Summary of Scientific Results: Evolution of the Accretionary Prism | Table of Contents


Stratigraphic Framework of Incoming Sediments
Results from Leg 190 help define several important spatial and temporal differences in the stratigraphy of the Shikoku Basin. These stratigraphic variations in turn influence patterns of deformation within the Nankai Subduction System. Beginning at the stratigraphic base, a volcaniclastic-rich facies overlies basalt basement at Site 1177 (Ashizuri Transect; Figs. 8A, 10). This facies differs from basal strata at Sites 808 and 1173 (Muroto Transect) in two important ways. First, the oldest strata at Site 1177 are early Miocene in age, rather than middle Miocene, and second, they contain considerable amounts of siliciclastic silt and sand in addition to volcanic sand and ash. The lower Miocene siliciclastic and volcaniclastic beds probably were eroded from the Japan Island arc and the Kyushu-Palau remnant arc. The age variation in basal sediment from west to east can be explained by proximity to the axis of the spreading ridge of Shikoku Basin. The ridge was active between 26 and 15 Ma, and seamount eruptions along the Kinan chain may have continued to 13—12 Ma. Temporal correlation of the Miocene ash beds remains uncertain. Given the existing biostratigraphic resolution, volcaniclastic beds along the Ashizuri Transect appear to be older than the thick rhyolitic tuff deposits that were cored at Site 808, so they may have been erupted from a different source.

Cores from Site 1177 include a package of lower(?) to upper Miocene siliciclastic turbidites with abundant woody organic matter. Deposition of correlative turbidites at DSDP Site 297 also began during the Miocene but continued into the Pliocene. The Miocene turbidites at Site 1177 were derived from a relatively large land mass, most likely southern Japan, and the dispersal system spread terrigenous sediment over a broad area of Shikoku Basin. During that same time interval, the seaward part of the Muroto Transect area sat above the basement high formed by the Kinan Seamounts. Higher seafloor relief evidently prevented the upslope deposition of turbidites.

Fine-grained hemipelagic deposits of Shikoku Basin display an unexpected characteristic of unusually high magnetic susceptibility. Magnetic susceptibility increases even within mudstone intervals between the Miocene turbidite packets at Site 1177. The mineralogic cause of this magnetic response in Shikoku Basin strata remains unknown.

A facies change from the lower to upper Shikoku Basin is defined at all sites by the absence to presence of ash beds containing recognizable volcanic glass shards. Most of the Pliocene volcanic ash beds probably were derived from the Honshu-Kyushu arc. Particle size, chemical composition, temperature, depth of burial, and time affect ash alteration and preservation. Thus, this unit boundary is time transgressive and sensitive to regional and temporal changes in the margin's thermal structure.

Shikoku Basin strata experienced diachronous burial during the Pliocene and Quaternary beneath an upward-coarsening and upward-thickening wedge of trench turbidites. The Nankai Trench wedge is thinner in the Muroto Transect area (above the basement high), but individual turbidites tend to be thinner and finer grained toward the southwest (DSDP Site 582). Quaternary sedimentation rates at Sites 1173 and 1174 were 600 and 760 m/m.y., respectively, more than one order of magnitude higher than the rate for Miocene turbidites at Site 1177 (35.5 m/m.y.). The main reasons for the higher rates of trench sedimentation include erosion from rapidly uplifting volcanic and metasedimentary terranes in the Honshu-Izu collision zone of central Japan and confinement of most sediment gravity flows to an axial dispersal system.

Summary of Scientific Results: Evolution of the Accretionary Prism | Table of Contents