The Nemuro Belt (根室帯, Nemuro-tai) is the easternmost tectonic unit or terrane[1]: 178 of Hokkaidō, Japan.[2]: 202 The boundary with the Tokoro Belt to the west is marked by the Abashiri Tectonic Line (網走構造線),[3]: 1114 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).[4]: 96 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".[5]: 1377 As dated by potassium–argon geochronology and radiolarians, the oldest sequences are Campanian‐Maastrichtian.[5]: 1377 The belt has rotated counterclockwise some 15–25° since the Late Cretaceous.[5]: 1389
Running roughly east to west,[5]: 1377 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),[2]: 210 which occur in the Shiranuka Hills and extend along the south coast from Kushiro and the Nemuro Peninsula into the South Kurils, to Zelyony (Shibotsu) and Shikotan,[6]: 607 and perhaps as far as the submarine Vityaz Ridge.[5]: 1377 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).[7]
Nemuro Group edit
The group includes the following formations, in ascending order:[6][8]
- 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 edit
The group includes the following formations, in ascending order:[7][9]
- 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.[7]
References edit
- ^ 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.
- ^ a b 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.
- ^ a b c d e 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.
- ^ a b 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.
- ^ a b c 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.