not reached, which were left open for further discussion at its 5th meeting.  Editorial comments on the English text of the Guidelines were produced during this inter-session period by Albuquerque, Astiz, Brekke, Carrera, Chan Chim Yuk, Croker, Lamont, Lu and Srinivasan.

The following members revised the translation of the Guidelines from English into other official languages of the United Nations:  Arabic translation (Beltagy); Chinese translation (Lu); French translation (Albuquerque, Betah, Chan Chim Yuk and Rio); Russian translation (Kazmin); and Spanish translation (Albuquerque, Astiz and Carrera).

The Guidelines were discussed and amended at the fifth session of the Commission and adopted on 13 May 1999.

The drafting of the Scientific and Technical Guidelines of the Commission on the Limits of the Continental Shelf during a relatively short period of time represents an important achievement towards the implementation of article 76 of the United Nations Convention on the Law of the Sea.

The Guidelines, which the Commission adopted by consensus, serve multiple purposes:  they are primarily intended to assist coastal States in preparing their submissions.  They are also designed to provide an important scientific and technical reference for the consideration of these submissions and the preparation of the Commission's own recommendations.  And last but not least, they form the basis on which the Commission shall provide advice, if requested by coastal States during the preparation of their necessary data.

The members of the Commission have an obligation to perform their duties honourably, faithfully, impartially and conscientiously.  These principles, which form the essence of their solemn declaration, have guided them in the preparation of their Scientific and Technical Guidelines.

The Commission expresses its gratitude to the Division for Ocean Affairs and the Law of the Sea under the leadership of Mr. Ismat Steiner, Director.  Special thanks goes to the Secretary of the Commission, Mr. Alexei Zinchenko, and to Lynette Cunningham, Vladimir Jares, Cynthia Hardeman and Josefa Velasco,  who ably assisted in the preparation of the Guidelines and in their expedient publication.

1.       Introduction

1.1.      The Commission on the Limits of the Continental Shelf recognizes the integral character of the United Nations Convention on the Law of the Sea (the Convention).  These Scientific and Technical Guidelines form the basis for the Commission to make its recommendations with respect to submissions prepared by States according to article 76 and Annex II to the Convention in a manner that is consistent with the Convention and international law.

1.2.      The Commission prepared these Guidelines for the purpose of providing direction to coastal States which intend to submit data and other material concerning the outer limits of the continental shelf in areas where those limits extend beyond 200 nautical miles from the baselines from which the breadth of the territorial sea is measured.  The Guidelines aim to clarify the scope and depth of admissible scientific and technical evidence to be examined by the Commission during its consideration of each submission for the purpose of making recommendations.

1.3.      With these Guidelines, the Commission aims also to clarify its interpretation of scientific, technical and legal terms contained in the Convention.  Clarification is required in particular because the Convention makes use of scientific terms in a legal context which at times departs significantly from accepted scientific definitions and terminology.  In other cases, clarification is required because various terms in the Convention might be left open to several possible and equally acceptable interpretations.  It is also possible that it may not have been felt necessary at the time of the Third United Nations Conference on the Law of the Sea to determine the precise definition of various scientific and technical terms.  In still other cases, the need for clarification arises as a result of the complexity of several provisions and the potential scientific and technical difficulties which might be encountered by States in making a single and unequivocal interpretation of each of them.

1.4.      The Commission designed these Guidelines with a view to ensuring a uniform and extended State practice during the preparation of scientific and technical evidence submitted by coastal States.  The Commission is aware that there might be other scientific and technical methodologies used by States to implement the provisions of article 76 to prepare a submission which may not be covered in this document.  These Guidelines are not intended to exhaust the full range of possible methodologies contemplated by States.  Whereas several scientific and technical avenues are available to develop an admissible body of evidence which may conform equally to all the relevant provisions contained in the Convention, the Commission has endeavoured to emphasize those which might minimize costs and result in the optimization of existing information and resources.

1.5.      The scientific nature and the order of the paragraphs in article 76 define the structure of the Guidelines.  Each chapter starts with a formulation of the problem posed by each of its provisions, followed by an in-depth discussion of its implementation.  Chapter 2 presents an overview of questions relating to the entitlement to an extended continental shelf and the delineation of its outer limits.  Chapter 3 reviews units of length and describes the geodetic methodology used to determine outer limits based on metrics.  Chapter 4 describes the hydrographic methodology used to determine the 2,500 metre isobath and other geomorphologic features.  Chapter 5 discusses the determination of the location of the foot of the continental slope as the point of maximum change in gradient at its base.  Chapter 6 examines the case in which evidence to the contrary might be presented as an alternative to the methodology described in chapter 5 to determine the location of the foot of the continental slope.  Chapter 7 discusses the classification and treatment of oceanic and submarine ridges, and other submarine elevations.  Chapter 8 discusses the geophysical methodology applied for the determination of sediment thickness and its error estimates.  Chapter 9 describes the data and other material to be included in a submission regarding the outer limits of the continental shelf.

1.6.      The Commission recognizes that the Convention poses in-depth requirements in several scientific disciplines and also poses the need for interdisciplinary scientific and technical cooperation for the preparation of data and materials in each submission.  These Guidelines are not aimed at describing in detail the scientific theories or precise technical methodologies involved in each discipline.  For that purpose, experts assigned to the preparation of submissions are advised to consult the contributions made by many scientific and technical, governmental and non-governmental organizations and disseminated through journals, conference proceedings and other publications.

1.7.      The annex provides a non-exhaustive list of international scientific and technical organizations whose data and information might be of interest to States which intend to prepare a submission.  Whereas those international organizations have the primary responsibility to promote the development of knowledge and research in their respective disciplines, the Commission has the sole responsibility to make recommendations and to provide scientific and technical advice in relation to submissions on the limits of extended continental shelves made by coastal States according to article 76 and Annex II to the United Nations Convention on the Law of the Sea.

