Кунашир аралының геологиялық құрылымы және геологиялық тарихы

Southernmost island in the Kuril islands

Kunashir Island (Кунаши́р; 国後島; ), possibly meaning Black Island or Grass Island in Ainu, is the southernmost island of the Kuril Islands archipelago. 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 (see Kuril Islands dispute).

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 comprised 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 was 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 (de Grave, 2015) 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 Island, 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 (Martynov et al.). The geology of Kunashir Island 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 texture (Vergunov; Vergunov and Vlasov; Sergeev; Piskunov and Rybin). 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 et al. ; Vitukhin et al.) 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 (e. g. Zhelubovsky and Pryaluhina; Bevz), although Martynov et al. 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. 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 (Martynov et al.). 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 and Rusinova). 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 (Syvorotkin and Rusinova, 1989). 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 Island, 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 (Syvorotkin and Rusinova, cit opt.). 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 Island. 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 (Martynov et al., 2010a). Among these, Tyatya Volcano stands out as one of the most active and best studied volcanoes in the Kuril Arc (Nakagawa et al.). 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 (Vergunov and Vlasov, cit. opt. ; Kovtunovich et al). The Prasolov plagiogranite (tonalite) diorite complex consisted of three distinct plutons: the Prasolov, Mechnikov and Lobanov massifs (Vergunov and Vlasov, 1964; Sergeev, 1976, cit. opt.). 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) (Rybin, 1994). 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).

The climate is humid continental with very heavy precipitation especially in the autumn and a strong seasonal lag with maximum temperatures in August and September. The vegetation mostly consists of spruce, pine, fir, and mixed deciduous forests with lianas and Kuril bamboo underbrush. The mountains are covered with birch and Siberian Dwarf Pine scrub, herbaceous flowers or bare rocks. Tree cores of century old oaks (Quercus crispula) were found in July 2001 on Kunashiri Island.

Kunashir, along with the neighbouring Lesser Kuril Chain of smaller islands, has been recognised as an Important Bird Area (IBA) by BirdLife International as the island supports populations of various threatened bird species, including many waterbirds, seabirds and waders.

The original inhabitants of the islandas with most of Hokkaido and the Kurilswere the Ainu. Europeans are first recorded visiting this part of the Kurils in 1643 when the Castricum under Maarten Gerritsz Vries was exploring Hokkaido and the surrounding area for the Dutch East India Company (VOC). Vries's account of the area was incredibly garbledincluding a fictitious continental extension dubbed Company Landbut while his imaginary Staten Island is usually connected to Iturup, its placement on most maps of the period more closely resembles the location of Kunashir. Vitus Bering's lieutenant Martin Spanberg mapped the actual locations of the Kurils including Kunashir in a series of voyages in 1738, 1739, and 1742 but Company Land and Staten Island continued to appear in European maps decades afterwards. The Japanese expanded north to Kunashir in the 18th century, with the Matsumae clan establishing a fishery and trading site called Kunashiri basho (国後場所) in 1754. Its headquarters was located in Tomari (present day Golovnino) and administered Kunashir, Iturup, and Urup. In 1789 Kunashir Island was one of the settings of the Menashi Kunashiri Battle in which native Ainu revolted against Yamato Japanese tradespeople and colonists. Russian navigator Vasily Golovnin attempted to map and explore the island in 1811, but was apprehended by Japanese authorities and spent two years in prison. On September 1, 1945, or one day before the surrender documents of World War II were signed on September 2, 1945, in accordance with understandings reached at the Yalta Conference, the Soviet Union invaded and occupied the Kuril Islands. This occurred after the Soviet Union renounced the Soviet–Japanese Neutrality Pact signed in April 1941, and declared war on Japan on August 9, 1945 (formally, the pact itself remained in effect until April 13, 1946). Although Japan agreed after deliberations to cede its claims to "the Kurile Islands" as part of the Treaty of San Francisco in 1951, the Japanese government has claimed since the mid 1950s that the southern islands were not part of the ceded Kuril Islands.

The largest settlement on Kunashir Island is Yuzhno Kurilsk, administrative center of Yuzhno Kurilsky District.

The primary economic activity is fishing. The island has a port next to Yuzhno Kurilsk. Kunashir Island enjoys a Mendeleevskaya GeoPP geothermal power plant with the capacity of 1.8 MW.

The island is served by Mendeleyevo Airport.

After the 1994 earthquake, about one third of Kunashir Island's population left and did not return. By 2002, the island's population was approximately 7,800. The total population of the disputed Kuril islands at that time was approximately 17,000.