Abstract

The Meiji sediment tongue is an elongate southeast-trending sedimentary body in the northwestern corner of the Pacific. The tongue is approximately 800 km long, 300 km wide, and as much as 1,800 m thick. It is thickest immediately south of Kamchatka Strait, which separates Kamchatka from the west end of the Aleutian Ridge, and it thins southeastward away from the strait. Studies of the sediment cored at Deep Sea Drilling Project site 192 indicate that the tongue began to form in earliest Miocene time and that most of it is clay-size terrigenous debris derived from eastern Siberia. Pelagic beds, chiefly diatomaceous debris, make up the remainder.

Today, clay-size erosional detritus is presumably swept into the northwestern Pacific by the Kamchatka Current, which flows from the Bering Sea southwestward through Kamchatka Strait. A large part of this flow is presumably deflected to the southeast along the axis of the sediment tongue. The tongue therefore signifies that the current, and presumably, the strait, formed at least by earliest Miocene time. In middle Miocene time, the accumulation rate of terrigenous clay (now claystone) at DSDP site 192 (the summit of Meiji Guyot) was 40 to 45 m/m.y. This rate implies that the guyot was near the strait in early Miocene time. Because the guyot is now near the strait, little of the tongue has been subducted beneath, or scraped off against, the Kamchatka continental margin. The Meiji sediment tongue is evidence that during the past 16 (probably 22) m.y. no more than 300 to 400 km of Pacific lithosphere has been subducted beneath Kamchatka (that is, American plate). Another sediment body in the North Pacific, the turbidite beds of the Aleutian Abyssal Plain, signifies that during the past 50 m.y. convergence between the Pacific and American plates has not exceeded about 500 km.

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