Izu-Bonin-Mariana Arc - Geology and Composition of The Westernmost Pacific Plate

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 is not 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 is bathymetrically ‘smooth’ and a southern portion that is 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.

The age of Western Pacific seafloor has been interpreted from seafloor magnetic anomalies correlated to the geomagnetic reversal timescale Nakanishi, Tamaki & Kobayashi 1992 and confirmed by Ocean Drilling Program scientific drilling. Three major sets of magnetic anomalies have been identified in the area of interest. Each of these lineation sets comprises M-series (mid-Jurassic to mid-Cretaceous) magnetic anomalies that are essentially "growth rings" of the Pacific Plate. These anomaly sets indicate that the small, roughly triangular Pacific plate grew by spreading along three ridges (Bartolini & Larson 2001). The oldest identifiable lineations are M33 to M35 (Nakanishi 1993) or perhaps even M38 (Handschumacher et al. 1988). It is difficult to say how old these lineations and the older crust might be; the oldest magnetic lineations for which ages have been assigned are M29 (157 Ma; (Channell et al. 1995). Magnetic lineations as old as M29 are not known from other oceans, and the area in the Western Pacific that lies inside the M29 lineation - that is, crust older than M29 - is on the order of 3x106 km2, about a third of the size of the United States. ODP site 801 lies on seafloor that isconsiderably older than M29 and the MORB basement there yields Ar-Ar ages of 167±5 Ma (Pringle 1992). The oldest sediments at site 801C are midde Jurassic, Callovian or latest Bathonian (~162 Ma; Gradstein, Ogg & Smith 2005).

Seafloor spreading in the Pacific during the Cretaceous evolved from a more E-W 'Tethyan' orientation to the modern N-S trend. This occurred during mid-Cretaceous time, a ~35-40 Ma interval characterized by a lack of magnetic reversals known as the Cretaceous Superchron or Quiet Zone. Subsequently, the location of N-S trending spreading ridges relative to the Pacific Basin migrated progressively to the east throughout Cretaceous and Tertiary time, resulting in the present marked asymmetry of the Pacific, with very young seafloor in the Eastern Pacific and very old seafloor in the Western Pacific.

Sediments being delivered to the IBM trench are not thick considering that this some of Earth's oldest seafloor. Away from seamounts, the pelagic sequence is dominated by chert and pelagic clay, with little carbonate. Carbonates are important near guyots, common in the southern part of the region. Interestingly, Cenozoic sediments are unimportant except for volcanic ash and Asian loess deposited adjacent to Japan and carbonate sediments associated with the relatively shallow Caroline Ridge and Caroline plate. Strong seafloor currents are probably responsible for this erosion or non-deposition.

The compositions of sediments being subducted beneath the northern and southern parts of the IBM arc are significantly different, because of the Cretaceous off-ridge volcanic succession in the south that is missing in the north. Lavas and volcaniclastics associated with an intense episode of intraplate volcanism correspond in time closely to the Cretaceous Superchron. Off-ridge volcanism became increasingly important approaching the Ontong-Java Plateau. There are 100–400 m thick tholeiitic sills in the East Mariana Basin and Pigafetta Basin (Abrams et al. 1993), and at least 650 m of tholeiitic flows and sills in the Nauru Basin, near ODP Site 462. Castillo, Pringle & Carlson 1994 suggest that this province may reflect the formation of a mid-Cretaceous spreading system in the Nauru and East Mariana basins. Farther north, deposits related to this episode consist of thick sequences of Aptian-Albian volcaniclastic turbidites shed from emerging volcanic islands, such as preserved at DSDP site 585 and ODP sites 800 and 801. A few hundred meters of volcaniclastic deposits probably characterizes the sedimentary succession in and around the East Mariana and Pigafetta basins. Farther north, at DSDP sites 196 and 307 and ODP site 1149, there is little evidence of mid-Cretaceous volcanic activity. It appears that the Aptian-Albian volcanic episode was largely restricted to the region south of present 20°N latitude. Paleomagnetic and plate kinematic considerations place this broad region of off-ridge volcanism in the present vicinity of Polynesia, where today off-ridge volcanism, shallow bathymetry, and thin lithosphere is known as the 'Superswell' (Menard 1984; McNutt et al. 1990).

The figure above shows the typical sediments drilled at Ocean Drilling Program site 1149, east of the Izu-Bonin segment. The sediments drilled at ODP site 1149 are about 400 m thick and are as old as 134 million years. The sedimentary section is a typical pelagic stratigraphy, accumulated mostly in the Cretaceous but also in the last 7 million years (late Neogene) built on a basement of Early Cretaceous oceanic crust. The lowermost portion is carbonate and chert, the next layer is very chert-rich, the third layer is clay-rich. This is followed by a long depositional hiatus before sedimentation resumes ~6.5 Ma (Late Miocene), with deposition of volcanic ash, clay, and windblown dust. The stratigraphy east of the Mariana segment differs from that being subducted beneath the Izu-Bonin segment in having a much greater abundance of Early Cretaceous intra-plate volcanics and flood basalts. About 470m of oceanic crust was penetrated at ODP site 801C during Legs 129 and 185. These are typical mid-ocean ridge basalt that were affected by low-temperature hydrothermal alteration. This crust is overlain by a 3 m thick, bright yellow hydrothermal deposit and about 60 m of alkali olivine basalt, 157.4±0.5 Ma old (Pringle 1992).