Shimanto belt

Japanese: 四万十帯 - しまんとたい
Shimanto belt

In the geotectonic division of Japan, this is one of the division names in the Southwest Japan Outer Zone and the Kanto Mountains. It is distributed in a belt shape, 15 to 100 kilometers wide and 1,500 kilometers long, from the Kanto Mountains to the Akaishi Mountains, Kii Peninsula, Shikoku, Kyushu, and Okinawa. "Shimanto" comes from the Shimanto River in Kochi Prefecture.

[Akihiro Murata February 17, 2016]

Divisions and boundaries

The Shimanto Belt is bordered at its northern limit by the Butsuzo Tectonic Line (thrust fault), and is thrust onto the limestone-dominated layer of the Sanbosan Belt (Southern Chichibu Belt). In some places, the strata of the Shimanto Belt directly contact the sedimentary rocks of the Northern Chichibu Belt and the metamorphic rocks of the Sanbagawa Belt.

The Shimanto Belt is divided into the Northern Belt, which is mainly made up of the Cretaceous System, and the Southern Belt, which is made up of the Paleogene to Lower Miocene Formations of the Neogene System. The Northern Belt is sometimes called the Shimanto Belt in the strict sense, and the Southern Belt the Setogawa Belt. The boundary between the two is often a thrust fault or reverse fault, and is called the Sasayama Tectonic Line in the Akaishi Mountains, the Gobo-Hagi Tectonic Line in the Kii Peninsula, the Aki Tectonic Line in Shikoku, and the Nobeoka Thrust Fault (Nobeoka Tectonic Line) in Kyushu. The Nobeoka Thrust Fault is so low-angled that it is nearly horizontal, so klippes and fensters are known.

[Akihiro Murata February 17, 2016]

Strata

The Cretaceous System of the Northern Belt is called by different names depending on the region, such as the Hidaka River Group in the Kii Peninsula and the Morotsuka Group in Kyushu. These strata consist of basaltic volcanic rocks, melange containing blocks of chert and sandstone, and alternating layers of sandstone and sandstone-mudstone, with sandstone being the most abundant. The Paleogene to Lower Miocene Series of the Southern Belt is also called by different names depending on the region, such as the Hyuga Group in Kyushu and the Muroto Peninsula Group in Shikoku. The strata of the Southern Belt also consist of melange containing blocks of basaltic volcanic rocks and pelagic siliceous mudstone, as well as alternating layers of sandstone and sandstone-mudstone. These strata distributed in the Shimanto Belt are collectively called the Shimanto Supergroup.

The strata of the Shimanto Belt were previously considered to be geosynclinal deposits, but are now considered to be accretionary deposits. The basaltic volcanic rocks, chert, and siliceous mudstone were deposited on an oceanic plate and are mixed with terrigenous sandstone and mudstone deposited in the trench. Shallow marine deposits are distributed in parts of the Shimanto Belt, and are considered to have been deposited in the forearc basin over the accretionary deposits. Since it has been determined that the strata of the Shimanto Belt are accretionary deposits, they are now often referred to as the Shimanto accretionary complex rather than the Shimanto Supergroup.

The strata of the Shimanto Belt, both in the Northern and Southern belts, tend to be north-dipping and north-upper, and are repeated by north-dipping thrust faults. In addition, the existence of a thrust structure called a duplex has been reported by some thrust faults. This is thought to have been formed by the bottoming out of trench sediments, which were stripped off and incorporated into the land side.

