Kunashir (), known in Japan as , is the southernmost island of the Kuril Islands. The island has been under Russian administration since the end of World War II, when Soviet forces took possession of the Kurils. It is claimed by Japan as part of its "Northern Territories". The 2021 Russian census recorded a population of 7,683 people.
Etymology According to the Hokkaido Government, the Japanese name comes from the Ainu (), which translates to "grass island".
Geography Kunashir is separated by the Catherine Strait (Kunashiri Suido) from the island of Iturup, which is located 22 km northeast; Kunashir Strait (upper Nemuro Kaikyo) separates Kunashir from Shiretoko Peninsula (Hokkaido Island), located 25 km to the west; Izmena Strait (Notsuke Suido or lower Nemuro Kaikyo) separates Kunashir from Notsuke Peninsula (Hokkaido Island), located 16 km to the southwest; and the South Kuril Strait (Yuzhno-Kurilski Proliv, Minami Chishima Kaikyo) separates Kunashir from Shikotan and the Habomai Islands, 50 km to the east. Kunashir is visible from the nearby Japanese island of Hokkaido, from which it is separated by the Nemuro Strait.
Area: Length: Width:
Kunashir is formed by four volcanoes which were separate islands but have since become joined together by low-lying areas with lakes and hot springs. All four volcanoes are still active: Tyatya (), Smirnov, Mendeleyeva (Rausu-yama), and Golovnin (Tomari-yama). The island is made up of volcanic and crystalline rocks.
Geology
Kunashir, the southernmost island of the Kuril Arc, is 123 km long (NE to SW), 7 to 30 km wide and has an area of 1,490 km². The island has several volcanic features, including active ones such as Tyatya (1,819 m), Ruruy (1,485 m), Mendeleev (886 m) and Golovnin (541 m). The terrain of Kunashir is rugged and mountainous, with young geomorphology, steep slopes and numerous waterfalls. Asymmetry in relief is notable, with the western (Okhotsk) shore being steep and elevated, in contrast to the eastern (Pacific) shore, which has a more level and flat topography.
In contrast to the Japan Arc, the absolute timing of the evolution of the primary Kuril Arc system has received comparatively little attention, particularly with respect to modern geochronological techniques. A recent study aimed to fill this gap by presenting the initial zircon U/Pb ages of the volcanic basement and thermochronological ages for the rocks of Kunashir, thus establishing a definitive temporal framework for their emplacement and subsequent exhumation. The volcanic rocks of the Kuril Islands generally exhibited a two-level structural classification: (1) a lower level consisting mainly of moderately deformed Neogene rocks, and (2) an upper level comprising Pleistocene to recent volcanics. These rocks exhibited a wide range of compositions from basalt to rhyolite, with a predominance of (basaltic) andesites.Vergunov, G.P. (1961). On Intrusive Rocks of the Southern Kuril Islands (Shikotan, Kunashir, Urup). Russian Geology and Geophysics, 5, 77–80 (in Russian).
The geology of Kunashir could be elucidated from this perspective by delineating two structural levels. The lower level, termed the Lower Complex, was characterised by yellowish to yellow-grey tuffs, tuffaceous sandstones and breccias, mainly of felsic to medium composition. Numerous subvolcanic stocks and plugs of andesite, dacite and rhyolite intruded these formations, with deeper intrusive rocks of granodiorite porphyry composition and textureVergunov, G.P., Vlasov, G.M. (1964). Magmatism and metamorphism/Kuril islands/ Intrusive rocks, pp. 567–581. In: Geology of the USSR, 31, Part 1, Kamchatka. Kuril and Komandor islands, Moscow, 733 (in Russian).Sergeev, K.F. (1976). Tectonics of the Kuril Island System. Nauka, Moscow, 240 (in Russian).Piskunov, B.N., Rybin, A.V. (2000). Petrogeochemical types of granitoids of the Kuril island arc. Doklady Earth Science, 371, 93–95.. The upper level, termed the Upper Complex, was characterised by basaltic and basaltic andesite flows, small sub-volcanic intrusive bodies and modern andesitic stratovolcanoes.
