天草下島北部の中新世貫入岩体の方向と応力解析

Abstract

The orientations of 76 sheet intrusions in northern Amakusa-Shimoshima, western Kyushu, were inverted to understand paleostress condition(s) in the backarc behind the junction of the SW Japan and Ryukyu arcs. The radiometric ages of intrusive rocks in the Amakusa area suggest that the sheet intrusions are 14-17 Ma, contemporaneous with the formation of the Japan Sea. The Eocene host formations are folded about a NNE-trending axis with an interlimb angle of ~140°, but the relative timing of the folding and magmatism is not constrained. The sheet intrusions have various orientations, but E-W-trending dikes are dominant. The tilt correction did not significantly affect the E-W-trending intrusions, because they meet the fold axis at high angles. Stress inversion using the mixed Bingham distribution yielded two stresses from in-situ and tilt-corrected orientations. The stresses calculated from the in-situ set of orientations appeared to fit better than those from the tilt-corrected set of orientations, because the stresses of the normal faulting and strike-slip faulting regimes from the in-situ set of orientations had nearly vertical stress axes. We thus suggest that magmatism postdated the folding.