Izu-Bonin-Mariana Arc Totally Explained

Everything about Izu-bonin-mariana Arc totally explained

Guam in the southern IBM arc system is where Magellan first landed after his epic crossing of the Pacific Ocean in 1521. The Bonin Islands were a significant stop for water and supplies for New England whaling during the early 19th century. At that time they were known as the Peel Islands. Terrible battles were fought on the islands of Saipan and Iwo Jima in 1944 and 1945; many young Japanese and American soldiers died in these battles. George H. W. Bush was shot down in 1945 near Chichijima in the Bonin Islands. Twelve Japanese seaman were stranded in June 1944 on volcanic Anatahan for seven years, along with the overseer of the abandoned plantation and an attractive young Japanese woman. Novel and 1963 movie Anatahan is based on these events. The B-29 bomber Enola Gay flew from Tinian to drop the first atomic bomb on Hiroshima in 1945. Sergant Shoichi Yokoi hid out in the wilds of Guam for 28 years before coming out of hiding in 1972. The Brown tree snake was accidentally introduced during World War II and has since devastated native birds on Guam.

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, up to 1,000 km from the present IBM trench. The IBM arc system is the surficial 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 Honshu, 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 is the only trench-trench-trench triple junction on Earth. The IBM arc system is bounded on the east by a very deep trench, which ranges from almost 11km deep in the Challenger Deep to less than 3 km where the Ogasawara Plateau enters the trench. The southern boundary is found where the IBM Trench meets the Kyushu-Palau Ridge near Belau. hus defined, the IBM arc system spans over 25° of latitude, from 11°N to 35°20’N

Plate motions

The IBM arc system is part of the Philippine Sea Plate, at least to the first approximation. Although the IBM arc deforms internally – and in fact in 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's 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 Factory. 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 doesn't feed the IBM Subduction Factory, so it isn't discussed further. The North American plate includes northern Japan, but relative motion between it and Eurasia is sufficiently small that relative motion between PH and EU explains the motion of interest. The Euler pole for PH-PA as inferred from the NUVEL-1A model for current plate motions [DeMetset al., 1994] 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 20-30 mm/y, whereas relative to the northernmost IBM, PA is moving WNW and twice as fast. At the south end of IBM, there's almost now convergence between the Caroline Plate and the Philippine Sea Plate. It should be noted that the IBM arc isn't 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 (Scholz and Campos, 1995). 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. McCaffrey (1996) noted that the arc-parallel slip rate in the forear reaches a maximum of 30mm/yr in the northern Marianas. According to McCaffrey, 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 three segments of IBM (figure to right) don't 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 Honshu 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 meters 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.

Behavior and composition of the Western Pacific plate

Everything on the Pacific plate that enters the IBM trench is subducted. The next section discusses some modifications of the lithosphere just prior to its descent and the age and composition of oceanic crust and sediments on the Pacific plate adjacent to the trench. In addition to subducted sediments and crust of the Pacific plate, there's also a very substantial volume of material from the overriding IBM forearc that's lost to the subduction zone by tectonic erosion (von Huene et al., 2004).

IBM Trench and outer trench swell

The oceanic trench and the associated outer trench swell mark where Pacific Plate begins its descent into the IBM Subduction Zone. The IBM trench is where the Pacific Plate lithosphere begins to sink. The IBM trench is devoid of any significant sediment fill; the ~400m or so thickness of sediments is completely subducted with the downgoing plate. The lithosphere that's about to descend into a trench starts to be bent just outboard of the trench; the seafloor is elevated into a broad swell that's a few hundred meters high and referred to as the "outer trench bulge" or “outer trench rise”. The IBM outer trench swell rises to about 300m above the surrounding seafloor just before the trench.