2.    Entitlement to an extended continental shelf and the delineation of its outer limits

2.1.    Formulation of the problem:  article 76
2.2.    Test of appurtenance
2.3.    Delineation of the outer limits of the continental shelf

2.1.  Formulation of the problem:  article 76

3.1.  Formulation of the problem:  paragraphs 1, 4, 5 and 7

3.3.5. The Commission feels that there is a uniform and extended State practice which justifies the acceptance of multiple interpretations of the low water line. All of them are regarded as equally valid in a submission.

3.3.6. The Commission is aware that there are different chart datum transfer techniques designed to provide the location of the low water line at sites along the coastline other than at tide gauge sites. The Commission may require background technical information in relation to the methodology used by coastal States for this purpose in each submission.

3.3.7. Articles 7, 9, 10 and 47 entitle States to delineate straight, closing and archipelagic baselines. The Convention does not specify the geodetic definition of these baselines. In the case of straight baselines delineated in accordance with the provisions of article 7, at least two definitions, loxodromes and ellipsoidal geodesics, have been adopted in State practice (United Nations, 1989).

3.3.8. In accordance with established State practice, the Commission shall accept the definition of straight, closing and archipelagic baselines as either geodesics or loxodromes. However, only one line definition can be consistently selected by a submitting State for all of its baselines. In the case of loxodromes, the Commission shall use the definition of a line of a constant azimuth on the surface of a geodetic reference ellipsoid (Bowring, 1985). The Commission strongly discourages the use of apparent straight lines as literally drawn on various paper nautical charts employing a variety of map projections.

3.3.9. The Commission remains open to consider all forms and combinations of methods used to determine the position of baselines by a State in a submission. The Commission may request during the consideration of a submission the following geodetic information about baselines:

• Source of the data;
• Positioning survey technique;
• Time and date of the survey;
• Corrections applied to the data;
• A priori or a posteriori estimates of random and systematic errors;
• Geodetic reference system; and
• Geometric definition of straight, archipelagic and closing lines.

3.4.1. The delineation of the outer limits of the extended continental shelf according to article 76 requires the determination of up to four outer limits delineated:

• By reference to the outermost fixed points at each of 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;
• At a distance of 60 M from the foot of the slope;
• At a distance of 350 M from baselines from which the breadth of the territorial sea is measured; and
• At a distance of 100 M from the 2,500 m isobath.

3.4.2. Boggs (1930) originally defined the technique used to determine the outer limits of the territorial sea from a selection of points along baselines as the method of envelopes of arcs. It was first introduced as a proposal for codification in international law by the United States delegation at the Hague Codification Conference in 1930. This method provides an outer limit every point of which is located at a prescribed distance from the nearest point on the coast. Shalowitz (1962, p. 171) has advanced a more refined definition of this method, where the outer limit:

"is the locus of the centre of a circle the circumference of which is always in contact with the coastline, that is, with the low water line or the seaward limits of inland waters."

3.4.3. The application of the method of envelopes of arcs is independent of the actual breadth of the limit. Thus, although the method was originally designed as a tool to determine the outer limit of the territorial sea, its mathematical application remains equally valid to determine the outer limit of other maritime spaces based on metric criteria.

3.4.4. The Commission regards the application of the method of envelopes of arcs on the surface of the geodetic reference ellipsoid in a submission as an admissible methodology to determine outer limits based on distances from the nearest points located on baselines, the 2,500 m isobath and the foot of the continental slope. The actual implementation of this method is carried out by solving iteratively a system of linearized distance equations in a resection mathematical model. An exhaustive combinatorial search algorithm is recommended in order to ensure that all possible combinations of pairs of points are analysed and that the nearest points are the ones which generate the outer limit.

3.4.5. The distance resection model described above relies on the solution of the direct and inverse positioning problems formulated in geodesy. A large number of solutions to these classic problems have been developed over the last two centuries. They can be broadly classified into three groups, based upon: the integration of differential equations; transferring an ellipsoid polar triangle to a concentric sphere; and using a conformal projection from the ellipsoid to the sphere (Schnadelbach, 1974). The Commission does not have a preference for the use of a particular solution and it is aware that the correct application of several of them must produce identical results.

3.4.6. The Commission acknowledges the existence of the method of tracés parallèles to determine the outer limits of maritime spaces from straight baselines as prescribed by the International Court of Justice in the Anglo-Norwegian Fisheries case of 1951. This method is a generalization of the method of envelopes of arcs for the cases of continuous straight, closing and archipelagic baselines.

3.4.7. The Commission regards the application of the method of tracés parallèles on the surface of the geodetic reference ellipsoid used in each submission as an admissible methodology to determine outer limits at distances of 200 M and 350 M from the nearest points located on straight, closing and archipelagic baselines from which the breadth of the territorial sea is measured.

3.4.8. The mathematical model for the determination of outer limits from straight baselines on the surface of a geodetic reference ellipsoid is computationally more intensive than the method of envelopes of arcs. It involves the successive application of the direct and inverse problems described above over a large series of discrete points along straight, closing and archipelagic baselines.

3.4.9. For the sake of simplicity, the two formulations described above have not made any reference to the introduction of a priori statistical information about the position of the baselines. In practice, this information must be incorporated in order to derive the confidence region associated with the limit (Sjoberg, 1996). It is clear, however, that the derived offshore limit will never surpass the accuracy of the positions of the baselines themselves, and therefore those States aiming to achieve the highest standards of accuracy in the determination of their outer limits should focus first on the accuracy of their baselines.

3.4.10. The Commission strongly discourages the application of the methods of envelopes of arcs and tracés parallèles through the use of manual graphical procedures on the surface of paper nautical charts. The distortions produced by inherent scale factors in map projections and the inapplicability of Euclidean geometry principles on the surface of a geodetic ellipsoid rule out the admissibility of this manual methodology.

