By : Neetu Singh
Island arc systems are formed when oceanic lithosphere is subducted beneath oceanic or continental lithosphere. They are consequently typical of the margins of shrinking oceans such as the Pacific, where the majority of island arcs are located. They also occur in the western Atlantic, where the Lesser Antilles (Caribbean) and Scotia arcs are found at the eastern margins of small oceanic plates isolated by transform faults against the general westward trend of movement.
Island arcs are recognized as tectonically active belts of intense seismic activity containing a chain or arc of active volcanoes. As early as the 19th century, W. J. Sollas drew attention to the correspondence of the arc-like forms of the Aleutians/Alaskan Peninsula, the East Indies (Indonesia), and several mountain chains to a series of great circles, and C. Lapworth discussed the ‘Volcanic Girdle of the Pacific’ (the Pacific ‘Ring of Fire’) as a continuous ‘septum’ separating ‘plates’ with different histories and thicknesses.
The deepest parts of the oceans, the deep-sea trenches, were located on the oceanward side of these arcs. As the nature of the ‘ring of fire’ was examined, it was realised that a line, called the andesite line, could be drawn around the Pacific outside which andesitic occurred (named after their type area in the Andes) and inside which basalts predominated.
Little could be done to discover the origin of island arcs until geophysical data were acquired. It was not until 1949, when H. Benioff showed that earthquake epicentres became progressively deeper as one went from the ocean side of the trench to the volcanic arc, that the idea of a relatively simple, steeply dipping thrust plane extending from near the trench to a depth of as much as 700 km was clearly established.
By the 1950s, substantial geophysical data had been acquired around the Pacific, off Indonesia, and in the Caribbean suggesting that large slabs might be dragged down beneath island arcs along subduction zones (also known as Benioff zones). It was not until 1968 that the next significant advance was made. The hypothesis of ocean-floor spreading in the 1960s had postulated that new lithosphere was being continuously created. It was recognised that unless the Earth was expanding, an equal amount of lithosphere must be being lost, and this seemed most likely to happen at the subduction zones. As the slab of oceanic lithosphere goes down, it melts partially at about 150–200 km depth, giving birth to magmas that rise and are extruded in volcanoes located 150–200 km from the axis of the trench.
The term ‘island arc’ is commonly used as a synonym for ‘volcanic arc’, yet the two terms are not quite the same. Volcanic arcs include all volcanically active belts located above a subduction zone, whether they are situated as islands in the middle of oceans or on continents, as along the west coasts of Central and South America. True island arcs include only those separated from the land by a stretch of water, such as those in the Caribbean. There is therefore a continuum of island-arc types:
1. some are truly intra oceanic, being situated entirely within the oceans, for example the Marianas, New Hebrides, Solomon , and Tonga in the Pacific; the Antilles and Scotia arcs in the Atlantic
2. others are separated from major continents by small ocean or marginal basins with a crust that is intermediate between continental and oceanic (Andaman islands, Banda, Japan, Kuril, and Sulawesi)
3. at the extreme end of the spectrum are those arcs built against continental crust, such as the Burmese and Sumatra/Java portions of the Burmese Andaman-Indonesian arc
4. finally the Andean chain, where the volcanic belt is located entirely within the continent and is not therefore an island arc. The age also varies. Some are very young: less than 10 Ma (10 x 106 years). Others are much older, dating back at least to the Tertiary or Cretaceous eras.
The exposed island arc is only one of a number of features of tectonic zones that extend from the trench at the ocean ward end to the marginal or back-arc basin on the continental side .
Proceeding from the oceanward side of the system, a bulge about 500 m high occurs about 120–150 km from the trench. The fore-arc region comprises the trench itself, the subduction complex (the ‘first arc’ or accretionary wedge or prism) and the fore-arc basin. The subduction complex is constructed of thrust slices of trench fill sediments and also possibly oceanic crust, which have been scraped off the downgoing slab by the leading edge of the overriding plate. The contact between the accretionary wedge and fore-arc basin is often a region of back-thrusting. The fore-arc basin is a region of tranquil, flat-bedded sedimentation between the fore-arc ridge and island arc. The island arc (‘second arc’) is made up of an outer sedimentary arc and an inner volcanic arc.