Previous studies Davydov, M.N., Dunichev, V.M., Taboyakov, A. Ya, 1968. Recent data on neogene sediments of southern Group of large kuril islands (Urup, iturup, kunashir). Russian Geology and Geophysics 12, 112e116.Vitukhin, D.I., Oreshkina, T.V., Pushcharovskii, Yu. M., Tsukanov, N.V., 1996. New data on the geology of the Iturup island (Kuril island arc). Stratigraphy and Geological Correlation 4 (6), 61e74. suggested a late Miocene to Pliocene age for the Lower Complex, and accordingly older rocks were either absent or not clearly identified on the island. The Lower Complex has been further subdivided into the Rybakov and Kamuy formations,Zhelubovsky, Yu. S., Pryaluhina, A.F., 1964. Stratigraphy/Kuril islands, p. 527e567. In: Geology of the USSR, 31, Part 1, Kamchatka. Kuril and Komandor islands, Moscow, p. 733 (in Russian).Bevz, V.E., 1971. Stratigraphy of Cenozoic deposits of the Iturup island (Kuril islands). Geology and Mineral resources of the Sakhalin and Kuril islands. Yuzhno-Sakhalinsk 19e31 (in Russian). although Martynov et al.Martynov, Yu.A., Khanchuk, A.I., Kimura, J.I., Rybin, A.V., Martynov, A.Yu., 2010b. Geochemistry and petrogenesis of volcanic rocks in the Kuril Island Arc.Petrology 18, 489-513. used a different classification and terminology (see below). The Rybakov Formation, located in the most uplifted and deeply exhumed blocks, especially in the northern part of the island, was mainly an andesitic volcanic complex. This formation correlates with what Martynov et al.Martynov, A.Yu., Kimura, J.I., Martynov, Yu.A., Rybin, A.V., 2010a. Geochemistry of late Cenozoic lavas on Kunashir Island, Kurile arc. The Island Arc 19, 86-104. termed the Miocene Greentuff Formation. The Kamuy Formation, which forms the backbone of the island, consisted mainly of flysch-like volcanic-sedimentary deposits rich in felsic pumice of (rhyo)dacitic composition, with a thickness exceeding 1100 m. The Upper Complex, represented by the Fregat Formation, overlies the Rybakov and Kamuy Formations (Lower Complex). A distinct structural and erosional unconformity separated these complexes. The Fregat Formation comprised subaerial (basaltic) andesitic lava flows interbedded with tuffs, hyaloclastites and basaltic breccias.Syvorotkin, V.L., Rusinova, S.V., 1989. Is there a lava plateau on the Kunashir island? Pacific Geology 4, 103e108. This suggests that the Fregat volcanic rocks are predominantly of sub-aerial origin, occasionally deposited at shallow depths below sea level. The Fregat volcanic deposits formed a distinctive volcanic plateau, formed near sea level and affected by subsequent tectonic movements, resulting in the fragmented table-top mountainous topography of Kunashir. The age of the Fregat Formation was determined to be late Pliocene to early Pleistocene based on diatoms (Neodenticula kamtschatica – N. Koizumii) and palynology (Dunichev, 1969), in agreement with K-Ar data from the Fregat basalts.
On Kunashir, the Late Miocene-Early Pliocene Rybakov Formation was exclusively formed under subaqueous conditions. The volcanic rocks of the middle Pliocene Kamuy Formation, assigned to subaerial conditions in the northern and central parts of Kunashir, indicate a subaqueous environment in the southern part. Intensive uplift, beginning in the second half of the Pliocene, led to significant tectonic movements between the accumulation of the Kamuy and Fregat formations in the late Pliocene. This resulted in a pronounced unconformity between the two, with the Fregat lava plateau experiencing differential vertical movement. The northern part of Kunashir showed substantial uplift of over 1 km, in contrast to the more modest elevation change of 200–300 m in the southern region. Sergeev reported a mid to late Miocene deformation event at Kunashir, associated with folding of the Kunashir 'basement' and emplacement of Miocene intrusions, accompanied by at least 1.5 km of uplift and denudation.
