(Table 1) Magnetic properties of sediments from ODP Hole 166-1007B

A detailed rock magnetic study was conducted on the top six cores (above 57.5 meters below seafloor) at Site 1007. The calcareous nannofossil biostratigraphy assigns the studied intervals to an age ranging from early Pleistocene to Holocene. We found two significant conversion points in the Pleistocene based on isothermal remanent magnetization (IRM) acquisition and on alternating field demagnetization of anhysteretic remanent magnetization and IRM. These conversion points correlate strongly with changes in sedimentation rate and lithology. We conclude that the paleomagnetic signals reflect changes of rate of sediment supply and of paleoenvironment on the western edge of the Great Bahama Bank

Topographic Anaglyphs from Detailed Digital Elevation Models Covering Inland and Seafloor for the Tectonic Geomorphology Studies in and around Yoron Island, Ryukyu Arc, Japan

Anaglyphs produced using a digital elevation model (DEM) are effective to identify the characteristic tectono–geomorphic features. The objective of this study is to reinvestigate the tectonic geomorphology and to present novel tectonic maps of the late Quaternary in and around the Yoron island based on the interpretation of extensive topographical anaglyphs along the map areas that cover the inland and seafloor. Vintage aerial photographs are used to produce the 3-m mesh inland digital surface model (DSM); further, the 0.6-s to 2-s-mesh seafloor DEM is processed using the cloud point data generated through previous surveys. Thus, we identify anticlinal deformation on both the Pleistocene marine terrace and the seafloor to the north of the island. The deformation axis extends in a line and is parallel to the general trend of the island shelf. The Tsujimiya fault cuts the marine terraces, which extend to the Yoron basin’s seafloor. If we assume that the horizontal compressive stress axis is perpendicular to the island shelf, these properties can easily explain the distribution and style of the active faults and deformation. This study presents an effective methodology to understand the island arc tectonics, especially in case of small isolated islands

High-density surveys conducted to reveal active deformations of the upper forearc slope along the Ryukyu Trench, western Pacific, Japan

Abstract In this study, which was conducted to investigate present-day geological deformations occurring off the central Ryukyu Arc along the Ryukyu Trench (Okinawa-jima) in the vicinity of Japan, we obtained and examined dense, high-resolution, 2D grid (high-density) multichannel seismic reflection data. The forearc slope of the central Ryukyu Arc lacks a large forearc basin but has a steep narrow slope that dips to the southeast in the direction opposite to the plate convergence. However, while the seismic profiles obtained in our investigation do not reveal any active structures indicative of compressional stress in this convergent subduction zone, recent tectonic deformations are characterized by normal faults that strike perpendicular to the Ryukyu Trench axis. Examinations of high-density geological data show that the recent relative motion of the overriding plate has resulted in an active, arc-parallel extensional field. Furthermore, the results of our observations indicate two stages of extensional stress beneath the forearc slope of the central Ryukyu Arc that resulted from back-arc rifting in the Okinawa Trough

Long-distance magma transport from arc volcanoes inferred from the submarine eruptive fissures offshore Izu-Oshima volcano, Izu–Bonin arc

Long-distance lateral magma transport away from volcanic centers in island arcs is emerging as a common phenomenon where the regional stress regime is favorable. It should also be recognized as an important factor in the construction and growth of island arcs, and a potential trigger for devastating eruptions. In this contribution, we report on recent investigations into the magma dynamics of Izu-Oshima volcano, an active basaltic volcano with an extensive fissure system.Izu-Oshima is flanked by numerous, subparallel NW–SE trending submarine ridges extending up to 22 km to the NW and the SE from the central vent. During a recent submersible survey we have identified that these ridges are fissures which erupted basaltic spatter and lava flows. Furthermore, lavas are petrographically similar along each ridge, while there are noticeable differences between ridges. The subparallel ridges are observed to transect a series of seamounts – the Izu-Tobu monogenetic volcanoes – which are dispersed across this area of the rear-arc. However, there are consistent petrographic and chemical differences between these seamounts and the ridges, indicating that they have different magma sources, yet, they are essentially bounding each other in dive tracks. The most appropriate scenario for their development is one where the Izu-Tobu Volcanoes are fed by an “in-situ” underlying source, while the NW–SE ridges are fed by lateral magma transport from Izu-Oshima.Magma erupted from each ridge is of a consistent geochemistry along its length, but has experienced crystal fractionation and some plagioclase accumulation. Compositions of the ridges are also very similar to lavas from the subaerial cones that can be traced down the flanks of Izu-Oshima. This implies that pairs of subaerial cones and submarine ridges represent the locus of magma transport events away from the storage system beneath Izu-Oshima. Hence, magma from this crustal reservoir moved upward to feed the on-edifice cones, as well as laterally to supply submarine fissures and dykes to a distance of 22 km