3.4.11. The Commission highlights three observations made by Gidel (1932, p. 510) more than half a century ago: first, that there is no parallelism between the coast and the limit; secondly, that the outer limit is simpler than the normal baseline; and, most importantly, that only a few points contribute to the delineation of the outer limit. There may be no need to submit the data on the full extent of the coastline, a full 2,500 m isobath or the continuous foot of the slope. Only the most seaward points which effectively contribute to the delineation of the outer limit need to be supported.

Figure 3.1

4.1. Formulation of the problem: paragraph 5
4.2. Sources of data and hydrographic measurements
4.3. Bathymetric model
4.4. Selection of points for the delineation of the 100 M limit

4.1.1. The Commission recognizes that the 2,500 m isobath is an essential feature for the implementation of article 76. It serves as the basis for the application of one of the constraint rules to the formulae lines in order to produce the outer limits of the continental shelf. According to paragraph 5, it is the reference baseline from which the 100 M line is measured:

"The fixed points comprising the line of the outer limits of the continental shelf on the sea-bed, drawn in accordance with paragraph 4 (a) (i) and (ii), either shall not exceed 350 nautical miles from the baselines from which the breadth of the territorial sea is measured or shall not exceed 100 nautical miles from the 2,500 metre isobath, which is a line connecting the depth of 2,500 metres."

4.1.2. A line determined at a distance of 100 M from the 2,500 metre isobath may not be used in the delineation of the outer limits of the extended continental shelf in the special case of submarine ridges. Paragraph 6 makes an exception of submarine elevations where it is required:

"Notwithstanding the provisions of paragraph 5, on submarine ridges, the outer limit of the continental shelf shall not exceed 350 nautical miles from the baselines from which the breadth of the territorial sea is measured. This paragraph does not apply to submarine elevations that are natural components of the continental margin, such as its plateaux, rises, caps, banks and spurs."

4.2.1. The complete bathymetric database used in the delineation of the 2,500 m isobath in a submission may only include a combination of the following data:

• Single-beam echo sounding measurements;
• Multi-beam echo sounding measurements;
• Bathymetric side-scan sonar measurements;
• Interferometric side-scan sonar measurements; and
• Seismic reflection-derived bathymetric measurements.

4.2.2. The Commission will consider single- and multi-beam echo sounding measurements as the primary source of evidence for the delineation of the 2,500 m isobath. All other admissible evidence provided by bathymetric and interferometric side-scan sonar measurements and seismic reflection-derived bathymetric measurements will be regarded as complementary information in general.

4.2.3. However, bathymetric information derived from seismic reflection and interferometric side-scan sonar measurements may be considered as the primary source in a submission for the purpose of delineating the 2,500 m isobath in special cases such as in ice-covered areas. The Commission may pay particular attention to the calibration and corrections applied to these data.

4.2.4. Bathymetric side-scan sonars are hybrid measuring systems, which collect both estimates of sea-floor slope and bathymetry. Whereas their sea-floor slope information might be relevant in other parts of a submission, potentially for the delineation of the foot of the slope, only their bathymetric component will be considered for the purpose of delineating the 2,500 m isobath.

4.2.5. The bathymetric data produced by light-detection-and-ranging (LIDAR) airborne systems may be particularly valuable to provide bathymetric information for the shallow regions of the seabed included in a submission. However, laser (light amplification by stimulated emission of radiation) profiling is clearly inapplicable to the delineation of the 2,500 m isobath, or the seabed region associated with the base of the continental slope.

4.2.6. Other sources of evidence, such as satellite altimetry-derived bathymetric data or imaging side-scan sonar information, will not be regarded as admissible for the purpose of delineating the 2,500 m isobath. This information, however, might be useful as additional qualitative information in support of other parts of a submission but will not be considered during the determination of this or any other isobaths. However, this data will be considered admissible as supporting information in a submission.

4.2.7. A full technical description of the bathymetric database used in the delineation of the 2,500 metre isobath will include the following information:

• Source of the data;
• Sounding survey techniques and classification;
• Geodetic reference system, navigational positioning methods and their errors;
• Time and date of the survey;
• Corrections applied to the data, such as ray path sound velocity, calibration, tides and other; and
• A priori or a posteriori estimates of random and systematic errors.

4.2.8. A priori depth error estimates, s, may be computed by means of the following internationally accepted formulae:

where:

with a 95 per cent confidence interval (IHO, 1998).

4.2.9. A posteriori errors may be obtained from the estimated covariance matrix of the estimated depth parameters, which results from an adjustment of an overdetermined system of linear equations formed by cross-over analysis of sounding lines (cf. Vanícek and Krakiwsky, 1982, p. 213).

4.2.10. Coastal States may use the a posteriori error estimation method where there is redundant information in order to assess the quality of historical bathymetric data whose positioning, survey technique and technical description are not available.

4.3.1. The submission will include the necessary cartographic products derived from the compiled bathymetric database to portray the 2,500 metre isobath. These cartographic products may include the following analytic or digital forms:

• Two-dimensional bathymetric profiles;
• Three-dimensional bathymetric models;
• Nautical charts and maps with contours.

4.3.2. Each cartographic product, including nautical charts officially recognized by the State, will be accompanied by a detailed description of the mathematical methodology and data used to produce it. The Commission will pay particular attention to the transit from numerical soundings to analytical functions.

4.3.3. The coastal State will be required to document the following information:

• Interpolation or approximation method;
• Density of measured bathymetric data;
• Perceptual elements such as map projections, vertical and horizontal scales, contour intervals, units, colours and symbols.

4.3.4. Wherever the bathymetric information presented to the Commission may be a filtered or smoothed subset of the original data, a full description of the methodology employed to produce it will be reported by the coastal State.

4.3.5. Full bathymetric models in three dimensions may be required in order to gain a spatial understanding of natural prolongation and may be essential in selecting the full extent of the 2,500 m isobath relevant to the determination of the 100 M outer limit.

