K-Ar ages of the andesites intruding along the Median Tectonic Line in northwestern Shikoku, Japan

Abstract

Many andesitic intrusions are exposed along the Median Tectonic Line (MTL) in northwestern Shikoku, Japan. Three dikes along the MTL and one intrusion occurring close to it were dated by the K-Ar method to estimate when the fault movement ceased. The dikes and intrusion are not deformed by the movement. The results give 15.1 to 15.4 Ma excluding severely altered one. The MTL in northwestern Shikoku region has not been active since about 15 Ma. Although a similar attenuation of active movement of the MTL during middle Miocene has been reported, the fault movement has continued in the other areas of southwestern Japan as demonstrated by Active Fault Research Group (1980). This may suggest that block displacement has been dominant since middle Miocene in the studied area. The age of the samples also indicates that the volcanic activity of Seto-uchi region started 15 Ma in northwestern Shikoku.