西南日本内帯における新第三紀後半の岩脈群と広域応力場

抄録

The preferred orientations of dike swarms of four areas formed from Late Miocene to Pliocene are investigated in order to know the age of change of regional tectonic stress orientation in the inner belt of Southwest Japan. As the mechanism of dike formation is considered to be a natural magma-fracturing experiment and dikes are regarded as fossil tension cracks formed parallel to the direction of horizontal maximum compression stress (σ_Hmax) of regional tectonic origin, the preferred direction of σ_Hmx could be determined from the most concentrated trend of dikes of a swarm. The most concentrated trend of dikes at northern Tango Peninsula, Shimane Peninsula, Susa area and Myoken area are N-S, N-S, N10°W and N30°W, respectively. The directions of σ_Hmax obtained from the preferred orientation of dike swarms of the four areas coincide with the direction of the maximum compressional principal tectonic stress obtained from the strike and the sense of motion of faults and folds formed in the same age. This result gives the estimation that the average direction of σ_Hmax in the inner belt of Southwest Japan was N15°W±15° from Late Miocene to Pliocene.