Abstract
Plio-Pleistocene paleostress field that allowed the deposition of Au-and-Ag-bearing, quartz vein-type ore in southwestern Kyushu, Japan, is still controversial. We studied mesoscale faults to understand temporal and spatial variation of stress in mid Pliocene sedimentary sequence, called the Hitoyoshi Formation, just to the northeast of the metallogenic province. Consequently, extensional and transtensional tectonic regimes were found from the fault-slip data, and both of them have the common minimum compressive stress axis in a NW-SE to NNW-SSE trend. The stress regimes are concordant with the known paleostresses found from mesoscale faults in the Pliocene Miyazaki Group on the Pacific side of Kyushu, suggesting that they represent regional stress fields in southern Kyushu. The observed minimum compressive axis is trending perpendicular and, therefore, concordant to known preferred orientation of the mineral veins in southwestern Kyushu. The coincidence suggests that the formation of veintype ore deposits was accompanied by extensional or transtensional tectonics.