Four modern and active (Pleistocene to Holocene) stratovolcanoes have shaped the modern landscape of Kunashir. These volcanoes, characterised by a typical alternation of lava flows and tuff deposits, exhibited mainly basaltic-andesitic to andesitic compositions, with minor amounts of more differentiated magmas of dacitic affinity. This modern magmatic activity was closely linked to the subduction of the Pacific Plate beneath the Okhotsk Plate. Among these, Tyatya Volcano stands out as one of the most active and best studied volcanoes in the Kuril Arc.Nakagawa, M., Ishizuka, Y., Kudo, T., Yoshimoto, M., Hirose, W., Ishizaki, Y., Gouchi, H., Katsui, Y., Solovyow, A.W., Steinberg, G.S., Abdurakhmanov, A.I., 2002. Tyatya volcano, southwestern Kuril arc: recent eruptive activity inferred from widespread tephra. The Island Arc 11, 236e254.
Both the Lower and Upper Complexes were characterized by numerous sub-volcanic intrusions, with the Lower Complex also containing deeper intrusions. In the Rybakov Formation these intrusions manifested as stocks, dikes and sills of basaltic, andesitic and dacitic composition. The Kamuy Formation contained mainly dacite stocks, dikes and sills. Deeper-seated igneous rocks without volcanic analogs were represented by two distinct intrusive complexes: (1) the Prasolov Complex, which included gabbro, diorite, quartz diorite, granodiorite, and tonalite or plagiogranite, and (2) the Dokuchaev Complex, which included three small bodies of fine-grained porphyritic granodiorite and tonalite.Kovtunovich, P. Yu., Safronov, A.D., Udodov, V.V., Rashepkina, E.V., Rudenko, E.A., 2002. Explanatory Notes on the Geology Map of Russia, Scale 1/200 000, Kuril Series, Sheets L-55-XXII, L-55-XXVIII, L-55-XXXIV, L-55-XXIII, L-55-XXIX, L-55XXXIII, K-55-II, L-55-XXXII. VSEGEI, St-Peterburg, p. 268 (in Russian). The Prasolov plagiogranite (tonalite) – diorite complex consisted of three distinct plutons: the Prasolov, Mechnikov and Lobanov massifs. The Prasolov Massif, the largest with an area of 18 km², located in NE Kunashir, acted as a stock. Intrusive contacts with the rocks of the Rybakov Formation were mainly tectonic, with rare intrusive contacts showing contact aureoles characterised by quartz-biotite hornfels. The Prasolov Complex was inferred to be of relatively late Miocene to Pliocene age based on intrusive contacts, the presence of pebbles from Prasolov Complex granitoids in the Kamuy Formation sediments, and K-Ar ages ranging from 61 ± 12 to about 10 Ma (most common ages found between 11e10 Ma).
The Dokuchaev Granitoid Complex, younger than the Prasolov Complex, comprised three small stocks that cut across the Kamuy Formation. Intrusive contacts were observed and Kamuy xenoliths were present (Vergunov and Vlasov, cit. opt.). The Dokuchaev Complex, consisting of tonalite-porphyry, granodiorite-porphyry and diorite-porphyry, is an amalgamation of the Dokuchaev, Valentina and Orlov massifs. The Valentina Massif, a tonalite-porphyry stock along the Okhotsk coast in the northern part of Kunashir, showed tectonic rather than intrusive contacts. A Pliocene age for Dokuchaev has been proposed on the basis of geological and K-Ar data, with rocks from the Dokuchaev complex having reported K-Ar ages of 6.5e4.2 Ma, corresponding to late Miocene to early Pliocene age.Rybin, A.V., 1994. Intrusive rocks of the Main Kuril ridge: petrography and petrogenesis. Yuzhno-Sakhalinsk 58 (in Russian) preprint, Institute of Marine Geology and Geophysics FEB RAS.
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