Tectonic Development of the Chichibu Belt, Southwest Japan

TOMINAGA Ryozo

Journal of science of the Hiroshima University. Series C, Geology and mineralogy

Tectonic development of the Chichibu belt, Southwest Japan was discussed in this paper, on the basis of analysis of tectonostratigraphic units and tectonic development of individual units of the Chichibu belt in Shikoku, clarifying following points.
The Chichibu belt in Shikoku consists of three terranes, the Sawadani Terrane, the Kurosegawa Terrane and the Sambosan Terrane. The Sawadani Terrane is composed of three tectonostratigraphic units, which develop as nappes separated by southward dipping thrusts and show the northward younging age polarity. They in Eastern Shikoku correspond to the Sawadani unit of early to middle Early Jurassic time, Higashiura unit of middle to late Early Jurassic time and Kenzan unit of Middle Jurassic time in descending order, respectively. The Ohnogahara and Kanogawa nappes in Western Shikoku are correlated with the Sawadani and Higashiura nappes respectively. The ages of the constituent rocks of the nappes show following rules: (1) In each unit the age of the constituent rocks arrange in the order of limestone, chert, siliceous mudstone and mudstone from older to younger. (2) Each pair of limestone and chert, chert and siliceous mudstone and siliceous mudstone and mudstone of individual units tends to show the overlap of age. (3) The rocks of a kind in the lower unit are younger in age than those of the same kind in the upper unit. (4) The same sort of rocks in the different units tend to show the overlap of age. The Sawadani Terrane in Shikoku is defined as a belt occupied by Early to early Middle Jurassic accretionary prism formed along the northern (inner) front of the Kurosegawa Island-arc. The Kurosegawa Terrane is defined as a realm converted through Jurassic diastrophism from an older island-arc (Kurosegawa Island-arc), which consists of the Siluro-Devonian basement comlex of continental crust type, Permo-Triassic continental shelf type sediments, Permo-Triassic accretionary prism etc. The Sambosan Terrane of the Hiradani - Shiraishi area, Eastern Shikoku, has been divided from north to south into the Torinosu Group and the A, Bl, B2 and C tectonostratigraphic units. The A, B1, B2 and C units have lithological and chronological sequences regarded as accretionary prisms which show a younging age polarity from north to south. The Sambosan Terrane is defined as a belt which is occupied by Middle Jurassic to Early Cretaceous accretionary prism formed along the southern front of the Kurosegawa Terrane.
Late Paleozoic to Mesozoic tectonic development of the Chichibu belt, Southwest Japan, is divided into nine stages. The initial structure of the Sawadani Terrane and the Sambosan Terrane were built up during the stages from 2 to 4 and the stages 5 to 7, respectively. And the diastrophism throughout the Chichibu belt had a climax at the change of the accretionary site from the northern (inner) side to the southern (outer) side across the Kurosegawa Terrane.

eng

HIROSHIMA UNIVERSITY

[ISSN] 0075-4374

[NCID] AA00706718