Reconstruction of Fujikawa Trough in Mio-Pliocene Age and its Geotectonic Implication

Abstract

The late Miocene to Pliocene Minobu Formation and its equivalents in the Southern Fossa Magna Region, central Japan, consist of turbidites and associated coarse clastic deposits. The thickness of the Minobu Formation and its equivalents attains up to 3 km. In those sediments, sixteen sedimentary facies are recognized, and they are grouped further into eight facies associations; Conglomeratic, Conglomeratic-sandy, Conglomerate-Sandstone, Sandy, Silty, Sandy-silty, Silty and Pebbly siltstone associations. Among them, the first four associations are interpreted as channel fill sediments. While the second three associations are thought to be interchannel sediments. Interpretation of such facies associations as mentioned above is based on the following points; 1) comparative studies of sedimentary facies and facies association in the study area with those reported from modern and ancient turbidite basins, 2) morphologic analysis of sedimentary body of facies association and 3) dispersal pattern of sediments. The last of those associations is assumed from its characteristic sedimentary features to be slump deposits which flowed down along such steep slope as fault scarp. Result of such interpretations leads further to the conclusion that sedimentary environments of the Minobu Formation and its equivalents is fillings of a trough which extends in N-S direction throughout the area. Furthermore, from sedimentological analysis of the trough fillings, feeder channel extending ENE-WSW direction, western boundary fault scarps and an axial channel of a few kilometres in width are revealed. Then it is named herein Fujikawa Trough. Petrography and dispersal pattern of clastic sediments indicate that the main source area was situated around the Kwanto mountainland, more than 40 kilometres away from Fujikawa Trough. Comparison of Fujikawa Trough with modern trough and trench is made. Consequently, Suruga Trough seems to show the closest similarity by the following reasons of 1) similarity of internal topographic features such as western boundary fault scarps and an axial channel of a few kilometres wide, 2) similarity of the axial direction of trough and 3) similarity of the sedimentary ratio of fill sediments. Such geomorphologic similarity of both trough is important, and suggests that Fujikawa Trough was formed as a consequence of collision of Izu-Bonin Arc into Southwest Japan Arc in the same mannar as Suruga Trough is developing now.