西南日本外帯の地殻構造 : 2002年四国―瀬戸内海横断地殻構造探査の成果 : 特集「地殻応力測定法の現状と解決するべき問題点」Part 2

Abstract

In 2002, an integrated seismic survey across the overall Outer zone of Southwest Japan was conducted in Shikoku and Seto Inland Sea. The seismic line, which was more than 130km long, transected all geologic constituents of the superficial Outer zone of Southwest Japan. The survey successfully provided the first crustal transection of the Outer zone of Southwest Japan. The transection gives the following important information on the crustal structure. 14 The upper crust of the Outer zone is characterized by N-dipping accretionary complexes, which have a S-younging sequence, as expected by the surface geology. 2. The Median Tectonic Line (MTL), the northern boundary of the Outer zone, dips northward at about 40 degrees, and reaches the lower crust whose lower boundary exhibits an upward convex shape. 3. The lower crust of the Outer zone is only 30km wide from the lower termination of the MTL. Most of the Outer zone has no lower crust. 4. The Philippine Sea plate (PHS) changes its subducting angle from 10 to 25 degrees around the southern border of the lower crust