New insights into deformation and fluid flow processes in the Nankai Trough accretionary prism: Results of Ocean Drilling Program Leg 190

The Nankai Trough accretionary prism is considered an end-member prism accr eting a coarse terrigenous sediment section in a setting with structural si mplicity, unparalleled resolution by seismic and other geophysical techniqu es, and large historic earthquakes. It therefore has been the focus of Ocea n Drilling Program (ODP) drilling to address several unresolved questions c oncerning accretionary processes and prism evolution. At six sites cored al ong two transects across the Nankai Trough accretionary prism during ODP Le g 190, lithostratigraphy and sediment diagenesis vary markedly. For the fir st time, reference sites at the seaward ends of the two transects defined t he stratigraphic framework of the accreting/subducting Shikoku Basin sedime ntary section. A thick section of Miocene turbidites and smectite-rich muds tone is present within the subducting section at the Ashizuri site. The tur bidites and mudstones are absent in the correlative section at the Muroto s ite; variations in lithology, mineralogy, and hydrologic properties of the incoming sediments probably contribute to the difference in prism wedge tap er between the two transects, while possibly controlling the seismic charac ter of the active plate boundary. The decollement in both transects is loca lized within a common stratigraphic unit (similar to5.9-7 Ma) within the lo wer Shikoku Basin facies. The decollement is also a major boundary for both physical and mechanical properties. A broad low-chloride pore water anomal y in the lower Shikoku Basin unit, first identified at Site 808, progressiv ely decreases in magnitude from prism to basin along the Muroto Transect. P hysical properties relationships, evidence for mineralogic changes in the s ediments, and pore fluid chemistry suggest that the chloride anomaly result s primarily from in situ diagenetic reactions in the sediments, possibly au gmented by flow of freshened fluid from depth. New constraints on stratigra phy and age of units along more landward parts of the Muroto Transect have dramatically changed our ideas about the tectonic evolution of the prism in this area. Growth of the seaward-most part of the prism took place very ra pidly, with 40 km of accretion within the past 2 Myr. This rate is at least 3 times greater than growth rates in a comparable prism.