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 accreting a coarse terrigenous sediment section in a setting with structural simplicity, unparalleled resolution by seismic and other geophysical techniques, and large historic earthquakes. It therefore has been the focus of Ocean Drilling Program (ODP) drilling to address several unresolved questions concerning accretionary processes and prism evolution. At six sites cored along two transects across the Nankai Trough accretionary prism during ODP Leg 190, lithostratigraphy and sediment diagenesis vary markedly. For the first time, reference sites at the seaward ends of the two transects defined 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 within a common stratigraphic unit (∼5.9–7 Ma) within the lower Shikoku Basin facies. The décollement is also a major boundary for both physical and mechanical properties. 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. Physical properties relationships, evidence for mineralogic changes in the sediments, and pore fluid chemistry suggest that the chloride anomaly results primarily from in situ diagenetic reactions in the sediments, possibly augmented by flow of freshened fluid from depth. New constraints on stratigraphy 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 rapidly, 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.