Geology and composition of the westernmost Pacific plate

The Pacific plate subducts in the IBM trench, so understanding what is subducted beneath IBM requires understanding the history of the western Pacific. The IBM arc system subducts mid-Jurassic to Early Cretaceous lithosphere, with younger lithosphere in the north and older lithosphere in the south. It isn't possible to directly know the composition of subducted materials presently being processed by the IBM Subduction Factory – what is now 130 km deep in the subduction zone entered the trench 4 – 10 million years ago. However, the composition of the western Pacific seafloor-oceanic crust - sediments, crust, and mantle lithosphere - varies sufficiently systematically that, to a first approximation, we can understand what is now being processed by studying what lies on the seafloor east of the IBM trench.
The Pacific plate seafloor east of the IBM arc system can be subdivided into a northern portion that's bathymetrically ‘smooth’ and a southern portion that's bathymetrically rugged, separated by the Ogasawara Plateau. These large-scale variations mark distinct geologic histories to the north and south. The featureless north is dominated by the Nadezhda Basin. In the south, crude alignments of seamounts, atolls, and islands define three great, WNW-ESE trending chains (Winterer et al., 1993): the Marcus Island-Wake Island-Ogasawara Plateau, the Magellan Seamounts Chain, and the Caroline Islands Ridge. The first two chains formed by off-ridge volcanism during Cretaceous time, whereas the Caroline Islands chain formed over the past 20 million years. Two important basins lie between these chains: the Pigafetta Basin lies between the Marcus-Wake and Magellan chains, and the East Mariana Basin lies between the Magellan and Caroline chains.
More recently, Engdahl et al. (1998) provided an earthquake catalog containing improved locations (Figure 10). This data set shows that, beneath northern IBM, the dip of the WBZ steepens smoothly from ~40° to ~80° southwards, and seismicity diminishes between depths of ~150 km and ~300 km (Figures 11a c). The subducted slab beneath central IBM (near 25°N; Fig. 11c) is delineated by reduced seismic activity that nevertheless defines a more vertical orientation that persists southward (Figures 11d f). Deep earthquakes, here defined as seismic events ≥300 km deep, are common beneath parts of the IBM arc system (Figures 10, 11). Deep events in the IBM system are less frequent than for most other subduction zones with deep seismicity, such as Tonga/Fiji/Kermadec and South America. Beneath northern IBM, deep seismicity extends southward to ~27.5°N, and a small pocket of events between 275 km and 325 km depth exists at ~22°N. There is narrow band of deep earthquakes beneath southern IBM between ~21°N and ~17°N, but south of this there are extremely few deep events. Although early studies assumed that seismicity demarcated the upper boundary of the slab, more recent evidence has shown that many of these earthquakes occur within the slab. For instance, a study by Nakamura et al. [1998] showed that a region of events beneath northernmost IBM region occur ~20 km beneath the top of the subducting plate. They propose that transformational faulting, which occurs when metastable olivine changes to a more compact spinel structure, produces this zone of seismicity. Indeed, the faulting mechanism for deep earthquakes is a hotly debated topic (for example, [Greenand Houston, 1995]), and has yet to be resolved. Double seismic zones (DSZs) have been detected in several parts of the IBM subduction zone, but their locations within the slab as well as interpretations for their existence vary dramatically. Beneath southern IBM, Samowitz and Forsyth [1981] found a DSZ lying 80 km and 120 km deep, with the two zones separated by 30 35 km. Earthquake focal mechanisms indicate that the upper zone, where most events occur, is in downdip compression, while the lower zone is in downdip extension. This DSZ is located at a depth where the curvature of slab is greatest; at greater depths it unbends into a more planar donfiguration. Samowitz and Forsyth [1981] suggested that unbending or thermal stresses in the upper 150 km of the slab may the primary cause of the seismicity. For northern IBM, Iidaka and Furukawa [1994] used a refined earthquake relocation scheme to detect a DSZ between depths of 300 km and 400 km, which also has a spacing of 30 35 km between the upper and lower zones. They interpreted data from S to P converted phases and thermal modeling to propose that the DSZ results from transformational faulting of a metastable olivine wedge in the slab. Recent work suggests that compositional variations in the subducting slab may also contribute to double seismic zone [Abers,1996], or that DSZs represent the locus of serpentine dehydration in the slab [Peacock,2001].

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