The sedimentary arc comprises coralline and volcaniclastic sediments underlain by volcanic rocks older than those found in the volcanic arc. This volcanic substrate may represent the initial site of volcanism as the relatively cool oceanic plate began its descent. As the ‘cold’ plate extended further into the asthenosphere, the position of extrusive igneous activity moved backwards to its steady state location now represented by the volcanic arc. The island arc and remnant arc (back-arc ridge or ‘third arc’) enclose a marginal sea (back-arc basin) behind the island arc. Such marginal seas are generally 200–600 km in width. In some island arc systems there may be up to three generations of marginal seas developed on the landward side of the island arc.
A subduction zone is identified by seismic foci, the seismic activity being concentrated on the upper surface of the down- going slab of lithosphere. The seismic activity defines the ‘seismic plane’ of the subduction zone, which may be up to 20–30 km wide. Subduction zones dip mostly at angles between 30º and 70º, but individual subduction zones dip more steeply with depth. The dip of the slab is related inversely to the velocity of convergence at the trench, and is a function of the time since the initiation of subduction. Because the down-going slab of lithosphere is heavier than the plastic asthenosphere below, it tends to sink passively; and the older the lithosphere, the steeper the dip.
Trenches are the deepest features of ocean basins, with depths ranging from 7,000 m to almost 11,000 m. The deepest are the Mariana and Tonga trenches. Most deep-sea trenches in the Pacific are formed of normal basaltic oceanic crust and are covered with thin layers of pelagic sediments and ash. This thin sedimentary layer is easily subducted under the overriding plate.
Ocean trenches are the result of under-thrusting oceanic lithosphere and are developed on the ocean side of both island arcs and Andean-type mountain ranges. They are remarkable for their depth and continuity, being the largest depressed features of the earth’s surface. The Peru-Chile trench is about 4,500 km long and reaches depths of 2–4 km below the surrounding ocean floor, so its base is 7–8 km below sea level. Trenches are generally 50–100 km in width. They have an asymmetric V-shaped cross-section, with the steeper side opposite the under-thrusting ocean crust. The sediment fill varies from almost nothing (e.g. Tonga-Kermadec) to almost complete (e.g. the Lesser Antilles Trenches).
Oceanic volcanic arcs are surrounded by large volcaniclastic aprons, kilometres thick. Most of the apron consists of pyroclastic fragments. As the submarine slopes of arc-related volcanoes are steep, there is great seismic activity and sedimentation is rapid, caused by slumping, sliding, and turbidity currents. Some of the results that have come from the study of both modern (e.g. the Lesser Antilles, New Hebrides) and ancient island arc systems have shown that:
1. the distribution of sediments around an arc is usually asymmetric, owing to the prevailing wind patterns, different arc slopes, and ocean currents (as in the Lesser Antilles where westerly winds blow most of the ash into the Atlantic)
2. the position of the island or individual volcanoes can migrate, often oceanward, towards the trench or along the arc;
3. as the volcano grows into shallow water and emerges, eruptions become more explosive, with ash dispersed over greater distances from the volcano;
4. sedimentary processes continually sort volcaniclastic fragments by grain size and density into a proximal coarsegrained facies of pillow breccias, debris-avalanche, and lahar (mudflow) deposits; a medial debris-flow facies; and a distal facies consisting of thin distal turbidites and fallout ashes.
As island arcs develop, enlarge, and become more mature, as in Japan and the North Island of New Zealand, terrestrial sediments and plants abound, and lagoons and lakes develop, especially within the calderas of the volcanoes.
