The Nemuro Belt (根室帯, Nemuro-tai) is the easternmost tectonic unit or terrane of Hokkaidō, Japan. The boundary with the Tokoro Belt to the west is marked by the Abashiri Tectonic Line (網走構造線), which runs from the area of Abashiri on the north coast to that of Urahoro on the south coast, the southern portion coinciding with the Urahoro Fault (浦幌断層). The belt is composed of volcanogenic sediments and volcanic rocks — for the most part, basalt and andesites; these may be remnants of an island arc that took shape over an "east or southeast dipping intraoceanic subduction zone". As dated by potassium–argon geochronology and radiolarians, the oldest sequences are Campanian‐Maastrichtian. The belt has rotated counterclockwise some 15–25° since the Late Cretaceous.
Running roughly east to west, the primary rock strata of the Nemuro Belt are the Late Cretaceous (Campanian) to early Palaeogene (Palaeocene and, in places, Eocene) deposits of the Nemuro Group (根室層群), which occur in the Shiranuka Hills [ja] and extend along the south coast from Kushiro and the Nemuro Peninsula into the South Kurils, to Zelyony (Shibotsu) and Shikotan, and perhaps as far as the submarine Vityaz Ridge. The Nemuro Group is in part overlain by the Middle Eocene Urahoro Group (浦幌層群), which is in turn overlain by the Upper Eocene to Lower Oligocene Onbetsu Group (音別層群).
Nemuro Group
The group includes the following formations, in ascending order:
- Nokkamappu Formation (ノッカマップ層)
- Ōtamura Formation (太田村層)
- Monshizu Formation (門静層)
- Oborogawa Formation (尾幌川層)
- Hamanaka Formation (浜中層)
- Akkeshi Formation (厚岸層)
- Tokotan Formation (床潭層)
- Kiritappu Formation (霧多布層)
Urahoro Group
The group includes the following formations, in ascending order:
- Beppo Formation (別保層)
- Harutori Formation (春採層)
- Tenneru Formation (天寧層)
- Yūbetsu Formation (雄別層)
- Shitakara Formation (舌辛層)
- Shakubetsu Formation (尺別層)
At the western end, the Rushin Formation (留真層) correlates with the Beppo, Harutori, and Tenneru Formations.
References
- Barnes, Gina (2013). "Origins of Japan—the 'Big Picture' Revisited: A Review of New Plate Tectonics Research". Japan Review. 25. International Research Center for Japanese Studies: 169–184. doi:10.15055/00000180.
- ^ Ueda Hayato (2016). "Hokkaido". In Moreno, T.; Wallis, S.; Kojima, T.; Gibbons, W. (eds.). The Geology of Japan. London: The Geological Society. pp. 201–221. ISBN 978-1-86239-743-9.
- Kanamatsu Toshiya; et al. (1992). 白糠丘陵西部地域,網走構造線西縁に分布する帰属不明の先第三系:北海道中生界,根室帯と常呂帯の構造関係についての一考察 [Pre-Tertiary Systems on the western side of the Abashiri Tectonic Line in the Shiranuka area, eastern Hokkaido, Japan: Implications to the tectonic relationship between the Nemuro and Tokoro Belts]. Journal of the Geological Society of Japan (in Japanese and English). 98 (12): 1113–1128. doi:10.5575/geosoc.98.1113.
- Kimura Gaku (1981). "Abashiri Tectonic Line — with special reference to the tectonic significance of the southwestern margin of the Kurile Arc". Journal of the Faculty of Science, Hokkaido University. Series IV. 20 (1). Hokkaido University: 95–111. hdl:2115/36710.
- ^ Vaes, B.; et al. (2019). "Reconstruction of Subduction and Back‐Arc Spreading in the NW Pacific and Aleutian Basin: Clues to Causes of Cretaceous and Eocene Plate Reorganizations". Tectonics. 38: 1367–1413. doi:10.1029/2018TC005164.
- ^ Kiminami Kazuo (1983). "Sedimentary History of the Late Cretaceous-Paleocene Nemuro Group, Hokkaido, Japan: a Forearc Basin of the Paleo-Kuril Arc-Trench System". Journal of the Geological Society of Japan. 89 (11): 607–624. doi:10.5575/geosoc.89.607.
- ^ Katagiri Takahiro; et al. (2020). "Collisional bending of the western Paleo‐Kuril Arc deduced from paleomagnetic analysis and U–Pb age determination". Island Arc. 29 (1). Wiley-Blackwell. doi:10.1111/iar.12329. hdl:2433/245933. e12329.
- Kiminami Kazuo (1978). 根室層群の層序の再検討 [Stratigraphic Re-examination of the Nemuro Group]. Earth Science (Chikyū Kagaku) (in Japanese and English). 32 (3): 120–132. doi:10.15080/agcjchikyukagaku.32.3_120.
- Sawaki Takayuki; et al. (2012). 釧路海岸の浦幌層群 [The Urahoro Formation on the Kushiro Coast] (PDF). GSJ 地質ニュース (in Japanese). 1 (12). Geological Survey of Japan: 363–368.
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