Accretion and suturing
The fundamental principle of article 76 is that the continental shelf is the
submarine prolongation of the coastal State’s land mass. On a geologic
time scale, the land mass and continental shelf are not static but evolve in
shape and size as a result of plate tectonic forces.
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 geological crust usually distinctly different from that of the deep ocean floor. The deep ocean floor is the result of sea floor spreading, and is characterised by basaltic rocks. Land masses are not always composed of continental-type rocks. Iceland and other volcanic islands are examples of land masses that are composed of basaltic rocks, and many land masses include rocks that are oceanic in origin. It is clear, therefore, that land masses and their prolongations cannot be distinguished from the deep ocean floor solely on the basis of rock type. This is recognised in article 76 by the lack of reference to rock type in definitions of the continental shelf.
Continents grow at plate margins by accretion and suturing of new material, by deposition in sedimentary basins, and by volcanic activity. This paper discusses how continents grow due to plate tectonic collisions along their margins.
Continents are part of the plate tectonic conveyor system, and the greatest increases in their extent take place along convergent plate margins. Where continental rocks and oceanic rocks collide, the oceanic crust is generally denser and is subducted beneath the continent. Where two continents collide neither is subducted and the result is thickening of the crust and formation of a mountain range. Collision of other types of crust can result in subduction, accretion, or a combination of both.
The nature of the subduction process—how the subducting plate interacts with the overriding plate—affects how the prolongation of the land mass is interpreted. Consider these subduction scenarios
In scenario a), the collision of two continental blocks results in their amalgamation and an extension of the land mass and continental shelf. Even if it remains active, the plate boundary is irrelevant for the determination of foot of the continental slope positions. The land mass and continental shelf are on both sides of the plate boundary and therefore the plate boundary cannot interrupt the continuity of their morphology or geology. The Himalayas and the South Island of New Zealand are examples of this type of margin.
In scenario b) the oceanic crust outboard of the subduction trench is
clearly comprised of “rocks of the deep ocean floor”, there is no
connection between them and the land mass, and therefore the continental
shelf cannot extend across the trench. The foot of the continental slope
positions lie along the trench axis. Scenario b) is recognised in the CLCS
guidelines (1999) (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 … 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.” The west coast of South America is an
example of this type of margin. Von Huene and Scholl estimate that
accretionary wedges are developed along 57% (about 24,500 km) of the
Earth’s convergent subduction margins, with non-accretionary wedges along
the remaining 19,000 km.
In scenario c), because of the geometry of the plate margin or the nature
and buoyancy of the rocks on the subducting plate, pieces of the subducting
plate—sometimes including oceanic crust—are scraped off (“accreted”)
onto, into or beneath the continent. The extent of the material accreted at
the plate margin—terranes—can be small or very large, depending on the
nature, density and thickness of rocks arriving at the subduction zone, and
the subduction dynamics. Accretion of this material adds to the extent of
the land mass and/or continental shelf. This is recognised in the CLCS
guidelines (1999) (7.3.1): “In active margins, a natural process by which
a continent grows is the accretion of sediments and crustal material of
oceanic, island arc or continental origin onto the continental margin.
Therefore, any crustal fragment or sedimentary wedge that is accreted to the
continental margin should be regarded as a natural component of that
Accretion is not entirely straightforward, however. The Louisville Ridge seamount chain northeast of New Zealand is entering the Kermadec Trench near 26º S. The rocks and sediments of the seamount chain fill much of the trench, and at least some of the volcanic seamounts are being scraped off the subducting crustal slab and accreted to the Kermadec margin. Although material from individual seamounts is being accreted to the margin and the seamounts have a ‘rise’ that appears to overlap and encompass the entire chain, it would be difficult to argue that the entire chain is a natural prolongation of the Kermadec margin.
The basement rocks of New Zealand consist of Paleozoic rocks of Gondwana (550–250 million year old, orange colours), intrusive igneous rocks (red) and rocks that have been accreted to the margin in the last 230 million years (yellow colours and purple).
Figure 5 (modified from Kasuga et al. 2000) shows a similar situation
across an arc/trench system where a large buoyant aseismic ridge is
colliding with the trench. The authors comment that for profile CD the
situation is ambiguous, and the “aseismic ridge … could be taken to
constitute the natural prolongation of the landmass of a coastal State”.
The main landmass in this case presumably occurs on the margin above the
inner trench wall. They further comment that “the foot of the continental
slope could be defined around the margin of the aseismic ridge itself”.
Figure 6. White Island, about 50 km north of the North Island of New Zealand, is an example of an active island arc volcano.
In scenario d), motion along a margin such as that illustrated in Figure 2 has stopped. Oceanic crust underlies the margin and extends outboard of the fossil trench. The seafloor outboard of the fossil trench axis rocks would still be considered “rocks of the deep ocean floor” and the foot of the continental slope positions would be picked along this axis.
In scenario e), subduction of anomalous crust (e.g., seamounts, volcanic plateaus, ridges) along a margin as illustrated in Figure 3 has stopped. The anomalous crust underlies the margin and extends outboard of the fossil trench such that a boundary with true oceanic crust lies seaward of the fossil plate boundary. The anomalous crust is not comprised of “rocks of the deep ocean floor”, and cessation of subduction results in suturing of the material to the land mass and continental shelf. The anomalous crust now forms part of the continental landmass. According to paragraph (7.3.1) of the CLCS guidelines (1999) accreted crustal fragments should be regarded as natural components of the continental margin. The foot of the continental slope positions are therefore defined along the boundary between the anomalous crust and the true deep ocean floor.
The Hikurangi Plateau is a large igneous province that is similar in composition to oceanic crust, but the result of voluminous volcanic activity similar to that which has formed Iceland. The plateau is about 2,000 m shallower than the adjacent deep ocean floor. The boundary between them is a scarp about 1,000 m high except where it is buried by sediments.
The Hikurangi Plateau collided with the subduction margin along the New Zealand portion of the Gondwana margin sometime before about 85 million years ago. The plateau can be traced south beneath the Chatham Rise for at least 150 km. It is overlain by rocks of the forearc: schist, greywacke sandstone, and volcanics, most of which are exposed on the Chatham Islands.
The plateau probably formed not long before it reached the subduction zone, and was still quite hot. Subduction along the New Zealand portion of the Gondwana margin stopped because the plateau was too buoyant to be subducted, and there was a re-organisation of the plate boundaries. When subduction stopped the rocks of the plateau were sutured to the New Zealand continent.
The physical properties of the Hikurangi Plateau have changed as it
cooled over the last 85 million years, and it is now being subducted beneath
the North Island of New Zealand.
|Figure 7. The Hikurangi Plateau is a large igneous province that extends beneath the Chatham Rise, sutured to New Zealand by cessation of subduction along the Gondwana margin 105 million years ago.|