赤石構造帯の形成過程 : 中新世における西南日本弧東部の地殻改変と関連して  [in Japanese] Formation of the Akaishi Tectonic Zone and its significance on the crustal recontruction of the eastern part of Southwest Japan Arc during the Miocene  [in Japanese]

The Akaishi Tectonic Zone is situated on the western Akaishi Mountains in easternmost Southwest Japan where the ENE-WSW trending tectonic grains begin to bend northward. This zone, more than 50 km long in the N-S direction and 3 to 10 km wide in the E-W direction, is a composite fault zone which is bounded by the Komyo Fault on the east, the Akaishi Tectonic Line on the west and the Median Tectonic Line on the northwest. The northern part of this zone is characterized by the aggregation of many tectonic slivers of km-order scale which include the Sanbagawa metamorphic rocks, the strata of the Chichibu Belt, plus additional Lower Cretaceous and Lower Miocene formations. The central part mainly consists of the Upper Cretaceous strata of the Shimanto Belt the trend of which is rotated counterclockwise with respect to the trend of the strata of the same belt to the east of this zone. The Akaishi Tectonic Line in this partisa shallowly dipping fault, whereas in the northern and southern parts it dips steeply eastward or westward. In the southernpart, the Komyo Fault and the Akaishi Tectonic Line bifurcate into several faults and the deformed Lower to lower-Middle Miocene formations occur between these faults. Structural analyses of the brittly fractured fault rocks suggest that most of the faults have moved by left-lateral strike-slip. The horizontal displacement of the tectonic grains of Southwest Japan along the zone totals more than 50 km. These characteristics of the Akaishi Tectonic Zone clearly indicate that the zone was formed as a giant-scale strike-slip duplex. The radiometric age analysis of the fault rocks suggests that this zone was probably initially formed during the late Oligocene to earliest Miocene, before deposition of subsequent early Miocene formations. The main activity is inferred to have occurred during the late Early Miocene to the Middle Miocene, resulting in the formation of a strike-slip duplex. This activity may be intimately related to the clockwise rotation of the Southwest Japan Arc due to collision of the Izu-Bonin Arc with Japan during the Miocene.