ABSTRACT We describe and interpret a system of well-preserved normal and reverse faults in the Kayo Formation of the Miocene Shimanto belt, an exhumed accretionary complex exposed on Okinawa Island. The normal and reverse fault systems both strike NE-SW, suggesting systematic horizontal stress variations between compression and extension. Temperature and pressure conditions for the normal and reverse fault systems were estimated from the densities of water in fluid inclusions in the veins along the faults, and previously reported maximum paleotemperature based on values of vitrinite reflectance and illite crystallinity. The fluid inclusion analyses yielded similar estimates for water density in both normal and reverse fault systems. The minimum geothermal gradient was constrained to a narrow range of 40–50 °C/km. These results suggest that the normal and reverse fault systems developed at a similar depth within the seismogenic zone. This can be interpreted as a change between horizontal compression and horizontal extension occurring at a maximum depth of 3.8–7.5 km below the seafloor, assuming lithostatic fluid pressure. This 90° rotation of the principal stress could be controlled by the seismic cycle, as exemplified by the rotation of stresses that occurred after the Tohoku-Oki earthquake.

Abstract The Shimanto Belt is one of the most-studied ancient accretionary complexes in the world and yields an opportunity to investigate deep plate boundary processes including seismogenesis in a subduction zone. It is extensively exposed south-westwards from central Japan through the Kii Peninsula, Shikoku, Kyushu and out to the Ryukyu islands (Fig. 2d.1). In this chapter, we overview recent research progress on this classic Cretaceous-Neogene accretionary unit, emphasizing how its tectonic history informs us about the ongoing processes in modern subduction zones.