Sediment Recycling at Convergent Margins: Constraints from the Cosmogenic Isotope 10Be
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Multichannel seismic (MCS) profiles and bathymetric data from the central Mariana and Izu‐Bonin subduction systems image the subducting Pacific Plate from the outer trench slope to beneath serpentinite seamounts on the outer fore arc. Subducting oceanic crust varies along the Mariana margin from 5.3 to 7 km thick and is covered by 0.5–2 km thick sediments and numerous seamounts. Oceanic crustal thickness east of the Izu‐Bonin Trench is ∼6 km. Faulting resulting from flexure of the incoming Pacific Plate begins up to 100 km east of the trench axis, near the 6 km depth contour. The plate is cut by normal faults that reactivate inherited tectonic fabric where that fabric strikes <25° to the trench. Where the strike is >25°, incoming crust breaks along new faults with a trench‐parallel strike. The Mariana Trench axis is commonly a graben that accommodates an abrupt change (within <25 km) of plate dip from <4° (commonly ≤2°) on the incoming plate to >8° beneath the outer fore arc. We infer that the plate fails there rather than simply bends under the applied loads. Along portions of the Mariana margin, subducting seamounts displace the trench axis westward and uplift the toe of the slope. Surprisingly, west of the toe, there is no geophysical evidence of disturbance of the upper plate in response to seamount subduction, nor of significant subduction erosion or sediment underplating. MCS profiles across the base of the Mariana inner trench slope provide evidence for both complete subduction and small‐scale accretion of Pacific Plate sediments; however, we found no evidence for long‐term sediment accretion. The subducting plate dips 9–12° beneath serpentinite seamounts on the Izu‐Bonin and Mariana fore arcs. Along the Mariana margin, the majority of these seamounts are located ∼50–70 km west of the trench where the mantle wedge is 3–7 km thick between 8–10 km thick fore‐arc crust and the top of the subducting plate. The apparent lack of significant deformation of the Mariana fore arc crust by subducting seamounts may be the result of a weak serpentinized mantle wedge and/or progressive fracturing as the subducting plate increases in dip as it passes through the trench graben.
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This paper describes the petrology and geochemistry of rocks from the Yap Trench acquired by three dives of the Jiaolong research submarine. Combining the geophysical data and submersible observations, this paper describes the geomorphology, shallow structures, and sedimentology of the Yap Trench and further discusses the tectonics and activities of this region. Two obvious slope breaks are found on the landward slope, and horsts and grabens with small fault offsets are observed in the ocean-ward slope of the trench. Peridotites sampled from the Yap Trench inner wall are highly depleted subduction-related mantle residues. Volcanic rocks in the northern segment of the trench have subduction-related characteristics that Yap fore-arc rocks underwent metasomatism during Cenozoic subduction. The rocks with remarkable lithologic difference from lithospheric mantle and upper crust sampled in the break slopes suggest that the slope break area may represent a lithologic boundary or transition zone. The landward slope of the Yap Trench was removed by subduction erosion as a result of collision with the Caroline Ridge. The bending of the down-going plate caused normal faults, horsts, and grabens with little or no sediments indicating that the Caroline Ridge is subducting beneath the Yap arc along the trench even though the convergence rate is very slow.
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Abstract The possible occurrence of hydrocarbon deposits in complex paleo subduction zones has recently been advanced in various literature. In this connection it is of interest to compare the outer-arc ridge of western and eastern Indonesia in order to evaluate the hydrocarbon potentials in the chaotic wedges in both areas. The arc-trench system in western Indonesia was formed by subduction of the Indian Ocean oceanic crust beneath the Eurasian continental crust. No significant hydrocarbon occurrence was reported in the accretionary wedge of western Indonesia. The fore-arc basin, west of Sumatra, typically lacks the coarse quartz rich sediments necessary for clastic reservoir formation. The heatflow and hydrocarbon source rocks are immature. The arc-trench system of eastern Indonesia shows an entirely different character. Two distinct phases can be discerned in the development of the Banda Arc. In an earlier phase, oceanic crust of the Indian-Australian plate was subducted under the Banda oceanic plate, and in a later phase followed by subduction of the Australian continental crust into the Banda Arc subduction zone. Whereas the oceanic crust dipping in the Sumatra-Java Trench is only covered by relatively thin pelagic sediments, large parts of the shelf and slope sequences of the Arafura Platform are carried passively on top of the Australian lithospheric plate down into the Tanimbar Trench and Aru Through. The more consolidated lower part of the sequence has a greater shear strength and consequently little material from these sequences was scraped off and incorporated in the chaotic wedge. If these older sediments are rich in organic material, the tectonic processes in the trench and beneath the chaotic wedge in combination with increasing burial depth will enhance the maturity of the organic material. If reservoir rocks exist in front of the chaotic wedge, upslope migration and accumulation must be considered as possible within the faulted blocks in these rocks. Due to the similarity in geological, tectonic and stratigraphic conditions, the oil and gas occurrence in the subduction complex of eastern Sulawesi might be explained in the same way, although the basic problem which has to be solved first is the past geotectonic position of Sulawesi vis a vis the Australian continental crust of the Vogelkop area in Irian Jaya.
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