4.3.6. The Commission is aware that the seabed can exhibit fractal properties in two and three dimensions (Mandlebrot, 1977). It is also aware that the generation of an analytical model, be it that described by means of contours on a chart or a mathematical expression, results in the generalization of line and surface features at various scales (Fox and Hayes, 1985). The Commission may require geostatistical, fractal, wavelet or other tests and analyses, as it feels appropriate, in order to determine the degree of uncertainty underlying a particular bathymetric model.

4.3.7. The Commission is aware that issues relating to scale, colour, type and others fall within the realm of perception. These issues will be taken into account in order to assess correctly the perception of important technical details.

4.4.1. The line determined at a distance of 100 M from the 2,500 metre isobath becomes effective as a constraint over the outer limits of the continental shelf wherever this isobath is located at a distance of 250 M or greater from the baselines from which the territorial sea is measured.

4.4.2. The selection of the most salient points along the 2,500 m isobath for the purpose of delineating the 100 M limit may be straightforward when isobaths are simple. However, when isobaths are complex or repeated in multiples, the selection of points along the 2,500 m isobath becomes difficult. Such situations arise as a result of geological and tectonic processes shaping the present continental margins. They can create multiple repetitions of the 2,500 m isobath, for example, by faulting, folding and thrusting along continental margins. Unless there is evidence to the contrary, the Commission may recommend the use of the first 2,500 m isobath from the baselines from which the breadth of the territorial sea is measured that conforms to the general configuration of the continental margin.

5.1. Formulation of the problem: paragraph 4
5.2. Sources of data
5.3. Filtering and smoothing
5.4. Delineation of the foot of the continental slope

5.1.1. The Commission recognizes that the foot of the continental slope is an essential feature that serves as the basis for entitlement to the extended continental shelf and the delineation of its outer limits. According to paragraph 4 (a) (i) and (ii), it is the reference baseline from which the breadths of the limits specified by formulae rules are measured:

"(i) a line delineated in accordance with paragraph 7 by reference to the outermost fixed points at each of 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."

5.1.2. Paragraph 4 (b) provides a dual regime for the determination of the 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 in the gradient at its base."

5.1.3. The Commission interprets the determination of the foot of the continental slope by means of the point of maximum change in gradient at its base, as a provision with the character of a general rule. The fundamental requirements posed by this provision are:

• The identification of the region defined as the base of the continental slope; and
• The determination of the location of the point of maximum change in the gradient at the base of the continental slope.

5.1.4. Its implementation will be guided by bathymetric, geomorphologic, geologic and geophysical sources of evidence.

5.2.1. Bathymetric and geological data provide the evidence to be used in the geomorphological analysis conducted to identify the region defined as the base of the continental slope. Only bathymetric information will be used to determine the location of the point of maximum change in the gradient at the base of the continental slope.

5.2.2. The bathymetric database used in the delineation of the foot of the slope in a submission may include only one, or a combination of the following data:

• Single-beam echo sounding measurements;
• Multi-beam echo sounding measurements;
• Hybrid side-scan sonar measurements;
• Interferometric side-scan sonar measurements; and
• Seismic reflection-derived bathymetric measurements.

5.2.3. The Commission will require a full technical description of the bathymetric database used in the implementation of this provision. It will also determine the relative value from each of these sources of data in a manner that is consistent with that applied to the determination of the 2,500 metre isobath (refer to section 4.2).

5.2.4. The Commission will also consider as admissible evidence synthetic bathymetric data produced in the form of grids and profiles derived from cartographic and analog sources officially recognized by the coastal State. These cartographic and analog sources may only be based, in turn, on a combination of the bathymetric measurements listed above. Synthetic bathymetric data will be accompanied by a detailed and complete technical description of the method applied and the bathymetric measurements used to produce the cartographic and analog sources from which it stems.

5.2.5. The coastal State will be required to document the following information about cartographic and analog sources:

• Interpolation or approximation methods;
• Spatial density and position of measured bathymetric data;
• Information on perceptual elements such as map projections, vertical and horizontal scales, contour intervals, units, colours and symbols.

5.2.6. The geological and geophysical database used in the identification of the region defined as the base of the foot of the continental slope in a submission may include a combination of the following sources of data:

• In situ samples and measurements;
• Geochemical and radiometric data;
• Geophysical measurements; and
• Side-scan imagery.

5.2.7. Evidence collected as in situ core samples will be accompanied by a complete technical description and their catalogue information will also be enclosed. In situ measurements may include any borehole or ocean-bottom geophysical measurements and their technical description.

5.2.8. Evidence collected in the form of geochemical and radiometric data will also be accompanied by a full technical description and their catalogue information will be enclosed.

5.2.9. Evidence collected in the form of geophysical measurements includes the full range of geophysical methods, including, but not limited to, seismic, gravity, magnetic, palaeomagnetic and side-scan sonar imagery data.

5.3.1. The Commission recognizes that filtering and smoothing of bathymetric data might be required in order to facilitate the identification of the location of the foot of the continental slope at the point of maximum change in the gradient at its base. This procedure might be required in some instances because the use of second derivatives of the bathymetric surface produces an enhancement of all features which may obscure the exact location of the foot of the slope.

5.3.2. Filtering in signal theory presupposes a clear differentiation between signal and noise, that is, what is to be regarded as wanted and unwanted information. In the context of the application of paragraph 4 (b), the shelf, the slope and the rise are signal. Any other information that obstructs the location of these features is regarded as noise.

5.3.3. The Commission is aware that the application of some filtering procedures presupposes the use of regularly spaced data. Bathymetric data are seldom collected in the field at equally spaced intervals. In these instances, a coastal State might produce a regularly spaced data set from irregularly spaced data. The Commission is aware that there are many approaches to performing this operation. It will pay close attention to the methodology employed to produce a regularly spaced data set, and it might request the original irregularly spaced data, details about the mathematical technique employed and the output comprised of regularly spaced data.

