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Legs 127 and 128

Japan Sea

The Japan Sea is located between the subduction zones of the western Pacific and Himalayan convergence. At least four major plates and several microplates converge in this area. Three structural provinces have been recognized: basinal areas, block-faulted ridges, and a compressional northeastern margin. Although initial extension is probably no younger than 10 Ma, folding along Japan Sea's east margin occurred in the late Plio-Pleistocene and thrust faulting is currently active. Quaternary volcanism in the Japanese islands and the eastern margin of the Japan Sea is a consequence of the continued westward subduction of the Pacific and Philippine plates. During Leg 127, four sites were drilled in the Yamato (Sites 794 and 797) and Japan (Sites 795 and 796) basins. During Leg 128, two sites were drilled on two structural highs, the Oki Ridge (Site 798) and the Kita-Yamato Trough (Site 799). At Leg 127 Site 794, an additional 190.5 m was cored during Leg 128 and two multi-ship geophysical experiments were conducted.

Sediments in the Yamato Basin are underlain by a sediment/basaltic-doleritic sill complex at least 350 m thick, whereas the acoustic basement in the Japan Basin is composed of calc-alkaline basalt and basaltic and andesitic lava flows. Drilling in the Yamato Basin confirmed that the southeast Japan Sea formed by rifting of a continental arc sometime before the early Miocene (19 Ma). Initial rifting was accompanied by intrusion and eruption of alkali and high-Al basalts associated with deltaic or shallow-marine clastic deposits. Subsidence then followed rapidly and, in the late early Miocene, the basin widened and deepened in conjunction with continued intrusion and extrusion of high-Al basalts for at least another 3.5 m.y. In contrast, basement in the Japan Basin is dated at only ~14 Ma.

The sedimentary sequences overlying basement in both the Yamato and Japan basins show regional similarities and form five distinct lithological associations which record major tectonic and paleoceanographic changes during the evolution of the sea, including i) Lower Miocene delta-front sands and siltstones rich in plant debris, which were deposited rapidly during initial backarc rifting and basin subsidence, ii) wide-spread Middle Miocene siliceous and carbonate-rich hemipelagic claystones and volcanic tuffs, marking a period when the rate of basin subsidence outpaced the rate of sediment accumulation, iii) Upper Miocene claystones and porcellanites, iv) Pliocene diatom oozes, evidence of climatic cooling, upwelling, and increased primary productivity, and v) light/dark rhythmic Plio-Pleistocene sediments, reflecting rapid oscillations in the oceanographic behavior of the Japan Sea from stagnant, near-anoxic conditions to the fully oxic and hyperventilated conditions exemplified by the sea at present. Significantly, the latter cyclic sequences are best developed in the upper Pleistocene sediments of the past 400 k.y. when basin sills and climatically induced periods of low sea level may have periodically isolated the Japan Sea from the open Pacific. The relatively recent emplacement of these shallow bathymetric sills or straits is one manifestation of a convergent plate boundary apparently developing along the eastern margin of the sea, as is evidenced by uplifted en echelon ridges, thrust faults, and large compressional earthquakes.

Drilling on the Okushiri Ridge showed conclusively that compressional tectonics began to collapse the northeast margin of the Japan Sea beginning about 1.8 Ma, the first exact age data on the initiation of the convergence along this margin. This major episode of tectonic deformation in the east Japan Sea caused uplift of many of the ridges and banks, including the Oki Ridge, and is manifested by a ubiquitous unconformity separating deformed pre-late Pleistocene strata from younger, relatively undeformed sediments. The 113 discrete ash beds recorded during drilling on the Oki Ridge offer a detailed record of volcanism in this region, beginning at 4-3.5 Ma, dramatically increasing between 1.3 and 0.9 Ma, and followed by major pulses between 0.9 and 0.3 Ma. Probable sources for the ash are the volcanoes of the southwest and northeast Japan Arc (Izu, Kyushu, and Tohoku) and the Oki-Dogo and Ulleung-Do volcanic islands.

The Kita-Yamato Trough, a failed rift within the larger Yamato Rise, is a graben with multiple normal faults bounding its central axis. The unusually thick sediment implies that the trough experienced pronounced subsidence, accommodated by continental crust beneath the Yamato Rise as opposed to the surrounding oceanic and transitional crust presumably underlying adjacent basins. A late early Miocene major and proximal volcanic event occurred in the trough area. The resultant altered rhyolitic tuff and tuff breccias are similar to the rhyolitic rocks associated with Kuroko massive sulfide deposits.

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