1. Leg 190 Summary¹

Shipboard Scientific Party²

ABSTRACT

Six sites along two transects across the Nankai Trough accretionary prism were successfully drilled during Leg 190, satisfying most of the leg objectives. Two reference sites at the seaward ends of the Muroto Transect (Site 1173) and the Ashizuri Transect (Site 1177) define the stratigraphic framework of the accreting/subducting Shikoku Basin sedimentary section. A thick section of Miocene turbidites and smectite-rich mudstone is present within the subducting section at the Ashizuri site. The turbidites and mudstones are absent in the correlative section at the Muroto site; variations in lithology, mineralogy, and hydrologic properties of the incoming sediments probably contribute to the difference in prism wedge taper between the two transects, while possibly controlling the seismic character of the active plate boundary.

The décollement in both transects is localized along a stratigraphic unit (~5.9-7 Ma) within the lower Shikoku Basin facies. This horizon can be correlated across both transects through its magnetic susceptibility signature.

A broad low-chloride pore-water anomaly in the lower Shikoku Basin unit, first identified at Site 808, progressively decreases in magnitude from prism to basin along the Muroto Transect. It is unclear whether this landward-freshening trend is due to in situ diagenesis, lateral fluid flow, or a combination of the two.

Our ideas of the tectonic evolution of the prism in the Muroto Transect have dramatically changed. Accretion of a Miocene and Pliocene turbidite package forms a large thrust-slice zone (LTSZ). This event is associated with a shift from a transverse sediment transport system that delivered coarse material from the arc to the trench to an axial transport system that delivers sediment down the trench axis from the Izu collision zone to the east. Growth of the prism from the LTSZ to the toe of the slope (a distance of 40 km) took place rapidly within the past 2 m.y.