ROCKMAGNETIC ANALYSIS OF SEDIMENTARY SEQUENCES FOR PALEO-ENVIRONMENTAL RESEARCH

Rockmagnetic analyses are conducted to examine their utility for paleo-environmental research on the following three sedimentary sequences : (1) samples from the Omma Formation of central Japan; (2) cores drilled off the Noosa Heads, Australia; (3) a core drilled from Honnor Submarine Valley, Antarctica. The sequences (1) and (2) show clear variation of magnetic properties consistent with the changes of depositional environment which correspond to the change of sedimentary circumstances such as Quaternary glacial-interglacial change. Three drilled cores of sequence (2) show high values in magnetic susceptibility during the time range from 14 to 18ka. The age of the sequence (3) is inferred to be beyond the Holocene, however, a preliminary result of magnetic susceptibility for the core shows no remarkable variation

Geological history of the land area between Okinawa Jima and Miyako Jima of the Ryukyu Islands, Japan, and its phylogeographical significance for the terrestrial organisms of these and adjacent islands

Abstract The modern and Late Pleistocene terrestrial fauna of Miyako Jima and adjacent islands (the Miyako Islands) in the southern Ryukyu Islands, southwestern Japan, includes some endemic taxa or genetically unique populations that exclusively have closest allies in the more isolated Okinawa Jima and adjacent islands (the Okinawa Islands) than in the Yaeyama Islands, which are located southwest of the Miyako Islands with much narrower intervening straits. Those taxa or populations include representatives of lineages that have physiologically highly limited ability for over-sea dispersal and the Miyako Islands are currently separated from the Okinawa Islands by at least 300 km of open water; therefore, the formation of this phylogeographical pattern is perplexing. In this study, we review the late Cenozoic geology of the Miyako Islands, southern Okinawa Jima, the Okinawa–Miyako submarine plateau (OMSP; a plateau located between Okinawa Jima and Miyako Jima), and the Kerama gap, which is a depression between the OMSP and Okinawa Jima. We then consider the origin of the modern and Late Pleistocene terrestrial animals, including a number of non-volant vertebrates on the Miyako Islands. Finally, we propose a new hypothesis (the OMSP hypothesis) to explain the enigmatic composition of modern and Late Pleistocene terrestrial vertebrate fauna of the islands. Southern Okinawa Jima was uplifted and emerged after ca. 2 Ma and was temporarily connected to the OMSP, which is likely to have emerged earlier than southern Okinawa Jima, to form a large island extending from Okinawa Jima to the Miyako Islands with a NE–SW direction of ~ 400 km. Subsequently, Okinawa Jima became separated from the OMSP when the Ryukyu Group—which is composed of Quaternary reef and associated fore-reef and shelf deposits—began to accumulate around the island at 1.7–1.4 Ma. During the interval from 2.0 to 1.7–1.4 Ma, numerous terrestrial animals, including flightless vertebrates, extended their distribution to the OMSP. Although the Miyako Islands repeatedly underwent complete submergence during deposition of the main part of the Ryukyu Group (1.25–0.4 Ma), they were uplifted and emerged to become a land area after ca. 0.4 Ma. In contrast, the OMSP subsided after ca. 0.4 Ma and was almost completely submerged after 0.27 Ma. During ca. 0.4–0.27 Ma, terrestrial animals migrated from the OMSP to the Miyako Islands