5.3.4. The Commission is aware that filter design is a wide field and that the frequency response functions of various filters can be very different even if they are designed to cut off information at specific thresholds. The Commission will pay special attention to the admittance function of the filters used in the wavelength or wave number domain which might be applied to two-dimensional bathymetric profiles and three-dimensional bathymetric surfaces.

5.3.5. The Commission shall not accept the artificial amplification or enhancement of any information at wavelengths at which the bathymetric information can be decomposed. Only the removal of unwanted noise at wavelengths shorter than those relevant to the description of the shelf, the slope and the rise will be regarded as admissible. The Commission might request full disclosure of the original unfiltered information, the mathematical details of the filter and the filtered data produced when filtering is applied.

5.3.6. Smoothing is an empirical procedure which also might have an important role to play in facilitating the identification of the main features of the continental margin. It might have a particularly useful application when other bathymetric features might have similar wavelengths to those which define the location of the foot of the continental slope.

5.3.7. The Commission is aware that the full array of empirical data smoothing techniques is vast. It remains open to considering the application of any smoothing technique, but it will examine closely the proper application of each one in this context. The Commission might request full disclosure of the original data, the mathematical details of the smoothing algorithm and the output data.

5.4.1. The methodology used to determine the foot of the continental slope by means of the point of maximum change in gradient at its base can be regarded also as a two- or three-dimensional problem. This mathematical methodology has some similarities to the second derivative technique employed in the enhancement of potential field maps produced routinely in gravity and magnetic geophysical prospecting. The Commission recognizes the usefulness and complementarity of the use of both two- and three-dimensional approaches.

5.4.2. The Commission is aware of the large number of techniques and methods available for sea-floor classification and roughness analyses (e.g., Fox and Hayes, 1985; Stewart et al. 1992; and Herzfeld, 1993). Many methods based on, for example, fractal and geostatistical analysis have been developed.

5.4.3. The Commission will not prescribe the use of a single mathematical methodology based on bathymetric data for the identification of the region defined as base of the continental slope. It will make recommendations based on the mathematical methodology applied on a case-by-case basis, and in view of all other geological and geophysical evidence presented by the coastal State.

5.4.4. For the purpose of identifying the region defined as the base, the Commission defines the continental slope as the outer portion of the continental margin that extends from the shelf edge to the upper part of the rise or to the deep ocean floor where a rise is not developed. The rise, in turn, is the wedge-shaped sedimentary body having a smaller gradient than the continental slope. Many continental margins, however, depart from this ideal picture (see chap. 6, sect. 6.2, and figs. 6.1A-6.1F), and in such cases geological and geophysical data may be used to assist in identifying the region referred to here as the base of the continental slope.

5.4.5. The Commission defines the base of the continental slope as a region where the lower part of the slope merges into the top of the continental rise, or into the top of the deep ocean floor where a continental rise does not exist. The Commission recommends that the search for the base of the continental slope be carried out by means of a two-step approach. First, the search for its seaward edge should start from the rise, or from the deep ocean floor where a rise is not developed, in a direction towards the continental slope. Secondly, the search for its landward edge should start from the lower part of the slope in the direction of the continental rise, or the deep ocean floor where a rise is not developed.

5.4.6. As a general rule, whenever the base of the continental slope can be clearly determined on the basis of morphological and bathymetric evidence, the Commission recommends the application of that evidence. Geological and geophysical data can also be submitted by coastal States to supplement proof that the base of the continental slope is found at that location.

5.4.7. The determination of the location of the point of maximum change in the gradient at the base of the continental slope will be conducted by means of the mathematical analyses of two-dimensional profiles, three-dimensional bathymetric models and preferably both. Methods based on a purely visual perception of bathymetric data will not be accepted by the Commission.

5.4.8. The determination of the location of the point of maximum change in the gradient was envisaged originally by its proponent as a two-dimensional problem based on the mathematical analyses of two-dimensional bathymetric profiles (Hedberg, 1976). This methodology is acceptable to the Commission with the provision that their three-dimensional location on a bathymetric map or nautical chart is provided at all times. The Commission recommends that the orientation of this profile be such that it runs in a perpendicular direction to the isobaths located at the point of maximum change in the gradient at the base of the continental slope.

5.4.9. The Commission is aware that several three-dimensional techniques have been designed in the past to produce a continuous trace of the foot of the continental slope. These techniques are based on the determination of the total curvature surface (Vanícek and Ou, 1996), the second derivative surface in the direction of the gradient (Bennet, 1996) and other second derivative-based analyses.

5.4.10. The Commission is also aware that the application of different two-dimensional and three-dimensional methodologies might yield different results in a given submission using the same data set, but it is ready to consider the application of one or several of them. In these cases, the Commission might perform comparative analyses of the results obtained by the application of two-dimensional methods, three-dimensional methods or both.

5.4.11. The Commission will request a complete technical description of the original three-dimensional bathymetric model, the details of the mathematical methodology and the output surface and the point or line defining the foot of the continental slope.

5.4.12. Where more than a single change in the gradient is located at the base of the continental slope, the Commission recognizes as a general rule the selection of the point of maximum change in the gradient as the method to identify the location of the foot of the continental slope. The selection of any other local change in the gradient at its base, i.e., any change other than the maximum, will be regarded by the Commission as an exception. The justification for the application of this exception will require the presentation of evidence to the contrary to the general rule, as described in the following chapter.

