Introduction | Table of Contents

SCIENTIFIC REPORT

ABSTRACT

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

The décollement in both transects is localized along a stratigraphic unit (~5.9—7 Ma) within the lower Shikoku Basin. This horizon is correlative across both transects through its magnetic susceptibility.

The broad low-chloride zone in the lower Shikoku Basin unit, first identified at Site 808, progressively decreases across the Muroto Transect. This landward-freshening trend is due to both enhanced diagenetic reaction and fluid flow.

Our ideas of the tectonic evolution of the Muroto Transect have been dramatically changed. Accretion of a Miocene and Pliocene turbidite package forms the 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 east. Growth of the prism from the LTSZ to the toe of the slope (40 km) took place rapidly within the past 2 m.y.

Introduction | Table of Contents