Geological Structure of the Tertiary Deposits in the Itsukaichi Basin, Tokyo Prefecture, Japan

Abstract

The Tertiary deposits in the Itsukaichi Basin, Tokyo Prefecture, rest unconformably upon the greatly deformed Chichibu-Torinosu System, which has suffered subsequently, intensive disturbance after the deposition of the top conglomerate (T₅) of the Tertiary rocks., In the Tertiary deposits is recognized a dislocation zone consisting of four areas (Fig., 4) with steep dips and a general trend of NW-SE., This fact suggests some genetic relationship between the formation of the abnormal dislocation zone in the Tertiary rocks and revival in activity of the previous fault lines of the basement., In the southwestern part of the basin, the dislocation is accompanied with an interesting mode of deformation and shearing which simulates plastic deformation., (Figs., 5∼7)., This is considered important in interpreting the process of dislocation of the Tertiary deposits., The numerous minor faults found in the Tertiary deposits are classified into three types, namely, A, B and C, according to their fault pattern (Fig., 8)., The distribution and characters of the fault patterns (size, shape, internal structure and external relation) of each respective type are discussed in detail., From field data it is inferred that the factors causing the different structural phemonena (dislocation, deformation and fault pattern) in the Tertiary deposits are the process of disturbance in the basement rocks and the difference in the physico-geological conditions of the Tertiary rocks.,