Marginal seas (back-arc basins) are small ocean basins lying on the inner, concave sides of island arcs, bounded on the side opposite the arc by a back-arc ridge (remnant arc). They are most common in the Western Pacific but are also found in the Atlantic behind the Caribbean and Scotia arcs.
Marginal basins may develop in response to tensional tectonics whereby an existing island arc is rifted along its length, and the two halves separate to give rise to the marginal basin. A striking feature of the western Pacific Ocean is the enormous area covered by a large and complex pattern of basins that lie behind the volcanic arcs and are marginal to the continent. These marginal basins have been a source of controversy ever since it was realised that their crusts, while usually having a thickness close to that of continental crust, have seismic velocities closer to those of oceanic crust. Most marginal basins are now known to be old ocean floor trapped behind an island arc and are recognised not only in the western Pacific but also in the Andaman Sea behind the Burmese- Indonesian volcanic arc, and behind the Antillean and Scotia arcs.
They range in age from very young back arc basins that have developed within oceanic crust relatively recently (intra oceanic back-arc basins) to those mature basins adjacent to continents, such as the Japan Sea, which is inactive at present (continental back-arc basins).
Island arc systems exhibit intense volcanic activity. A large number of events take place on a plane which dips on average at an angle of about 45° away from the under-thrusting oceanic plate. The plane is known as the Benioff (or Benioff-Wadati) zone, after its discoverer(s), and earthquakes on it extend from the surface, at the trench, down to a maximum depth of about 680 km.
At these depths earthquakes occur as a result of the internal deformation of the strong descending slab of lithosphere, so that the majority of events lie about 30–40 km beneath the top of the slab. The presence of earthquakes at depths in excess of 70 km
Ridges similar to mid-ocean ridges occur at the margins of oceans; the East Pacific Rise is an example. There are other spreading ridges behind the volcanic arcs of subduction zones. These are usually termed back-arc spreading centres.
The reason why the ridges are elevated above the ocean floor is that they consist of rock that is hotter and less dense than the older, colder plate. Hot mantle material wells up beneath the ridges to fill the gap created by the separating plates; as this material rises it is decompressed and undergoes partial melting.
At the ridge crest the lithosphere may consist only of oceanic crust, which is basaltic in composition. As it moves away from the spreading centre, it cools and contracts and the peridotitic (olivine-rich) mantle component grows rapidly. The contraction leads to an increase in water depth.
When subducting oceanic lithosphere reaches a depth of over 80 km, an island arc is created at the surface by volcanic and plutonic activity some 150–200 km from the trench axis. Most of the world’s island arcs are situated along the western and northern margins of the Pacific Ocean, although there are others, such as those in the Caribbean.
Relatively young island arcs are structurally simple e.g. Tonga Kermadec, the New Hebrides, Aleutians, and Lesser Antilles. Older, mature island arcs are more complex as they were formed on earlier subducting plates. They are generally underlain by thicker crust 20–35 km thick, e.g. the Japanese and Indonesian arcs. The Lesser Antilles Subduction Zone The Eastern Caribbean shows all the main features of an island arc .The Atlantic Oceanic crust is subducting at a rate of c. 20 mm/year. The ocean trench is largely filled by sediment from the Orinoco River in Venezuela. These sediments have been deformed into a large accretionary wedge, over 20 km thick, known as the Barbados Ridge. Between the ridge and the island arc is the Tobago Trough, a fore-arc basin. The island arc stretches from Sombrero to Grenada, and comprises an outer sedimentary arc and an inner volcanic arc. These merge at Guadeloupe. The Grenada Trough – a back-arc basin – flanks the inner side of the island arc, and is bounded to the west by the Aves Ridge – possibly a remnant island arc.
Island arcs are associated with the subduction of oceanic crust. They are areas of intense volcanic and earthquake activity. Research suggests that there is considerable variety in island arc systems and that they are complex features. The Caribbean (Lesser Antilles) island arc system is an excellent example of an island arc system, and there are many ‘classic’ examples in the Pacific, too.