http://www.gns.cri.nz

 

http://www.niwa.cri.nz

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

Prolongation of the land mass

The fundamental principle of article 76 is that the continental shelf includes the submarine prolongation of the coastal State’s land mass. Paragraph 1 of article 76 says

The continental shelf of a coastal State comprises the seabed 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.

and paragraph 3 of article 76 says

The continental margin comprises the submerged prolongation of the land mass of the coastal State, and consists of the seabed 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.

The problem is to determine what is meant by prolongation of the land mass and how to apply it to real world continental margins. 

Prolongation in a physical sense concerns how far something extends. When it refers to a land mass there are three aspects of prolongation: morphologic, the seafloor continues the shape of the land mass; geologic, the rocks beneath the sea floor are the same as or related to those of the land mass; and tectonic, the rocks beneath the sea floor share their history with those of the land mass. 

A land mass is above sea level because of differences in the composition and density of the rocks beneath the land mass from those of the rocks of the deep ocean, and/or because of active tectonic forces that continuously drive changes in the shape of the Earth’s surface. Moreover, the geological and tectonic factors that form the land masses do not stop at the beach. Land masses, whether they are huge continents or small islands, prolong – morphologically, geologically, and tectonically – beneath the ocean. 

A bathymetric map of the globe shows the morphological extension of the land masses and the often vast areas of intervening deep ocean floor.

Geophysical and geological data show that the morphological prolongation of the land mass is a manifestation of the other two senses of prolongation. The morphology of the margin of a land mass is described in terms of a shelf, a slope, and a rise. Beyond the rise lies the deep ocean floor. This morphological transition, from land mass to deep ocean floor, is the result of the composition and density of the rocks beneath the sea floor, the geologic processes that form and shape them, and the tectonic forces that act on them. Land masses are comprised of rocks that are less dense, on average, than those of the deep ocean. These less dense rocks extend beneath the ocean and contribute to the relatively shallow depth of the shelf, slope and rise. Tectonic and geologic processes such as subduction, volcanism, and sedimentation also contribute to the shallow depth of the margin. The fundamental distinction between land masses and the deep oceans, therefore, is geological and tectonic in origin.

Geological prolongation of a land mass cannot be established solely on the basis of rock type. Land masses do not generally have a single characteristic rock type, but are the result of igneous, metamorphic, sedimentary, and tectonic processes that form complex ‘continental’ crust, distinctly different from the crust of the deep ocean floor. Rocks of the deep ocean floor are basaltic, formed by sea floor spreading and are compositionally simpler than continental rocks. They generally show only minor compositional variations, reflecting the relatively homogenous mantle input to the magmatic processes.

Land masses are not always composed of continental-type rocks. For example, oceanic rocks can be thrust onto or accreted to a land mass or continent by tectonic movements. Some land masses are composed entirely of non-continental rocks: Iceland and other mid-ocean volcanic islands are examples. Other land masses, such as Cyprus, include large areas of obducted oceanic crust, or ophiolites. Although a land mass such as Iceland may have continuity and uniformity of geological basalt rock type with that of the deep ocean floor, there is a difference in tectonic history and hence morphology between such an island and the surrounding deep ocean floor. This break in morphology and tectonic history may mean that the extent of the continental shelf is limited to 350 nautical miles (the limit for a submarine ridge) and not 100 nautical miles from the 2,500 mete isobath (the limit for a submarine elevation).

The occurrence of basaltic rocks formed as the result of sea floor spreading is not diagnostic because they may have been incorporated into a land mass by tectonic forces, but that does not mean that rock type cannot be used to distinguish between prolongations of the land mass and the deep ocean floor. Rocks such as granite, marble and sandstone only form as part of continent building processes and can be used to infer that the area is not part of the deep ocean floor.

Tectonic prolongation implies that the rocks beneath the sea floor have a shared history with the rocks of the land mass. Shared history refers to the processes of continent building: sediment deposition, igneous activity, and suturing or accretion of rocks along plate margins. Disruption of this history results from fragmentation of the land mass by the tectonic processes of rifting and sea floor spreading. Continents and land masses are part of the plate tectonic conveyor system, and their extent and composition changes with time as the result of interactions along the plate margins.

The geologic, morphologic and tectonic boundaries are seldom, if ever, abrupt, and implicit in the determination of prolongation is an assessment of the degree of the continuity or uniformity of the geology, morphology and tectonic history. The estimated degree of continuity/uniformity is necessarily dependent on factors such as the nature and amount of information available. It is also not necessary to have continuity in all aspects of prolongation to demonstrate natural prolongation of the land mass. As mentioned above, the aspect of prolongation may have implications for the extent of the continental shelf.

Cartoon depiction of a crustal block, variously related to the mainland, but encompassed by the mainland rise.

Imagine for instance the situation of a high-standing bathymetric feature (Figure 1), variously related to the mainland. In the model A in the figure above, the isolated bathymetric high is assumed to be geologically and tectonically related to the land mass and continental plateau but separated from them by a bathymetric saddle. The intervening crust has been thinned by an extensional episode. In model B the stretching proceeded to the stage where the basement rocks linking the mainland and the bathymetric high are interpreted to be oceanic crust formed by sea floor spreading. In model C the bathymetric high is geologically and tectonically unrelated to the mainland. For all three instances a sediment apron or ‘rise’, extends along the plateau margin and fills the gap between the mainland and the bathymetric high. It links the mainland and entirely surrounds the isolated plateau. Possible foot of the continental slope positions lie along the margins of both the land mass and continental plateau and the isolated plateau.

In each of the above scenarios the morphological prolongation is complicated by a deep trough between the bathymetric feature and the land mass. For the latter two examples the geological prolongation is complicated by the occurrence of oceanic crust  and the tectonic prolongation is complicated by an episode of sea floor spreading. Do these complications and decreasing continuity interrupt the ‘natural prolongation’ of the land mass to the bathymetric feature? According to paragraph 3 of article 76 (see above), probably not. Both features are encompassed by the same ’rise’, and according to that paragraph they are therefore part of the same continental margin. The presence of the rise around the features means that morphological continuity between the two land masses exists even though geological continuity does not, and prolongation of the land mass extends to the isolated plateau.