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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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