Geotectonic Setting Of Japans Hydrate Deposits

Japan rests on a volcanic island arc of tectonic and magmatic thickening of the crust related to westerly subduction of oceanic crust in the northwest Pacific Ocean. This arc forms the margin of the continent here. However, the tectonic framework is complex and will not be discussed here in detail.

The youngest and most seaward of the accretionary terranes along the N and NW Pacific rim, which is a continuation of the island arc stretching to the

SW from Alaska, is in the process of becoming amalgamated with older subduction complexes that lie along the eastern Asian cratonic cores displayed from China to easternmost Russia. The accretionary prism margin of Japan bifurcates offshore central Honshu, SE Japan, where the younger Izu-Bonin Trench diverges from the northeastern continental margin of Japan to extend along the eastern margin of the Izu-Bonin Ridge, which is itself an accretionary prism and volcanic arc that is emergent from the sea immediately to the south of Japan (Fig. 1).

The Pacific Plate is in collision with both the Eurasian and Philippine Sea Plates. Whereas the Pacific Plate directly abuts Japan in the north, albeit with a possible accretionary relic of the North American Plate or an independent tectonosedimentary terrane similar to those (e.g. Wrangellia-like) accreted to North America along the Canadian and Alaskan Pacific margins, the Pacific Plate does not yet abut Japan directly. The Philippine Sea Plate, or an attenuated relic of it and the sediments riding upon it that have not been consumed in subduction beneath Japan or shortened by tectonic thickening along the eastern margin of the plate that carries the Izu-Bonin Ridge is in collision with Japan along the Nankai Trough (Fig. 1). The subduction of oceanic crust beneath the Izu-Bonin Ridge began in late Middle Miocene to Early Pliocene, and an accretional prism caused by offscarping of incoming sediments on the Philippine Sea Plate has been well developed (e.g., Ashi and Tokuyama, 1997). Subduction along the Nankai Trough confrontation zone is older. Both subduction zones may structurally underlie at least part of the southern part of Japan.

The mechanism of plate collision, subduction, and the formation of accretionary prisms under a compressional stress framework (within which significant lateral and vertical fault movements are commonly part of the tectonic process) is an excellent environment for the migration of methane and associated fluids and other gases, especially from deep sources, and their concentration as hydrate in the upper part of the accretionary prism. The geotectonic situation of Japan is similar to that of the Cascadia Margin (Chapter 15) and the continental shelf margin of the Palmer Peninsula of Antarctica (Chapter 16), where substantial hydrate has also been identified.

The continental margin of Japan, however, is tectonically complex because of the jump of the active north circum-Pacific subduction zone to the Izu-Bonin Trench at the annealing triple point off SE Honshu (Figs. 1, 2). Active subduction in the northernmost part of the Ryukyu Trough, (Nankai Trough) appears to have been retarded by the development of the Izu-Bonin Trench system. The trench in the northern end does not maintain a sharply defined bathymetric depression comparable with the active Japan-Izu-Bonin Trench system, nor is the water depth within the relic trench indicative of strong subduction. The northern part of the Nankai Trough and the Philippine Sea Plate may be subject to tectonic elevation owing to obduction above the west-plunging Japan-Izu-Bonin subduction system.

One of the important features of the eastern continental margin of Japan is that the shallow water shelf is very narrow. In addition, sediment derived from the nearby high relief volcanic mountains of Japan can be swept to the

Figure 1. Regional tectonic plate and geomorphic framework in the vicinity of Japan, after Tsuji et al. (1998). B, Possible terrane boundary separating the southern Japan accretionary prism from an amalgamated terrane of North American affinity (N); H, Honshu, the main island of the archipelago of Japan.

Figure 1. Regional tectonic plate and geomorphic framework in the vicinity of Japan, after Tsuji et al. (1998). B, Possible terrane boundary separating the southern Japan accretionary prism from an amalgamated terrane of North American affinity (N); H, Honshu, the main island of the archipelago of Japan.

shelf edge and onto the continental slope and remain more immature than sediment winnowed across a wider shelf. Sandy clastic sediment that can be swept rapidly over the shelf edge and deposited on the slope provides for good porosity and permeability sediments in which hydrate may form. Shelf sediment offshore here, for instance, has a well differentiated sand-silty bedding (Tsuji et al., 1998) that can be traced in drill holes and on seismic records from land to the upper slope.

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