ABSTRACT This study examined the thermal history of the Kumano forearc basin, as well as slope basin sediments and the accretionary prism, in the Nankai Trough, offshore Japan, using vitrinite reflectance measurements of sediments collected from Integrated Ocean Drilling Program Sites C0004, C0007, and C0009. We detected a paleothermal anomaly in the Kumano forearc basin at Site C0009 that has a 200-m-wide peak with a maximum temperature of 79 °C, ranging from 1000 to 1200 m below seafloor (mbsf). We concluded that thermal fluid is passing through the peak zone based on a curvilinear paleotemperature structure with a wide peak zone. Estimation of reflectance increase through vitrinite reaction promotion suggests that 80–100 °C thermal fluid had passed within at least 100 k.y., thus causing the anomaly. The thermal fluid upwelling could relate to thrusts and ancient splay fault activity near Site C0009. The thermal anomaly zone in the Kumano forearc basin at Site C0009 coincides with the currently active fluid conduit zone imaged on reflection seismic profiles. These results indicate that massive fluid circulation occurs spatially and temporally through thrusts in the subduction zone.

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.

Abstract We conducted a palaeomagnetic study on the Cenozoic sedimentary sequences of the Nankai Trough, recovered by the Integrated Ocean Drilling Program Expedition 322 in SE Japan. Sedimentary sections of Late Miocene age from the two subduction input sites (sites C0011 and C0012) recorded a pattern of magnetic polarity reversals that correlates well with the known magnetic polarity time scale. The polarity of characteristic remanent magnetization could be identified throughout the majority of the recovered cores of the two sites, following removal of a low-stability drilling-induced remanence. Most of the observed magnetostratigraphy from the characteristic directions is in good agreement with that to be expected from the stratigraphic position of the sequence deduced from the biostratigraphic data. Palaeomagnetic data from both shipboard and shore-based studies indicate changes in the rate of sedimentation from 9.5 to 2.7 cm/kyr at about 11 Ma, suggesting that some fundamental palaeoenvironmental change in the Shikoku Basin and/or significant tectonic event may have occurred in Late Miocene.

The relationships between mountain building, surface erosion, sediment supply to the trench, and growth of the accretionary prism are examined in southwest Japan and the Nankai Trough. Mountain building caused by the subduction of the Philippine Sea plate in the Nankai Trough and collision in central Japan has resulted in a rock uplift rate of ∼4 mm/yr. Surface denudation rates in the mountain regions are on the order of 3–4 mm/yr, resulting from the heavy rainfall of the Asian monsoon. This fact suggests that mountain building is almost in an equilibrium stage in which surface erosion and rock uplift balance each other, resulting in a constant altitude of ∼2000 m. Several drainage systems on land and in offshore submarine canyons enable the transport of eroded sediments directly into the Nankai Trough. Most of the terrigenous sediments supplied to the Nankai Trough are accreted in the subduction zone of the Philippine Sea plate. The accretion rate of the sediments in the eastern Nankai Trough is ∼1.68 × 10 7 m 3 /yr, which is consistent with the denudation rate of the Akaishi Mountains, contributing to the supply of 1.72 × 10 7 m 3 /yr of sediment in central Japan. The growth of the accretionary prism is an important controlling factor for the onset of large earthquakes in the Nankai Trough, because the hanging wall of the rupture area of the seismogenic zone is composed entirely of the accretionary prism. Repeated large earthquakes with a recurrence time of ∼100–200 yr, which are well recorded in the Nankai Trough, in turn promote surface erosion through consecutive landslides and tsunamis. Southwest Japan, with its extensive record of both erosional processes and seismic events, shows the intimate long-term relationships between tectonically driven mountain building, surface erosion under the Asian monsoon climate, growth of the accretionary prism in the trench, and the generation of large earthquakes.