6. Foot of the continental slope determined by means of evidence to the contrary to the general rule

6.1. Formulation of the problem: paragraph 4 (b)
6.2. Geological and geophysical evidence
6.3. Determination of the foot of the continental slope
6.4. Considerations to be given with respect to evidence to the contrary

6.1.1. The Commission recognizes that 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. However, article 76, paragraph 4 (b), also incorporates a possible exception when evidence to the contrary of this general rule might be submitted by a coastal State:

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

6.1.2. The Commission interprets the determination of the foot of the continental slope when evidence to the contrary to the general rule is invoked, as a provision with the character of an exception to the rule. This provision not only does not oppose, but in fact complements, the general rule established by the determination of the foot of the continental slope as the point of maximum change in the gradient at its base. Both approaches aim to find the foot of the continental slope at its base.

6.1.3. The complementary character of this provision is emphasized by the fact that in addition to bathymetric and geomorphological evidence, all other necessary and sufficient geological and geophysical evidence must also be included as part of a submission by a coastal State.

6.1.4. The Commission feels it important to outline the breadth and scope of the necessary and sufficient evidence which will be required from States that might deem it appropriate to invoke this provision. The clarification of relevant scientific terms precedes the description of this evidence below.

6.1.5. The Commission acknowledges that article 76 makes use of scientific terms in a legal context, which at times departs significantly from accepted scientific definitions and terminology. The trend for the creation of separate interpretations of terms can be traced back to the work carried out for the first United Nations Conference on the Law of the Sea by the International Law Commission (Oxman, 1969). Article 76, paragraph 1, which defines the legal concept of the continental shelf by means of a reference to the outer edge of the continental margin, provides a measure of the current gap between the juridical and the scientific use of terms.

6.1.6. The definition of the continental margin in the earth sciences had a geomorphologic inception at the time of its adoption by various scientific organizations (Wiseman and Ovey, 1953). Current scientific knowledge about the nature and extent of the continental margin has evolved greatly from its original definition. It incorporates many additional geological and geophysical concepts within the framework provided by plate tectonics (COSOD II, 1987; ODP/JOIDES, 1996).

6.1.7. Although article 76 refers to the continental shelf as a juridical term, it defines its outer limit with a reference to the outer edge of the continental margin with its natural components such as the shelf, the slope and the rise as geological and geomorphological features. According to article 76, paragraph 1:

"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, or to a distance of 200 nautical miles from the baselines from which the breadth of the territorial sea is measured where the outer edge of the continental margin does not extend up to that distance."

6.1.8. Article 76, paragraph 3, provides further guidance to the Commission:

"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."

6.1.9. These paragraphs are valuable to the Commission on several grounds. They help clarify concepts such as natural prolongation of the land territory to the outer edge of the continental margin in the geological sense of these terms, which require the consideration of tectonics, sedimentology and other aspects of geology. But also, they provide guidance to the Commission in interpreting the meaning of the term "evidence to the contrary" to the general rule if this provision, with the character of an exception, is invoked by a coastal State in a submission to determine the foot of the continental slope.

6.1.10. The Convention does not prescribe the application of a specific scientific methodology to define the location of the foot of the continental slope when evidence to the contrary to the general rule is invoked. The Commission interprets this provision as an opportunity for coastal States to use the best geological and geophysical evidence available to them to locate the foot of the continental slope at its base when the geomorphological evidence given by the maximum change in the gradient as a general rule does not or can not locate reliably the foot of the continental slope.

6.2.1. 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. The rise is normally a wedge-shaped sedimentary body having a smaller gradient than the continental slope. The rise developed predominantly in a rifted margin realm with sufficient supply of sediments from the continent after breakup and commencement of sea-floor spreading.

6.2.2. From a geomorphological perspective, the shelf in ideal cases is the part of the seabed adjacent to the continent, which forms a large submerged terrace that dips gently seaward. The breadth of the shelf depends on the geological evolution of the adjacent continent. The continental shelf extends seaward to the continental slope, which is characterized by a marked increase in gradient. 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.

6.2.3. The shelf and the continental slope have characteristics typical of continental crust, often including thick layers of sediments. 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.

6.2.4. The Commission is aware of the difficulties arising from the determination of the foot of the continental slope and the edge of the continental margin from a geological perspective. Continental crust is compositionally distinct from oceanic crust, but the boundary between these two crustal types may not be clearly defined. Simple subdivision of margins into shelf, slope and rise may not always exist owing to the variety of geological and geomorphological continental margin types resulting from different tectonic and geological settings.

6.2.5. It is difficult to generalize the geological and geomorphological parameters that a coastal State may consider to establish the foot of the continental slope at its base by means of evidence to the contrary to the general rule. However, some examples and definitions are presented here based on evidence provided by plate tectonics. The Commission is well aware that these considerations may not exhaust all possible geological and geomorphologic types of continental margins as examples.

6.2.6. Over the last 20 years, geoscientific studies and activities such as the International Deep Sea Drilling Project/Ocean Drilling Programme (DSDP/ODP) have demonstrated the presence of a variety of continental margins (e.g. COSOD II, 1987), which can be grouped into three major categories:

    (a) Convergent (active) continental margins are formed along plate boundaries linked to active and inactive subduction zones often, but not always, associated with a trench (e.g. ODP/JOIDES, 1996; Bally, 1988; Taylor and Natland, 1995). The convergent continental margins comprise three different types:

(i) The accretionary convergent continental margin consists of a wide wedge of accreted sediments which were scraped off from the downgoing (lower) plate (fig. 6.1A);

(ii) The poor- or non-accretionary convergent continental margin is characterized by a poorly developed accretionary wedge. Most of the incoming sediments are underplated beneath the upper plate or are removed by the downgoing (subducting) plate (fig. 6.1B);

(iii) The destructive convergent continental margin shows no accretion. Material from the upper plate is eroded ("tectonic erosion") at the foot and from the base of the upper plate by the subducting lower plate (fig. 6.1C);

    (b) Rifted (extensional, passive) continental margins were formed along incipient plate boundaries during continental breakup and subsequent initial production of oceanic crust by sea-floor spreading (e.g. Bally, 1988; Edwards and Santogrossi, 1990; von Rad et al., 1982; Coffin and Eldholm, 1991). The category of rifted continental margins can be subdivided into two types:

