Sakawa orogeny - Sakawa orogeny

A series of crustal movements that occurred widely across the Japanese archipelago during the Cretaceous Period of the Mesozoic Era (approximately 80 million years, from 145 million to 65 million years ago). It is also called the Sagawa Series Orogeny. "Sagawa" is the name of a place in the central-western part of Kochi Prefecture, and it is derived from the discovery of a Cretaceous thrust movement in the Chichibu Belt in this area. It is believed that these crustal movements were caused one after another by the oceanic plate (Kula plate) moving north in the Pacific Ocean at that time, colliding with the Japanese archipelago, which was located on the edge of the Eurasian plate at that time, and sinking obliquely underneath it. This crustal movement formed the framework of the present-day Japanese archipelago. At that time, the Sea of ​​Japan had not yet formed.

The main crustal movements are as follows:

(1) Andesitic volcanic activity in the Early Cretaceous period occurred in the area stretching from Moneron Island in southern Sakhalin to Rebun Island in Hokkaido and the Sanriku Coast.

(2) Intrusions of granite (such as Tono granite, Abukuma granite, Ryoke granite, and Chugoku granite) from the late Cretaceous period in the area extending from the Oshima Peninsula in Hokkaido through the Kitakami Mountains, Abukuma Mountains, Echigo Mountains, Ina Mountains, Kiso Mountains, Chugoku Mountains, Tsukushi Mountains, and Kyushu Mountains, as well as volcanic activity of rhyolite (such as Nohi rhyolite) in the Mino and Hida regions and Chugoku region.

(3) High-temperature metamorphism associated with the igneous activity of the granite (Ryoke metamorphic belt, Abukuma metamorphic belt), and high-pressure metamorphism in the area closer to the Pacific Ocean of the Ryoke metamorphic belt (Sanbagawa metamorphic belt and the northern edge of the northern Chichibu belt).

(4) The Median Tectonic Line begins to move (mid-Cretaceous).

(5) The formation of folded mountains through folding and uplift in the wide area on both sides of the then Median Tectonic Line, and the occurrence of large-scale submarine landslides due to this uplift.

(6) Formation of accretionary sediments in the central Hokkaido region – off the coast of Sanriku – off the coast of Abukuma – the southern half of the Kanto Mountains – the Akaishi Mountains – the Kii Mountains – the southern half of the Shikoku Mountains – the Kyushu Mountains – the Ryukyu Islands (including the Shimanto Supergroup).

[Yoshida Shizuo and Murata Akihiro]

Source: Shogakukan Encyclopedia Nipponica About Encyclopedia Nipponica Information | Legend

中生代白亜紀（1億4500万年前から6500万年前の、およそ8000万年間）に、当時の日本列島域に広くおこった一連の地殻変動。佐川系列造山運動ともいう。「佐川」は高知県中西部の地名であり、この周辺の秩父帯(ちちぶたい)で白亜紀の衝上(しょうじょう)運動が明らかにされたことに由来する。この地殻変動は、当時の太平洋を北進してきた海洋プレート（クラプレートKula plate）が、当時のユーラシアプレートの縁辺に位置した日本列島に衝突し、その下へ向かって斜めに沈み込んだことによって次々に引き起こされたものと推定される。この地殻変動によって現在の日本列島の骨格が形成された。なお当時、日本海はまだ形成されていなかった。

　その主要な地殻変動は次のとおりである。

(1)サハリン南部モネロン島―北海道礼文(れぶん)島―三陸海岸を連ねた地域におこった最初期白亜紀の安山岩質火山活動。

(2)北海道渡島(おしま)半島―北上(きたかみ)山地―阿武隈(あぶくま)山地―越後(えちご)山脈―伊那(いな)山地・木曽(きそ)山脈―中国山地―筑紫(つくし)山地・九州山地にわたる地域での、おもに白亜紀後半の花崗(かこう)岩類の貫入（遠野(とおの)花崗岩、阿武隈花崗岩、領家(りょうけ)花崗岩、中国花崗岩など）、および美濃(みの)・飛騨(ひだ)地域―中国地方における流紋岩類の火山活動（濃飛(のうひ)流紋岩など）。

(3)前記花崗岩の火成活動に関連した高温型の変成作用（領家変成帯、阿武隈変成帯）、および領家変成帯のより太平洋側の地帯における高圧型変成作用（三波川(さんばがわ)変成帯および秩父帯北帯(ほくたい)北縁部）。

(4)中央構造線の活動開始（白亜紀中期）。

(5)当時の中央構造線を軸としてその両側の幅広い地帯における褶曲(しゅうきょく)運動と隆起による褶曲山脈形成、およびこの隆起に伴う大規模海底地すべりの発生。

(6)北海道中軸域―三陸沖―阿武隈沖―関東山地南半―赤石山地―紀伊山地―四国山地南半―九州山地―琉球(りゅうきゅう)列島における付加堆積(たいせき)物の形成（四万十(しまんと)累層群など）。

［吉田鎮男・村田明広］

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