[Akihiro Murata February 17, 2016]

[References] | Aki Tectonic Line | Reverse fault | Klippe | System | Sanbosan Belt | Thrust fault | Southwest Japan Outer Zone | Setogawa Belt | Chert | Duplex | Nobeoka Thrust | Fenster | Accretionary complex | Butsuzo Tectonic Line | Mélange | Formation
Northern Shimanto Belt (Cretaceous)
©Shogakukan ">

Northern Shimanto Belt (Cretaceous)

Shimanto belt, southern belt
©Shogakukan ">

Shimanto belt, southern belt


Source: Shogakukan Encyclopedia Nipponica About Encyclopedia Nipponica Information | Legend

Japanese:

日本の地体構造区分上、西南日本外帯および関東山地における区分名の一つ。関東山地から、赤石山脈、紀伊半島、四国、九州、沖縄まで、15キロメートルから100キロメートルの幅で、延長1500キロメートルにわたって帯状に分布する。「四万十」は高知県の四万十川に由来する。

[村田明広 2016年2月17日]

区分と境界

四万十帯はその北限を仏像構造線(衝上(しょうじょう)断層)で境されており、三宝山(さんぼうさん)帯(秩父(ちちぶ)帯南帯)の石灰岩卓越層に衝上されている。四万十帯の地層は、一部では直接、秩父帯北帯の堆積岩(たいせきがん)類や三波川(さんばがわ)帯の変成岩類と接する。

 四万十帯は、おもに白亜系からなる北帯と、古第三系~新第三系下部中新統からなる南帯とに分けられる。なお、北帯を狭義の四万十帯、南帯を瀬戸川帯とよぶこともある。両者の境界は衝上断層あるいは逆断層であることが多く、赤石山脈では笹山(ささやま)構造線、紀伊半島では御坊(ごぼう)‐萩(はぎ)構造線、四国では安芸(あき)構造線、九州では延岡(のべおか)衝上断層(延岡構造線)とよばれている。延岡衝上断層は水平に近いほど低角であるため、クリッペやフェンスターが知られている。

[村田明広 2016年2月17日]

地層

北帯の白亜系は、紀伊半島では日高川層群、九州では諸塚(もろつか)層群など、それぞれの地方によって異なる地層名でよばれている。これらの地層は、玄武岩質火山岩類、チャートや砂岩をブロックとして含むメランジュ、砂岩および砂岩泥岩互層からなっているが、量的には砂岩がもっとも多い。南帯の古第三系~新第三系下部中新統も、九州で日向(ひゅうが)層群、四国で室戸(むろと)半島層群など各地方ごとに異なる地層名でよばれている。南帯の地層も玄武岩質火山岩類や遠洋性の珪質(けいしつ)泥岩をブロックとして含むメランジュと、砂岩および砂岩泥岩互層からなる。四万十帯に分布するこれらの地層はひとまとめにして四万十累層群とよばれる。

 四万十帯の地層は、以前は地向斜堆積物とされていたが、現在では付加堆積物であると考えられている。玄武岩質火山岩類やチャート、珪質泥岩は海洋プレート上に堆積したもので、それらが、海溝に堆積した陸源性の砂岩や泥岩と混ざり合っている。なお、四万十帯には一部に浅海堆積物が分布しており、付加堆積物を覆って堆積した前弧海盆のものとされている。四万十帯の地層は付加堆積物であることが明らかになったため、四万十累層群という呼称を使わずに、四万十付加コンプレックスとよばれることが多くなってきた。

 四万十帯の地層は、北帯・南帯のいずれでも、北傾斜・北上位であることが多く、北傾斜の衝上断層によって繰り返している。また、いくつかの衝上断層によって、デュープレックスという衝上構造の存在が報告されている。これは海溝堆積物が剥(は)ぎ取られて、陸側に取り込まれる底付け作用により形成されたものと考えられている。

[村田明広 2016年2月17日]

[参照項目] | 安芸構造線 | 逆断層 | クリッペ | | 三宝山帯 | 衝上断層 | 西南日本外帯 | 瀬戸川帯 | チャート | デュープレックス | 延岡衝上断層 | フェンスター | 付加コンプレックス | 仏像構造線 | メランジュ | 累層
四万十帯北帯(白亜紀)
©Shogakukan">

四万十帯北帯(白亜紀)

四万十帯南帯
©Shogakukan">

四万十帯南帯


出典 小学館 日本大百科全書(ニッポニカ)日本大百科全書(ニッポニカ)について 情報 | 凡例

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