(i) The wide, thin-crusted continental margin type (rifted non-volcanic margin) with widths of several hundreds of kilometres is characterized by a complex system of horsts and intervening grabens and half-grabens formed during the rifting and early drifting phases, and by a thinned continental crust (fig. 6.1D);

(ii) The narrow, thick-crusted continental margin type (rifted volcanic margin) is characterized by a thick lower-crustal lens with seismic velocities in the range of 7.2-7.6 km/s, and by a huge volcanic construction in the upper crustal level displayed in seismic sections by an average 100 km wide and several thousands of metres thick wedge of seaward-dipping reflectors (fig. 6.1E). Results of DSDP/ODP drilling have confirmed previous interpretations (e.g. Hinz, 1981) that the wedge of seaward-dipping reflections consists predominantly of basaltic lavas extruded in a shallow marine or subaerial environment. This voluminous volcanic body extending often continuously over distances of several thousands of kilometres along rifted continental margins was formed within a relatively short episode of transient volcanism during initial continental breakup. Recent studies have shown that approximately 70 per cent of the rifted Atlantic continental margins are volcanic continental margins;

    (c) Sheared continental margins were created along zones of translational continental rupturing during continental breakup and subsequent sea-floor spreading (fig. 6.1F).

6.3.1. Evidence to the contrary to the general rule in article 76, paragraph 4 (b), is interpreted by the Commission as 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 at its base when the geomorphological evidence given by the maximum change in the gradient does not or can not locate reliably the foot of the continental slope.

6.3.2. The inability of the general rule to locate the foot of the slope by means of the maximum change in the gradient at its base can be found in a number of scenarios. One of these scenarios can be envisaged, for example, when the curvature of the seabed along the base of the continental slope is constant. In this case, the maximum change in the gradient encompasses not only a point, but also a region.

6.3.3. Another scenario where the maximum change in the gradient may not clearly render the location of the foot of the continental slope at its base was already identified at the end of the previous chapter. In a scenario where irregular seabed topography reveals a number of local maxima in the change of the gradient at the base of the continental slope, it is possible that its maximum maximorum may not be indicative of the location of its foot.

6.3.4. In these exceptional cases, geological and geophysical evidence may be introduced as an alternative for determining the location of the foot of the continental slope at its base.

6.3.5. Article 76, paragraph 1, defines the breadth of the continental shelf with a reference to the edge of the geological continental margin. The Commission finds guidance in this paragraph to determine that any point identified on the basis of geological or geophysical evidence as the foot of the continental slope shall be located inside the geological continental margin.

(a) Convergent (active) continental margins

6.3.6. From a geoscientific perspective, the seaward extent of convergent continental margins is defined either by the seaward edge of the accretionary wedge (figs. 6.1A and 6.1B), or in the case of the destructive convergent margin type by the foot of the upper plate and by the foot of the inner trench wall, respectively (fig. 6.1C).

6.3.7. This distinct seaward limit or plate boundary can be determined with acceptable accuracy by modern multi-channel seismic techniques and modern bathymetric techniques (see chap. 8).

(b) Rifted (non-volcanic) and sheared continental margins

6.3.8. From a geoscientific perspective, the seaward limit of both the rifted non-volcanic continental margin and the sheared continental margin is defined as the transition between continental crust and oceanic crust created by sea-floor spreading and related volcanic/magmatic processes. Although continental crust has a different composition from that of oceanic crust, the boundary between the two crustal types may not be clearly defined; sometimes gradational or oceanic crust may even intrude into extended and downthrown continental crust.

6.3.9. Modern multi-channel and deep-penetrating seismic reflection studies and wide-angle reflection/refraction studies in parallel with magnetic and gravity measurements (see chap. 8) are needed to determine the location of the transitional zone of the rifted non-volcanic continental margins and of the sheared continental margins, especially in areas where magnetic sea-floor spreading anomalies are not well developed.

6.3.10. Apart from drilling, sampling and coring of crustal outcrops, including seamounts in the transitional area between continental and oceanic crust, can provide evidence of rock type or lithology and supply material for a variety of studies, for example, radiometric age dating, palaeontological age correlation, geochemical-isotope chemical analyses and palaeomagnetic studies. These results can be of use in determining the ocean-continent boundary along rifted non-volcanic and sheared continental margins. 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) zone (figs. 6.1D and 6.1F) 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.

(c) Rifted volcanic continental margins

6.3.11. Rifted volcanic continental margins are characterized by a thick low-crustal lens with high seismic velocities in the range of 7.0-7.6 km/s and a thick sequence of seaward-dipping reflectors (SDRS) beneath the basement surface. The SDRS merge seaward without a sharp boundary into oceanic crust created at a pre-existing oceanic ridge. Since the feather edge of the SDRS overlies rifted continental crust, a major part of the rifted volcanic continental margin can be considered as "the natural prolongation of the land territory" (article 76, paras. 1 and 3). The seaward extent of rifted volcanic continental margins can be defined as an area in which the SDRS terminate seaward and where the thickness of the igneous continental crust decreases to values typical of oceanic crust, i.e. less than 15 kilometres. Wide-angle reflection/refraction data and magnetic and multi-channel seismic reflection measurements are needed for determining the landward limit of the transitional zone (COT in fig. 6.1E) of the rifted volcanic continental margins, which might be considered by the Commission as an equivalent of the foot of the continental slope in the context of paragraph 4.

6.3.12. Although geological (plate tectonic) considerations are very important for coastal States in the determination of the foot of the continental slope, geomorphological aspects must also be considered. Among geological considerations, in addition to plate tectonics, it is recommended to consider also the sedimentary history of the margin resulting in the depositional build-up and geomorphological shaping of the margin.

