The Shatsky Rise is one of the largest oceanic plateaus, a class of volcanic features whose formation is poorly understood. It is also a plateau that was formed near spreading ridges, but the connection between the two features is unclear. The geologic structure of the Shatsky Rise can help us understand its formation. Deeply penetrating two-dimensional (2-D) multichannel seismic (MCS) reflection profiles were acquired over the southern half of the Shatsky Rise, and these data allow us to image its upper crustal structure with unprecedented detail. Synthetic seismograms constructed from core and log data from scientific drilling sites crossed by the MCS lines establish the seismic response to the geology. High-amplitude basement reflections result from the transition between sediment and underlying igneous rock. Intrabasement reflections are caused by alternations of lava flow packages with differing properties and by thick interflow sediment layers. MCS profiles show that two of the volcanic massifs within the Shatsky Rise are immense central volcanoes. The Tamu Massif, the largest (~450 km × 650 km) and oldest (ca. 145 Ma) volcano, is a single central volcano with a rounded shape and shallow flank slopes (<0.5°–1.5°), characterized by lava flows emanating from the volcano center and extending hundreds of kilometers down smooth, shallow flanks to the surrounding seafloor. The Ori Massif is a large volcano that is similar to, but smaller than, the Tamu Massif. The morphology of the massifs implies formation by extensive and far-ranging lava flows emplaced at small slope angles. The relatively smooth flanks of the massifs imply that the volcanoes were not greatly affected by rifting due to spreading ridge tectonics. Deep intrabasement reflectors parallel to the upper basement surface imply long-term isostasy with the balanced addition of material to the surface and subsurface. No evidence of subaerial erosion is found at the summits of the massifs, suggesting that they were never highly emergent.

The Shatsky Rise is an oceanic plateau consisting of three main massifs that were constructed in the Pacific Ocean by intense volcanism during the Late Jurassic to Early Cretaceous. In order to explore the sources of this oceanic plateau, we present noble gas compositions from fresh quenched glasses cored by ocean drilling at Integrated Ocean Drilling Program Site U1347 on the Tamu Massif and Site U1350 on the Ori Massif. The studied glasses are normal-type basalts, the most abundant of four types of basalts defined by trace element compositions. Possible disturbances of noble gas compositions by posteruption radiogenic ingrowth in aged glasses are assessed by extraction of gases from glass vesicles by stepwise crushing. The 3 He/ 4 He ratios in glasses from Site U1347 are lower than atmospheric 3 He/ 4 He, presumably owing to magma degassing coupled with radiogenic ingrowth of 4 He. In contrast, glasses from Site U1350 exhibit a limited range of 3 He/ 4 He (5.5–5.9 Ra). Uniform 3 He/ 4 He cannot be achieved if gases in glass vesicles have been affected by secondary contamination or posteruption radiogenic ingrowth. Therefore, the uniform 3 He/ 4 He in the normal-type basalts from Site U1350 is ascribed to their source characteristics. Relatively low 3 He/ 4 He among oceanic basalts suggests the involvement of recycled slab material in the source of the normal-type basalts. However, the depleted radiogenic isotope signatures are inconsistent with recycled slab being a distinct melting component. Instead, we propose that the normal-type basalts of the Shatsky Rise were sourced from a domain where subducted fertile material is dispersed in the mantle.

Abstract Abstract: In the Kimmeridge Clay Formation of the Wessex–Weald Basin, five organic-matter-rich intervals (ORIs), dated from Kimmeridgian–Tithonian times, can be correlated from distal depositional environments in Dorset and Yorkshire (UK) to the proximal environments in Boulonnais, northern France. The ORIs are superimposed on a meter-scale cyclic distribution of organic matter (OM), referred to as primary cyclicity, which is commonly interpreted to result from Milankovitch climate forcing. The present work addresses the distribution of redox-sensitive and/or sulfide-forming trace metals and selected major elements (Si, Al, and Fe) in Kimmeridge Clay shales from the Cleveland Basin (Yorkshire) and the Boulonnais cliffs with two objectives: (1) to determine whether the ORIs formed in similar paleoenvironments, and (2) to identify the mechanism(s) of OM accumulation. High-resolution geochemical data from primary cycles in the Yorkshire boreholes (Marton and Ebberstone boreholes) were studied and the results are then applied with lower-resolution sampling at the ORI scale in the Flixton borehole and Boulonnais cliff. Good correlations are found between total organic carbon (TOC) vs. Cu/Al and Ni/Al, but relationships between TOC and Mo/Al, V/ Al and U/Al are more complex. Cu and Ni enrichment is interpreted to have resulted from passive accumulation with OM in an oxygen-deficient basinal setting, which prevented the subsequent loss of Cu and Ni from the sediment. Mo and V were significantly enriched only in sediments where considerable amounts of OM (TOC > 7%) accumulated, the result of strongly reducing conditions and OM burial. At the scale of the Flixton ORIs, the samples with the highest Mo and V concentrations also show relative Fe enrichment, suggesting pyrite formation in the water column (combination of euxinic conditions and presumably low sedimentation rates). Samples from all ORIs were slightly enriched in Si relative to Al, interpreted as reflecting decreased sediment flux during transgressive and early-highstand systems tracts. The data show that in some ORIs, OM accumulation proceeded while productivity was not particularly high and sediments were not experiencing strong anoxia. In other ORIs, OM accumulation was accompanied by widespread anoxia and possibly euxinic conditions in distal settings. Though somewhat different from each other, all of the ORIs developed during episodes of reduced terrigenous supply (transgressive episodes). The common feature linking these contrasted episodes of enhanced OM storage (ORIs) must be the conjunction of productivity coupled with a decrease in the dilution effect by the land-derived supply, in a depositional environment prone to water stratification and, therefore, favorable to OM preservation and accumulation.

Abstract The Japan Trench off northern Honshu Island is associated with a backarc basin, a magmatic arc, a forearc basin, and a continental shelf that is submerged deeper than the usual shelf above 200 m water depth. The latest period of arc magmatism began to build the volcanic backbone of Honshu Island in late Oligocene to early Miocene time, and an intense Miocene period of volcanism produced green tuff, a thick complex of altered felsic to mafic volcanic rock interbedded with marine sediment. Other times of accelerated volcanism occurred in the Pliocene and Pleistocene (Cadet and Fujioka, 1980). The Pacific side of Honshu Island is built on an older continental framework encompassing the Kitakami massif, a large body of Mesozoic and Paleozoic rock overlain by a transgressive Cretaceous sediment sequence. In the adjacent submerged area is a deep basin filled with Cretaceous to Recent sediment (see diagrammatic section). The seaward flank of the basin is truncated by subaerial erosion as is the adjacent 35-km-wide section of Cretaceous rock that comprises the seaward part of the margin. The unconformity marking the top of the Cretaceous complex is prominent in many multichannel records across this margin, and it marks an abrupt change from rocks with a low to rocks with a high acoustic velocity. Paleogene sediment also fills part of the basin, but its seaward extent is limited. Neogene sediment with a basal Oligocene unit underlain in turn by the Cretaceous complex was penetrated at DSDP site 439 (see diagrammatic section), and geophysical