Abstract
Measurements of temporal changes in the three-dimensional (3D) orientation and density of intracrystalline healed and sealed (open) microcracks in quartz grains have enabled the reconstruction of paleostress fields and determination of the history of microcracking in the Late Cretaceous Nojima Granodiorite of northern Awaji Island. This in turn has allowed the determination of the relationship between these features and the Nojima Fault, which was seismically reactivated during the 1995 Kobe Earthquake. The 3D orientations of healed microcracks (HC) indicate that σ3 stress directions had N–S orientations at almost all locations, whereas sealed microcracks (SC) preserve evidence of NW–SE and N–S σ3 orientations. K–Ar biotite ages for the Nojima Granodiorite and fluid inclusion formation temperatures (c. 320–370°C) suggest that the healed microcracks formed at around 80 Ma. Fission-track ages of associated pseudotachylytes indicate that initial movements on the Nojima Fault probably occurred at around 56 Ma, with spatial variations in microcrack density indicating that initial faulting occurred after HC formation but before SC formation. This indicates a NW–SE σ3 orientation during HC formation (after restoration incorporating the clockwise rotation of SW Japan at c. 15 Ma), with the subsequent formation of NE–SW and E–W trending high-angle SCs filled with limonite or iron oxides. These SCs probably formed at shallower crustal depths than the HCs. The fact that the σHmax SC orientation is not consistent with the current NW–SE or WNW–ESE regional stress field means that SC formation must have ceased by the Quaternary. In addition, the HC σ3 orientation is nearly normal to the general trend of the Median Tectonic Line, suggesting the existence of a significant NW–SE extensional stress field prior to the clockwise rotation of SW Japan, rather than a NW–SE regional compressive field that would be conformable with oceanic plate movement directions during the Late Cretaceous.
