The Izu–Bonin–Mariana (IBM) arc system is a tectonic plate convergent boundary in Micronesia. The IBM arc system extends over south from Tokyo, Japan, to beyond Guam, and includes the Izu Islands, the Bonin Islands, and the Mariana Islands; much more of the IBM arc system is submerged below sea level. The IBM arc system lies along the eastern margin of the Philippine Sea plate in the Western Pacific Ocean. It is the site of the deepest gash in Earth's solid surface, the Challenger Deep in the Mariana Trench.
The IBM arc system formed as a result of subduction of the western Pacific plate. The IBM arc system now subducts mid-Jurassic to Early Cretaceous lithosphere, with younger lithosphere in the north and older lithosphere in the south, including the oldest (~170 million years old, or Ma) oceanic crust. Subduction rates vary from ~2 cm (1 inch) per year in the south to 6 cm (~2.5 inches) in the north.
The volcanic islands that comprise these island arcs are thought to have been formed from the release of volatiles (steam from trapped water, and other gases) being released from the subducted plate, as it reached sufficient depth for the temperature to cause release of these materials. The associated trenches are formed as the oldest (most western) part of the Pacific plate crust increases in density with age, and because of this process finally reaches its lowest point just as it subducts under the crust to the west of it.
The IBM arc system is an excellent example of an intra-oceanic convergent margin (IOCM). IOCMs are built on oceanic crust and contrast fundamentally with island arcs built on continental crust, such as Japan or the Andes. Because IOCM crust is thinner, denser, and more refractory than that beneath Andean-type margins, study of IOCM melts and fluids allows more confident assessment of mantle-to-crust fluxes and processes than is possible for Andean-type convergent margins. Because IOCMs are far removed from continents they are not affected by the large volume of alluvial and glacial sediments. The consequent thin sedimentary cover makes it much easier to study arc infrastructure and determine the mass and composition of subducted sediments. Active hydrothermal systems found on the submarine parts of IOCMs give us a chance to study how many of Earth's important ore deposits formed.
Boundaries of IBM arc system Crust and lithosphere produced by the IBM arc system during its ~50 Ma history are found today as far west as the Kyushu–Palau Ridge (just east of the West Philippine Sea Basin), up to from the present IBM trench. The IBM arc system is the expression of the operation of a subduction zone and this defines its vertical extent. The northern boundary of the IBM arc system follows the Nankai Trough northeastward and onto southern Honshū, joining up with a complex system of thrusts that continue offshore eastward to the Japan Trench. The intersection of the IBM, Japan, and Sagami trenches at the Boso triple junction, one of only two trench–trench–trench triple junctions on Earth. The IBM arc system is bounded on the east by a very deep trench, which ranges from almost deep in the Challenger Deep to less than { where the Ogasawara Plateau enters the trench. The southern boundary is found where the IBM Trench meets the Kyushu–Palau Ridge near Belau. So defined, the IBM arc system spans over 25° of latitude, from 11°N to 35°20’N.
Plate motions
The IBM arc system can be regarded as part of the Philippine Sea plate, at least to the first approximation. Although the IBM arc deforms internally and to the south a small plate known as the Mariana plate is separated from the Philippine Sea plate by a spreading ridge in the Mariana Trough – it is still useful to discuss approximate rates and directions of the Philippine Sea plate with its lithospheric neighbors, because these define, to a first order, how rapidly and along what streamlines material is fed into the subduction process. The Philippine Sea plate (PH) has four neighboring plates: Pacific (PA), Eurasian (EU), North American (NA), and Caroline (CR). There is minor relative motion between PH and CR; furthermore, CR does not feed the IBM subduction process, so it is not discussed further. The North American plate includes northern Japan, but relative motion between it and Eurasia is sufficiently small that the relative motion between PH and EU explains the process. The Euler pole for PH-PA as inferred from the NUVEL-1A model for current plate motions, lies about 8°N 137.3°E, near the southern end of the Philippine Sea Plate. PA rotates around this pole CCW ~1°/Ma with respect to PH. This means that relative to the southernmost IBM, PA is moving NW and being subducted at about /year, whereas relative to the northernmost IBM, PA is moving WNW and twice as fast. At the south end of IBM, there is almost no convergence between the Caroline plate and the Philippine Sea plate.
The IBM arc is not experiencing trench 'roll-back', that is, the migration of the oceanic trench towards the ocean. The trench is moving towards Eurasia, although a strongly extensional regime is maintained in the IBM arc system because of rapid PH-EU convergence. The nearly vertical orientation of the subducted plate beneath southern IBM exerts a strong "sea-anchor" force that strongly resists its lateral motion. Back-arc basin spreading is thought to be due to the combined effects of the sea-anchor force and rapid PH-EU convergence. Because of these different spreading rates caused by subducting slab pull, this might allow for sea floor spreading and magma upwelling to form new oceanic crust near the leading edges of these subduction zones.
