Izu-Bonin-Mariana Arc - IBM Arc System Components

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 200 km 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 1 and 4 km 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.

The back-arc regions of the three segments are quite different. The Izu segment is marked by several volcanic cross-chains which extend SW away from the magmatic front. The magmatically-starved Bonin arc segment has no back-arc basin, inter-arc rift, or rear-arc cross chains. The Mariana segment is characterized by an actively spreading back arc basin known as the Mariana Trough. The Mariana Trough shows marked variations along strike, with seafloor spreading south of 19°15’ and rifting farther north.

The IBM arc system southwest of Guam is markedly different than the region to the north. The forearc region is very narrow and the intersection of backarc basin spreading axis with the arc magmatic systems is complex.