8 research outputs found
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 IzuâBonin outer forearc region. The oceanic basement of the IzuâBonin forearc was created by supra-subduction zone seafloor spreading during early Eocene (c. 50â51 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 IzuâBonin 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 MioceneâPleistocene increased carbonate preservation, deepened CCD and tephra input from both the oceanic IzuâBonin arc and the continental margin Honshu arc. The IzuâBonin 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 continentâoceanic crust boundary. The IzuâBonin 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