小断層から推定された鉱床地域の古応力 : 鹿児島県永野地域の例  [in Japanese] Palaeostress analysis in metallogenic areas : the case of the Nagano area, southern Kyushu, Japan  [in Japanese]

Some 160 mesoscale faults were observed in Plio-Pleistocene strata to understand the state of stress associated with the formation of vein-type ores at 〜1.9 Ma in the Nagano area, Japan, where NE-SW trending strike-slip and oblique normal faults were found to be abundant; but faults with other orientations and senses were also common. The strike-slip faults showed sinistral and dextral senses, suggesting that this area experienced multiple stresses since the late Pliocene. Five datasets were chosen from the Nagano Formation and Chayagatake Andesite, the depositional ages of which cover the late Pliocene to earliest Pleistocene. Each dataset was inverted to estimate paleostresses. The two datasets from the lower and middle Nagano Formation yielded the strike-slip faulting regime of stress with NNE-SSW and WNW-ESE trending σ_1, and σ_3 axes, respectively, and with intermediate stress ratios. The three datasets from the upper horizon yielded the opposite stress, suggesting that the state of stress changed sometime around 2Ma. The veins around the Yamagano Mine have E-W trends, indicating a largely N-S trending σ_3 at the time of vein formation, consistent with the stress determined from the upper horizon. The ores were deposited presumably under the strike-slip faulting regime of stress.