A STUDY ON THE FORMATION AND THE TILTING OF THE MYOKO VOLCANO, CENTRAL JAPAN

Abstract

The Myoko Volcano is a double volcano formed in the Quaternary Era, which is situated at the northern extremity of the Fuji Volcanic Zone. Author has performed a tephrochronological research of the Myoko Volcano and has considered the process of its formation. Moreover, he has investigated the geological structure of the basement complexes and the geomorphological characteristics of the Myoko Volcano and its marginal area. As the result, he has come to conclude as follows : 1) The basement complexes of the Myoko Volcano are composed of the Neogene system. They have several anticlinal and synclinal axes stretching nearly from the northeast to the southwest with the complicated structure. The Myoko Volcano is located on the eastern wing of the anticlinorium on the top of which the mountains such as Hiuchi-san, Yake-yama Volcano, and others are distributed. 2) The ejectas of the Myoko Volcano are stratigraphicaly divided into some following units, namely Pyroclastic flow, Pumice flow, Middle loam, Upper loam, Mud flow I, Mud flow II, Tsubame lava, Older central cone pyroclastic flow, and Younger central cone pyroclastic flow in ascending order. Moreover, there are Somma lava and Central cone lava. 3) Comparing the volcanic ejectas of the Myoko Volcano with the type locality of Japanese Quaternary system in the Southern Kanto District, it may be assumed that Pyroclastic flow, Pumice flow and Middle loam of the Myoko Volcano are correlated to the deposits of Shimosueyoshi stage and Musashino loam. Myoko Upper loam and Mud flow I•II are correlated to Tachikawa loam. Tsubame lava and Older central cone pyroclastic flow and Younger one are considered to have been deposited in the Alluvial Epoch. 4) The forming processes of the Myoko Volcano are divided into some following stages, namely the construction of strato volcano (from Pyroclastic flow or Pumice flow to Upper loam, Somma lava), the formation of caldera (Mud flow I, 19600±600 Y.B.P. or 19300±200 Y.B.P.), the breaking of northern external wall of somma (Mud flow II, 17900±450 Y.B.P.), the effusion of Tsubame lava, the construction of central cone (Older central cone pyroclastic flow, 8640±100 Y.B.P.; Younger central cone pyroclastic flow, 4790±110 Y.B.P.). 5) It may be assumed that the body of the Myoko Volcano has been suffering the crustal movement tilting eastward throughout its constructing process. Such a movement of the Myoko Volcano is presumed to have been caused by the folding of the anticlinorium of the basement complexes. 6) The evidences for the above mentioned tilting are as follows ; a) Land features of the east side slope of the Myoko Volcano and its foot areas suggest that this area has been deformed by the internal forces pushing from east to west, b) the somma was broken only on the east side, and c) the valley system of east-west diretcion has developed in foot areas. 7) The folding of the anticlinorium of the basement complexes have the scale of more than 20 km in wave length. It may be assumed that the maximum amounts of subsidence of the east side slope and the maximum ones of uplift of the west side slope by the tilting respectively are about 220 m and 240 m, which have been caused during the time from the outflow of Pyroclastic flow and Pumice flow to the present. According to this assumption, the tilting rates have the order of the magnitude of 8.4×10-7 a year.