6.3.13. The Commission understands that some coastal States may have difficulties in obtaining the necessary data to define the oceanic-continental crust boundary, which in some cases is not clear.

6.4.1. If a State has given evidence to the contrary to the general rule against using the foot of the continental slope (article 76 (4) (b)) in its submission, the Commission will deal with, inter alia, the following questions:

(i) Is that evidence acceptable to the Commission?

(ii) Does that evidence pertain to the identification of the foot of the continental slope? Is that evidence purely bathymetric and/or morphological?

(iii) Does that evidence include subsurface information aimed at establishing that the limit obtained by the rule of maximum change in gradient would not, for example, equate to the limit of the geological continental margin?

(iv) If such evidence to the contrary is presented as part of a submission, the Commission will request that it be also accompanied by the results of applying the rule of maximum change in gradient.

Figure 6.1

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Figure 6.2

7. Ridges

7.1. Formulation of the problem: paragraphs 3 and 6
7.2. Oceanic ridges and submarine ridges
7.3. Submarine elevations

7.1. Formulation of the problem: paragraphs 3 and 6

7.1.1. The Commission is aware that oceanic and submarine ridges as well as submarine elevations are given special attention in article 76 with respect to issues of entitlement to an extended continental shelf and the delineation of its outer limits.

7.1.2. Article 76 mentions three types of sea floor highs:

• Oceanic ridges of the deep ocean floor (para. 3);
• Submarine ridges (para. 6);
• Submarine elevations (para. 6).

7.1.3. None of these terms is precisely defined. It seems that the term "ridge" is used on purpose, but the link between the "oceanic ridges" of paragraph 3 and the "submarine ridges" of paragraph 6 is unclear. Both terms are distinct from the term "submarine elevations" of paragraph 6.

7.1.4. Paragraph 3 establishes that the continental margin does not include the deep ocean floor with its oceanic ridges:

"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."

7.1.5. According to paragraph 6:

"Notwithstanding the provisions of paragraph 5, on submarine ridges, the outer limit of the continental shelf shall not exceed 350 nautical miles from the baselines from which the breadth of the territorial sea is measured. This paragraph does not apply to submarine elevations that are natural components of the continental margin, such as its plateaux, rises, caps, banks and spurs."

7.1.6. This seems to imply that "submarine ridges" and "submarine elevations" are also distinct legal categories, as they are subject to separate provisions regarding the maximum outer limit.

7.1.7. The constraints contained in paragraph 6 for submarine ridges do not apply to submarine elevations that are natural components of the continental margin, such as "plateaux, rises, caps, banks and spurs".

7.1.8. The distinction between the "submarine elevations" and "submarine ridges" or "oceanic ridges" shall not be based on their geographical denominations and names used so far in the preparation of the published maps and charts and other relevant literature. Such a distinction for the purpose of article 76 shall be made on the basis of scientific evidence taking into account the appropriate provisions of these Guidelines.

7.2.1. Ridges under the sea may be formed in a variety of geological processes, including:

• Ridges formed by the sea-floor spreading and associated volcanic-magmatic processes;
• Ridges formed along transform faults and created as an inherent part of the sea-floor spreading process;
• Ridges formed by later tectonic activity resulting in uplift of oceanic crust;
• Ridges formed by volcanic activity related to the movement of crust over a hot spot. These ridges are commonly composed of coalescing volcanic features or seamounts and generally occur on oceanic crust;
• Ridges formed by interaction of oceanic crustal plates;
• Ridges formed by regional excessive volcanism related to plumes of anomalously hot mantle;
• Ridges associated with active plate boundaries and the formation of island arc systems. They could occur as active and inactive (remnant) volcanic arcs, and forearc and back-arc ridges. Such ridges commonly reflect different stages in the progressive development of island arc systems and may result from variations in factors such as the rate and direction of convergence, and from the nature of the plate being subducted;
• Ridges formed by rifting (extension and thinning) of continental crust. This process commonly forms broader features, such as marginal plateaux and rises, but sometimes creates elongated slivers of continental crust separated by oceanic or highly extended continental crust.

7.2.2. This categorization of ridges is not exhaustive and complete owing to the variety of the tectonic settings of the sea floor.

7.2.3. In scientific literature the term "oceanic ridges" is not used in an entirely strict sense. In some cases it clearly refers to oceanic spreading ridges only, while in others it seems to apply to all ridges composed of oceanic basaltic rocks (i.e. the first five categories in the list above). Transform ridges, in cases where they develop through time from a continental crustal environment into an oceanic crustal environment, may be difficult to classify as only one or the other along their full length. The other ridge types, except perhaps for some back-arc ridges, have no relationship to oceanic crust.

7.2.4. Some ridges located within the continental margins have been present since the early evolution of the margin and have influenced it since then. Because of their presence, sediment dispersal and thickness and the morphology of the sea floor may have acquired a unique configuration and individualization within the regional context.

7.2.5. It should be noted that paragraph 6 makes reference to both the category of submarine ridges, and that of submarine elevations that are natural components of the continental margin. At the same time, the Convention recognizes that the provision of paragraph 6 concerning the maximum limit of 350 M applies to submarine ridges.

7.2.6. The Commission feels that the provisions of paragraphs 3 and 6 may create some difficulties in defining ridges for which the criterion of 350 M in paragraph 6 may apply on the basis of the origin of the ridges and their composition.

7.2.7. For example, if the oceanic ridges include the first five types of ridges mentioned above (composed of oceanic basaltic rocks), one may find some examples where the ridges formed along transform faults or by later tectonic activity infringe the continental margin of continents.

7.2.8. Some ridges (including active spreading ridges) may have islands on them. In such cases it would be difficult to consider that those parts of the ridge belong to the deep ocean floor.

7.2.9. Article 76 makes no systematic reference to the different types of the earth's crust. Instead it only makes reference to the two terms: "the natural prolongation of ... land territory" and "the submerged prolongation of the