The obliquity of convergence between PA and the IBM arc system change markedly along the IBM arc system. Plate convergence inferred from earthquake slip vectors is nearly strike-slip in the northernmost Marianas, adjacent to and south of the northern terminus of the Mariana Trough, where the arc has been 'bowed-out' by back-arc basin opening, resulting in a trench which strikes approximately parallel to the convergence vectors. Convergence is strongly oblique for most of the Mariana arc system but is more nearly orthogonal for the southernmost Marianas and most of the Izu–Bonin segments. In 1996 it was noted that the arc-parallel slip rate in the forearc reaches a maximum of /year in the northern Marianas. This is fast enough to have produced geologically significant effects, such as unroofing of high-grade metamorphic rocks, and provides one explanation for why the forearc in southern IBM is tectonically more active than that in northern IBM.
Geologic history of the IBM arc system
The evolution of the IBM arc system is among the best known of any convergent margin. Because IBM has always been an arc system under strong extension, its components encompass a broad area, from the Palau–Kyushu Ridge to the IBM trench (see first-right figure). In general, the oldest components are farthest west, but a complete record of evolution is preserved in the forearc. The IBM subduction zone began as part of a hemispheric-scale foundering of old, dense lithosphere in the Western Pacific. The beginning of true subduction localized the magmatic arc close to its present position, about away from the trench, and allowed the sub-forearc mantle to stabilize and cool. The arc stabilized until about 30 Ma, when it began to rift to form the Parece Vela Basin. Spreading also began in the northernmost part of the IBM arc about 25 Ma and propagated south to form the Shikoku Basin. Parece Vela and Shikoku basin spreading systems met about 20 Ma and the combined Parece Vela Basin-Shikioku Basin continued widening until about 15 Ma, ultimately producing Earth's largest back-arc basin. The arc was disrupted during rifting but began to build again as a distinct magmatic system once seafloor spreading began. Arc volcanism, especially explosive volcanism, waned during much of this episode, with a resurgence beginning about 20 Ma in the south and about 17 Ma in the north. Tephra from northern and southern IBM show that strong compositional differences observed for the modern arc have existed over most of the arc's history, with northern IBM being more depleted and southern IBM being relatively enriched. About 15 Ma, the northernmost IBM began to collide with Honshū, probably as a result of new subduction along the Nankai Trough.
A new episode of rifting to form the Mariana Trough back-arc basin began sometime after 10 Ma, with seafloor spreading beginning about 3–4 Ma. Because disruption of the arc is the first stage in forming any back-arc basin, the present Mariana arc volcanoes cannot be older than 3–4 Ma but the Izu–Bonin volcanoes could be as old as ~25 Ma. The Izu interarc rifts began to form about 2 Ma.
IBM arc system components
The three segments of IBM (figure to right) do not correspond to variations on the incoming plate. Boundaries are defined by the Sofugan Tectonic Line (~29°30’N) separating the Izu and Bonin segments, and by the northern end of the Mariana Trough back-arc basin (~23°N), that defines the boundary between the Bonin and Mariana segments. Forearc, active arc, and back arc are expressed differently on either side of these boundaries (see figure below).
The forearc is that part of the arc system between the trench and the magmatic front of the arc and includes uplifted sectors of the forearc situated near the magmatic front, sometimes called the 'frontal arc'. The IBM forearc from Guam to Japan is about wide. Uplifted portions of the forearc, composed of Eocene igneous basement surmounted by reef terraces of Eocene and younger age, produce the island chain from Guam north to Ferdinand de Medinilla in the Marianas. Similarly, the Bonin or Ogasawara Islands are mostly composed of Eocene igneous rocks. There is no accretionary prism associated with the IBM forearc or trench.
The magmatic axis of the arc is well defined from Honshū to Guam. This 'magmatic arc' is often submarine, with volcanoes built on a submarine platform that lies between water depth. Volcanic islands are common in the Izu segment, including O-shima, Hachijojima, and Miyakejima. The Izu segment farther south also contains several submarine felsic calderas. The Izu arc segment is also punctuated by inter-arc rifts. The Bonin segment to the south of the Sofugan Tectonic Line contains mostly submarine volcanoes and also some that rise slightly above sealevel, such as Nishino-shima. The Bonin segment is characterized by a deep basin, the Ogasawara Trough, between the magmatic arc and the Bonin Islands forearc uplift. The highest elevations in the IBM arc (not including the Izu Peninsula, where IBM comes onshore in Japan) are found in the southern part of the Bonin segment, where the extinct volcanic islands of Minami Iwo Jima and Kita Iwo Jima rise to almost 1000 m above sealevel. The bathymetric high associated with magmatic arc of the Izu and Bonin segments is often referred to as the Shichito Ridge in Japanese publications, and the Bonins are often referred to as the Ogasawara Islands. Volcanoes erupting lavas of unusual composition – the shoshonitic province – are found in the transition between the Bonin and Mariana arc segments, including Iwo Jima. The magmatic arc in the Marianas is submarine to the north of Uracas, south of which the Mariana arc includes volcanic islands (from north to south): Asuncion, Maug, Agrigan, Pagan, Alamagan, Guguan, Sarigan, and Anatahan. Mariana volcanoes again becomes submarine south of Anatahan.