Abstract High‐resolution seismic stratigraphy of the Yamato Basin, Japan Sea, was successfully established using core‐log‐seismic data integration. The construction of synthetic seismograms by the combination of physical properties and well‐log data from the Ocean Drilling Program (ODP) Site 797 was the key to accomplishing the high‐resolution seismic stratigraphy. To achieve resolution comparable with well‐log data and core lithology, single channel seismic reflection data taken from ODP underway geophysics were reprocessed, and then carefully compared with synthetic seismogram, core and well log profiles to identify seismic units. Ten seismic stratigraphic units were identified at the site, and seismic stratigraphic interpretation was successfully extended from the site to the nearby area along the Yamato Basin margin. The opal‐A/opal‐CT (biogenic silica/metastable diagenetic silica) boundary has different appearances at places from strong to weak, and mostly discontinuous. One of the significant results achieved from this study is clear distinction of the opal‐A/CT boundary from a very strong reflector, which appears at 22 m below the opal‐A/CT boundary. Through well‐log and physical properties characterization of the different units, resistivity was found to be the best indicator of diatom content and with gamma‐ray it also is an indicator of chert layers in the opal‐CT zone. Velocity is not greatly effected by diatom ooze in the opal‐A zone, however, it shows strong peaks and has an indirect relationship with gamma‐ray in the opal‐CT zone. Finally, successful correlation of Gamma‐ray Attenuation Porosity Evaluator density and resistivity peaks with strong seismic reflectors from upper and lower stratified layers may provide new information on the late Neogene paleoceanography of the Japan Sea in high‐resolution scale.
Research Article| July 01, 1986 Comment and Reply on “Two modes of back-arc spreading”: REPLY K. Tamaki K. Tamaki 1Geological Survey of Japan, Higashi, Yatabe, Ibaraki 305, Japan Search for other works by this author on: GSW Google Scholar Author and Article Information K. Tamaki 1Geological Survey of Japan, Higashi, Yatabe, Ibaraki 305, Japan Publisher: Geological Society of America First Online: 01 Jun 2017 Online ISSN: 1943-2682 Print ISSN: 0091-7613 Geological Society of America Geology (1986) 14 (7): 630–631. https://doi.org/10.1130/0091-7613(1986)14<630:CAROTM>2.0.CO;2 Article history First Online: 01 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn Email Permissions Search Site Citation K. Tamaki; Comment and Reply on “Two modes of back-arc spreading”: REPLY. Geology 1986;; 14 (7): 630–631. doi: https://doi.org/10.1130/0091-7613(1986)14<630:CAROTM>2.0.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract No Abstract Available. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.
To ascertain factors controlling melt production along a typical distal, ‘hotspot-interacting’ mid-ocean ridge, we investigated the extent and distribution of both plume-related and plume-unrelated basalt from the central Indian ridge (CIR) between 15°S and 20°S. Comprehensive geochemical data of fresh-quenched volcanic glasses and basalts were used. Variation of Sr, Nd, and Pb isotopic compositions and Nb/Zr, Ba/Nb, and Ba/La content were interpreted by mixing of three melt end members: the Indian depleted MORB mantle derived melt; radiogenic and enriched melt derived from source mantle for Rodrigues Ridge and the intermediate series of Mauritius Island (RE2, radiogenic enriched component 2); and radiogenic but depleted melt derived from source mantle for Gasitao Ridge (RD, radiogenic depleted component). On the basis of quantitative mantle melting and melt mixing model, results show that sources for RE2 and RD are geochemically distinct from those of the Réunion plume (RE1, radiogenic enriched melt component 1). Moreover, the geochemical variation of MORB of 15°S to 20°S is unrelated to contamination of the upper mantle by the Réunion plume. These results suggest strongly that plume-unrelated heterogeneity is widespread throughout the upper mantle. The chemical characteristics of RE2 are remarkably pronounced in basalt from the central portion of ridge segment 16 around 18°S, suggesting substantial magma production. The influence of RE2 decreases along with decreasing magma production to the north, and is only slightly identifiable in basalt from the northern part of segment 18. Although the influence of RE2 decreases somewhat to the south, basalts with extreme RE2 signature were produced in the center of segment 15 around 19°S, where magma production is high. In contrast to RE2, the geochemical signature of RD in basalt is geographically limited to two localities: the south end of segment 18 and the center of segment 15. However, these observations reveal that both RE2 and RD contribute strongly to magma production on segment 15. Results show that melting of ancient recycled plate materials with a low melting point regulates voluminous magma production along the CIR.
