ALBERT

Description: Numerous large igneous provinces formed in the Pacific Ocean during Early Cretaceous time, but their origins and relations are poorly understood. We present new geochronological and geochemical data on rocks from the Manihiki Plateau and compare these results to those for other Cretaceous Pacific plateaus. A dredged Manihiki basalt gives an 40Ar-39Ar age of 117.9+/-3.5 Ma (2 sigma), essentially contemporaneous with the Ontong Java Plateau ~2500 km to the west, and the possibly related Hikurangi Plateau ~3000 km to the south. Drilled Manihiki lavas are tholeiitic with incompatible trace element abundances similar to those of Ontong Java basalts. These lavas may result from high degrees of partial melting during the main eruptive phase of plateau formation. There are two categories of dredged lavas from the Danger Islands Troughs, which bisect the plateau. The first is alkalic lavas having strong enrichments in light rare earth and large-ion lithophile elements; these lavas may represent late-stage activity, as one sample yields an 40Ar-39Ar age of 99.5+/-0.7 Ma. The second category consists of tholeiitic basalts with U-shaped incompatible element patterns and unusually low abundances of several elements; these basalts record a mantle component not previously observed in Manihiki, Ontong Java, or Hikurangi lavas. Their trace element characteristics may result from extensive melting of depleted mantle wedge material mixed with small amounts of volcaniclastic sediment. We are unaware of comparable basalts elsewhere.

Description: The nodules used in this study were collected from two cruises in the western North Pacific by box coring (KH-79-4) and dredging (GH-78-1). After being dried at 110°C each sample extracted from the top, side and bottom of the nodule at various penetration levels were radiochemically analyzed for their Thorium isotopes contents by digesting about 100mg of a nodule sample without application of the successive separation treatment. Thorium in the solution was purified with an anion exchange resin and chemical separation techniques, and electrodeposited onto a silver disc from a hydrochloric acid and ethanol solution. The alpha activities of 228Th, 230Th and 232Th were counted with a spectrometer equipped with a surface barrier silicon detector. The chemical yield was determined by counting the 0 activity of 234Th added before the analysis. The yield ranged from 80 to 95%.

Description: The nodules used in this study were collected from two cruises in the western North Pacific by box coring (KH-79-4) and dredging (GH-78-1). After being dried at 110°C each sample extracted from the top, side and bottom of the nodule at various penetration levels, a chemical separation technique was used in a 6% acetic acid solution (fraction A). The residual was then submitted to a mixture 6% (IM) acetic acid and IM hydroxylamine hydrochloride (a reducing agent). The leachant is designated as fraction B and the dry residual (fraction B). On each part, acetylene-air flame atomic absorption spectrophotometry was used for the determination of Fe, Mn, Co and Ni. Aluminium was determined by acetylene-nitrous oxide flame atomic absorption spectrophotometry. Flameless atomic absorption spectrophotometry was used to determine Co, Ni and Cu. Dissolved silicate was determined by the standard colorimetric method of molybdenum yellow by adding boric acid as a masking agent for fluoride. The table presents the bulk composition as a recombination of the 3 parts analysed.

Description: An Mw 6.0 earthquake struck ~50 km offshore the Kii Peninsula of southwest Honshu, Japan on 1 April 2016. This earthquake occurred directly beneath a cabled offshore monitoring network at the Nankai Trough subduction zone and within 25-35 km of two borehole observatories installed as part of the International Ocean Discovery Program's NanTroSEIZE project. The earthquake's location close to the seafloor and subseafloor network offers a unique opportunity to evaluate dense seafloor geodetic and seismological data in the near field of a moderate-sized offshore earthquake. We use the offshore seismic network to locate the main shock and aftershocks, seafloor pressure sensors, and borehole observatory data to determine the detailed distribution of seafloor and subseafloor deformation, and seafloor pressure observations to model the resulting tsunami. Contractional strain estimated from formation pore pressure records in the borehole observatories (equivalent to 0.37 to 0.15 µstrain) provides a key to narrowing the possible range of fault plane solutions. Together, these data show that the rupture occurred on a landward dipping thrust fault at 9-10 km below the seafloor, most likely on the plate interface. Pore pressure changes recorded in one of the observatories also provide evidence for significant afterslip for at least a few days following the main shock. The earthquake and its aftershocks are located within the coseismic slip region of the 1944 Tonankai earthquake (Mw ~8.0), and immediately downdip of swarms of very low frequency earthquakes in this region, illustrating the complex distribution of megathrust slip behavior at a dominantly locked seismogenic zone.