Calcareous Nannofossil Biostratigraphy of the Nankai Trough

Abstract

ODP Leg 131 recovered nannofossil-bearing sediments from Site 808 in the Nankai Trough, western Pacific Ocean. Three holes were examined for nannofossils, 808A, 808B, and 808C. A total of 22 nannofossil events were recognized, of which 10 are used as zonal markers.The sediments recovered from Hole 808A (0-111.4 mbsf) contain Pleistocene nannofossil assemblages that are mostly well preserved. All samples from this hole were assigned to nannofossil Zone NN21.The nannofossil assemblages observed in Hole 808B (111.0-358.8 mbsf) are poorly to well preserved and were all assigned to the Pleistocene. The NN21/NN20 Boundary is placed at 230.7 ± 4.4 mbsf.Hole 808C was cored from 298.5 to 1327 mbsf and basalt was reached at 1289.9 mbsf. The sediments recovered range in age from the upper part of Zone NN20 of the Pleistocene to Zone NN5 of the middle Miocene and contain poorly to well-preserved nannofossil assemblages. The Pliocene/Pleistocene Boundary, marked by the FO Gephyrocapsa caribbeanica, was placed at 776.3 ±1.6 mbsf, and the Miocene/Pliocene Boundary is tentatively placed at 955.9 ±1.5 mbsf.The lowermost sediments above basement as well as a sediment sample intercalated between basalt flows are assigned to Zone NN5, with an age of approximately 15 Ma.Age estimates provided by nannofossils show that the sedimentation rate in the trench-fill deposits of the Nankai Trough was very high, 800-1350 m/m.y (0-0.46 Ma), whereas in the Shikoku Basin deposits (> 0.46 Ma), the sedimentation rate was much lower (24-200 m/m.y). These age estimates also provide an extrapolated age of approximately 15 Ma for the basaltic basement at Site 808.

Iodine isotope ratios and halide concentrations in fluids of the Satsuma-Iwojima volcano, Japan

“…Satsuma-Iwojima is related to the subduction of the Philippine Sea Plate under the Eurasian Plate in the Nankai Trough. The age of the oceanic crust at the trough is 15 Ma, while sediment ages under the decollement (ODP 808) where found to be between 5 and 15 Ma old (Olafsson, 1993). With an arc-trench gap of 150 km, an average subduction angle of 40 • and a convergence rate of 3.8 cm/yr (Urrutia and Morton, 1997), oceanic crust and sediments have increased in age by 5 Ma by the time they have reached the region under Satsuma-Iwojima.…”

“…Another important consideration in selecting Satsuma-Iwojima is the availability of data from ODP Site 808 in the Nankai Trough, roughly 500 km to the northeast. This drill site provides important data concerning the age of subducted sediments in this area (Olafsson, 1993) and on the composition of sediment pore fluids of the Philippine Sea Plate in the initial stages of subduction beneath the Eurasian plate (You et al, 1993, Kastner et al, 1993.…”

Clay Mineralogy of Mudstones from the Nankai Trough Reference Sites 1173 and 1177 and Frontal Accretionary Prism Site 1174

“…Age-depth relations at each site are sensitive to variations in sedimentation rate and thrusting at the prism toe, so nonuniform burial rates were input into the model. Our choices for age-depth inputs (Table T3) are constrained by paleomagnetic and nannofossil data (Shipboard Scientific Party, 1991Olafsson, 1993). The potassium concentration for each 10-m interval was taken from the nearest shipboard pore fluid measurement (Shipboard Scientific Party, 1991.…”

Preliminary report on the significance of magmatic enclaves in adakite genesis at Hakusan volcano, central Japan

“…Ujike, unpublished data). It is noted that the distribution of volcanoes that spewed adakitic rocks is restricted to the area where the young PHS plate (~ 15 Ma at the Nankai Trough off Shikoku Island; Olafsson, 1993) is subducting (Fig. 1).…”