Depositional setting, provenance and tectonic-volcanic setting of Eocene-Recent deep-sea sediments of the oceanic Izu-Bonin forearc, NW Pacific (IODP Expedition 352)

New biostratigraphical, geochemical, and magnetic evidence is synthesized with IODP Expedition 352 shipboard results to understand the sedimentary and tectono-magmatic development of the Izu–Bonin outer forearc region. The oceanic basement of the Izu–Bonin forearc was created by supra-subduction zone seafloor spreading during early Eocene (c. 50–51 Ma). Seafloor spreading created an irregular seafloor topography on which talus locally accumulated. Oxide-rich sediments accumulated above the igneous basement by mixing of hydrothermal and pelagic sediment. Basaltic volcanism was followed by a hiatus of up to 15 million years as a result of topographic isolation or sediment bypassing. Variably tuffaceous deep-sea sediments were deposited during Oligocene to early Miocene and from mid-Miocene to Pleistocene. The sediments ponded into extensional fault-controlled basins, whereas condensed sediments accumulated on a local basement high. Oligocene nannofossil ooze accumulated together with felsic tuff that was mainly derived from the nearby Izu–Bonin arc. Accumulation of radiolarian-bearing mud, silty clay, and hydrogenous metal oxides beneath the carbonate compensation depth (CCD) characterized the early Miocene, followed by middle Miocene–Pleistocene increased carbonate preservation, deepened CCD and tephra input from both the oceanic Izu–Bonin arc and the continental margin Honshu arc. The Izu–Bonin forearc basement formed in a near-equatorial setting, with late Mesozoic arc remnants to the west. Subduction-initiation magmatism is likely to have taken place near a pre-existing continent–oceanic crust boundary. The Izu–Bonin arc migrated northward and clockwise to collide with Honshu by early Miocene, strongly influencing regional sedimentation

Seismic Studies of Subsurface Structure in the Ewa Coastal Plain, Oahu, Hawaii

Seismic studies using well-logging, refraction, and reflection methods were carried out in 1965 in conjunction with a core-sample drilling project in the Ewa Coastal Plain, Oahu, Hawaii. The seismic well-logging technique gave a complicated velocity-depth profile, with higher velocities associated with reef limestone and lower velocities associated with mud deposits. The seismic refraction method showed a simpler velocity-depth profile with only a few distinct layers. The seismic reflection method corroborated the simpler profile obtained with the refraction method. The two profiles were reconciled, as the complicated profile can be averaged out into the simpler profile. The averaging-out process can be applied to the whole sedimentary column so that a P-wave velocity value may represent the sedimentary layer at any given locality. However, no single value can be assigned as typical for sedimentary layers for the entire Hawaiian area. The velocity values depend upon the composition of the layer, which is made up of varying proportions of mud, reef limestone, and weathered basalt. Layer 2 of the oceanic crust in the Hawaiian area has a rather uniform character, with seismic velocities ranging from 4.8 to 5.1 km/sec, and thicknesses from 4 to 8 km

Crustal and Upper Mantle Structure of the Solomon Islands as Revealed by Seismic Refraction Survey of November-December 1966

A seismic refraction survey was carried out in the waters around the Solomon Islands during November and December 1966. Three ships were involved in the survey: two, stationed at the end points of the traverses, acted as recording ships; the third steamed along the traverses and dropped explosives. Reflection profiling and magnetic surveys were simultaneously carried out with the refraction survey. The results show that (a) on the Ontong Java Plateau to the northwest of the islands the crust is about 25 km thick with subnormal crustal velocities; (b) southwest of the New Georgia Islands the crust is thinner than normal and is underlain by a mantle with low velocity; (c) southwest of Bougainville Island the crust is generally of normal oceanic structure underlain by a mantle with low velocity; and (d) mantle material in the Slot is found at a depth of 14 km