Paleostress orientation from 3‐D orientation distribution of microcracks in quartz from the Cretaceous granodiorite core samples drilled through the Nojima Fault, south‐west Japan

Abstract

Abstract The orientations of both healed extension microcracks and microcracks in quartz grains sealed mostly by carbonate minerals were measured from Cretaceous granodiorite core samples drilled along the Nojima Fault, southwest Japan. The preferred orientations of both healed and sealed microcracks consist of approximately three orthogonal sets, (components) A, B and C, in which A strikes NS–NW‐SE and dips vertically, B strikes EW–NE‐SW and dips vertically, and C is subhorizontal. Both the healed and sealed microcracks were possibly formed by hydraulic fracturing, and the successive release of tensile stress due to pore fluid overpressure in the principal stress directions could have caused this microcracking in mutually orthogonal directions. The quartz grains are also very moderately plastically deformed, which is indicated by the occurrence of kink bands and undulose extinction. The association of healed microcracks and kink bands in the quartz suggests that these microstructures formed under subgreenschist facies conditions (≈300°C) during hydrothermal activity that could have occurred immediately after the emplacement of granodiorite during the Late Cretaceous period. Based on both the preferred orientation of microcracks, and c‐axis fabrics of kinked and unkinked grains (so called kink method), it is inferred that the σ1‐and σ3‐axis were oriented horizontally in NS–NW‐SE and EW–NE‐SW directions, respectively. The inferred paleostress field does not conform to the east–west‐trending compression during the Quaternary period, but to the activation of EW–NE‐SW‐trending, left–lateral strike–slip faults during the Late Cretaceous period in southwest Japan.