Geology of the Kuril Islands
The Kuril Archipelago represents the central portion of the Kuril-Kamchatka Island Arc formation, which includes eastern Hokkaido, the Kuril Islands, and southern Kamchatka. It is composed of two main ridges: the Lesser Kuril Ridge and the Greater Kuril Ridge. The Lesser Kuril Ridge includes the Nemuro Peninsula of eastern Hokkaido, the Habomai Island group, and Shikotan, and continues to the northeast as the submarine Vityaz Ridge.
The Greater Kuril Ridge includes the Shiretoko Peninsula of eastern Hokkaido, all of the remaining Kuril Islands, from Kunashir north to Shumshu, and the southern tip of the Kamchatkan Peninsula. The formation of the Kuril Archipelago began in the late Cretaceous (approximately 90 million years before present) when the Okhotsk Terrane of the Kula Plate collided with the Siberian continent (Kimura and Tamaki, 1985) creating a subduction zone along the southeast margin of the Okhotsk Terrane. This subduction zone initiated the formation of the Kuril-Kamchatka Trench and the subsequent volcanism that created the Academy of Sciences Rise (now located in the central Sea of Okhotsk) and the Lesser Kuril Ridge. Volcanic activity and uplift in the region of the Lesser Kuril Ridge intensified during the Paleocene and Eocene as the Kula-Pacific Ridge was subducted into the Kuril-Kamchatka Trench, and it was probably during this period that the Lesser Kuril Ridge emerged from the sea (Kimura and Tamaki, 1985).
After the subduction of the Kula-Pacific Ridge, a volcanic hiatus ensued, and there is no evidence of further volcanic activity in the Lesser Kuril Ridge. During the hiatus of the late Eocene and Oligocene, the Okhotsk Plate was subsiding, and there is evidence that the Lesser Kuril Ridge may have been submerged during part of this period (Kimura and Tamaki, 1985). Late in the Oligocene (approximately 30 million years before present) the Okhotsk Terrane began to rotate clockwise and the back-arc basin that now forms the southern Sea of Okhotsk began to open to the west of the Kuril Arc. Volcanic activity resumed near the Kuril-Kamchatka Trench during this period, but was concentrated in the location of what is now the Greater Kuril Ridge. Consequently the oldest rocks in the main arc of the Kuril Islands are of late Oligocene and early Miocene age (Markhinin, 1968; Markov and Khotin, 1973; Savostin et al., 1983).
The back-arc basin was fully formed by the middle Miocene (Kimura and Tamaki, 1985). Although this period marks the beginning of the formation of the primary chain of the present-day Kuril Islands, sediment records indicate that they probably did not emerge above the sea surface until the early Pliocene (Kimura and Tamaki, 1985). During the past 10 million years, the Greater Kuril Ridge has experienced intense volcanic activity and crustal uplift (Markhinin, 1968; Yakushko and Nikonov, 1983; Gnibidenko, 1985). Although most of the islands along this ridge have not been studied closely, there is good evidence that the southern Kuril Islands (Kunashir and Iturup) emerged from the sea during the Pliocene or early Pleistocene and have been above sea level ever since (Bulgakov, 1996).
Some of the smaller islands, such as Atlasova, have been formed by recent volcanic activity and are therefore quite young (Melekestsev et al., 1990). Another major event in the geologic evolution of the Kuril Archipelago was the decoupling and subsequent southwestward migration of the Kuril Plate, which is located between the Greater Kuril Ridge and the Kuril-Kamchatka Trench. This decoupling was the result of stresses produced by the oblique subduction of the Pacific Plate in the southern region of the Archipelago (Kimura and Tamaki, 1985; Kimura, 1986). Although the timing of the initial decoupling is uncertain, the southwestern migration of this plate culminated in the collision of the Lesser Kuril Ridge with the Eurasian plate in the late Miocene (Kimura and Tamaki, 1985; Kimura, 1986; Bazhenov and Burtman, 1994).
This collision formed the Hidaka Mountains of Hokkaido and established the present-day location of the Lesser Kuril Islands. During the Late Pleistocene there were at least two major sea-level regressions associated with glacio-eustatic changes in this region (Leont'yev, 1970; Briggs, 1974; Korotkii, 1985). There is some disagreement about how much the sea level dropped during these periods, but most estimates suggest that both regressions were on the order of 100 m below present sea level (Leont'yev, 1970; Matthews, 1984; Korotkii, 1985), and possibly 200 to 300 m below present sea level (Briggs, 1974). Although deep trenches separate many of the Kuril Islands, those nearest to the northern and southern end of the Archipelago are relatively shallow. Hokkaido, Kunashir, Shikotan, and the Habomai Island group are presently separated by straits far less than 100 m deep, as are Kamchatka, Shumshu, and Paramushir. Depths of no greater than 230 m separate Iturup and Kunashir.
Therefore, it is highly likely that these southernmost and northernmost islands were connected with each other and to nearby Hokkaido and Kamchatka, respectively, during the sea-level minima of the Late Pleistocene. Additionally, Hokkaido would have been connected to Sakhalin and to the mainland during these periods (Kryvolutskaya, 1973). Because the last of these major sea-level regressions occurred 10,000-30,000 years ago, these islands have probably not been completely isolated for very long. Glaciers covered the northern and central islands during the glacial maxima of the Pleistocene, but probably did not extend any further south than central Iturup, as there are no traces of glaciation on Kunashir or in the Lesser Kuril Islands (Kryvolutskaya, 1973). In summary, the islands of the Lesser Kuril Ridge were formed during the Late Cretaceous and Paleocene, while the islands of the Greater Kuril Ridge began to form during the Late Oligocene and Miocene.
The volcanic activity that gave rise to the Lesser Kuril Ridge ceased during the Eocene, although the elevation of these islands continues to change due to eustatic crustal movements. The volcanic activity that gave rise to the Greater Kuril Ridge is still in progress, and the elevation of these islands continues to change with new eruptions and eustatic crustal movements. There is evidence that the Lesser Kuril Ridge emerged from the sea during the Paleocene or Eocene, but that it has been submerged one or more times since then. The Greater Kuril Ridge emerged during the early Pliocene. Some of the Kuril Islands have been above sea level since that time, while others have been formed by more recent volcanic activity. During the glacial sea-level regressions of the Late Pleistocene, Kunashir, the islands of the Lesser Kuril Ridge, and perhaps Iturup were connected to each other and to Hokkaido; and at the northern end, Shumshu and Paramushir were connected to each other and to the Kamchatka Peninsula. At the time of these regressions, glaciers covered most of the islands of the Archipelago, advancing as far south as central Iturup.
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