Abstract The Myojin Knoll is a submarine volcano that has a classically beautiful conical-shaped silicic caldera whose surface is covered by pumice. To determine the tectonic structure inside the caldera wall and beneath the caldera floor of this pumicious submarine volcano, we carried out a structural interpretation study using newly collected deep-penetrating multichannel seismic (MCS) reflection data. We also conducted a detailed velocity analysis of the MCS data, which facilitated the interpretation study. The results demonstrate that approximately 90% of the caldera wall is composed of pumiceous volcanic breccia. This finding supports those of previous researchers who, based on seafloor observations, single-channel seismic reflection, and gravity and geomagnetic data, concluded the Myojin Knoll is a knoll having a pumiceous caldera wall underlain by a pre-caldera rhyolitic stratovolcano edifice. We also determined a down-warping reflector approximately 800 m beneath the caldera floor. A seismic unit immediately above the reflector has a higher P -wave velocity than the pumice units and shows a chaotic seismic reflection pattern. We interpreted the reflector to be the bottom of a possible shallow magma chamber where the magma would undergo repeated expansion and contraction as a result of recurrent eruption activities.
Age of the Japan Sea has been controversial and its ambiguity makes an obstacle for the study of Cenozoic tectonic history of the Japanese Islands. Ages of the Japan and Yamato Basins of the Japan Sea were examined by three independent methods; stratigraphic consideration of the basin, age-depth relation, and age-heat flow relation. The stratigraphic constraints of the Yamato Basin are good by the presence of DSDP drilling holes in the central part and marginal part of the basin and bottom sampling at the outcrops of the lower sedimentary sequence. The age-depth relation of the open ocean is not applicable to the marginal seas, while the age-heat flow relation is valid in the marginal seas. Basement depths after sediment loading correction of the Japan Basin and the Yamato Basin were compared with those of other marginal basins in the Western Pacific whose ages have been well constrained. The age estimations of the Japan Basin by age-depth relation and age heat-flow relation show good coincidence. The age estimations of the Yamato Basin, however, show discrepancy among the three methods. The basement depth shows the age range of 6-0 Ma, the stratigraphic consideration shows that of around 10 Ma, and the heat flow data show estimated age older than 10 Ma. This discrepancy is due to thick accumulation of Neogene volcanonclastics (correlated to the Green Tuff Formation on Japanese Islands) which are represented by 3.5km/sec velocity layer on seismic refraction records and make an acoustic basement on seismic reflection record. As a preliminary conclusion, the age of both of the Japan Basin and the Yamato Basin is estimated to be roughly identical with the range of 30 Ma or older to 17 Ma or slightly younger. The identical age of the both basins contradicts the hypothesis of two stage of spreading of the Japan Sea; the older Japan Basin and the younger Yamato Basin. The presented age estimation is also against recently proposed clockwise rotation of the Southwest Japan at 15 Ma time with the duration less than 1 Ma which was documented by the paleomagnetic study and is believed to be a direct result of the opening of the Japan Sea. The discrepancy will be solved by a future more sophisticated understanding of tectonics of the Japan Sea.
The Ayu Trough lies on the southern boundary between the Philippine Sea Plate and Caroline Plate. Although this trough may be the best place to study the evolution and kinematics of the Philippine Sea Plate, the origin and evolution of this trough are poorly understood. Our geophysical and morphological surveys in the northern part of the Ayu Trough revealed that the trough shares morphological similarities with slow‐spreading mid‐ocean ridges. The seafloor ages and an average spreading rate of the trough were inferred from the average length of the ridge segments, distribution of sediment thickness, and basement subsidence. Based on the seismic section at 3°30′N, the opening of the trough started about 25 Ma with an average half‐spreading rate 4.1 mm/yr. This spreading rate is much slower than some previous estimates. The present magma production rate and activity of the trough is estimated to be even lower than the 25 m.y. average. The seafloor depth of the trough axis and relief of the axial valley are approximately twice as deep as that of active mid‐ocean ridges. The trough axis valley is covered with sediment in contrast with the axial valleys of active mid‐ocean ridges. Sedimentation at the trough axis shows that the spreading rate has been quite slow for the last 2.5 Ma.
