Samples with vein structures were taken from Sites 787 and 793 in the forearc basin of the Izu-Bonin island arc off Aoga Shima and Sumisu Jima, respectively, between the present volcanic front and the outer arc high. The samples were studied by thin section, X-ray radiograph, and magnetometer; they are discussed with respect to the tectonic implication of the vein structures to the island-arc development.Vein structures are developed in finer, more clayey, preferentially radiolarian-bearing mudstone, subvertical to the bedding plane, which is mostly horizontal. The veins are restricted to certain horizons: in the upper Oligocene at Site 787 and in the lower Miocene at Site 793. The veins are filled with a dominant clay mineral (montmorillonite), which flowed into the vein when the fracture and concomitant stress drop occurred. Some clay mineral was deposited from the fluid that invaded the vein. Some veins might have occurred as hydraulic fractures. The shape, mode of occurrence, and other structural features indicate that the veins originated either as extension fractures or shear cleavages, and then were rotated by the following shearing parallel to the bedding. Sometimes the bedding-parallel slip planes are dislocated by the veins, and sometimes vice versa. This suggests that the vein formation and bedding parallel slip alternately occurred within the same stress environment.Vein attitude was measured by a magnetometer, after alternating field demagnetization; we interpret that they originally formed as subvertical planes, the trends of which average to N45W. The quantity of samples studied was small, but the trends suggest that the stress field for veining might have had a relative extensional stress axis that lay subhorizontally and trended generally northeast. This stress orientation might be attributed to either bending or normal faulting in the forearc basin, at a time when the arc trended northwest.
IntroductionThis chapter includes information on shipboard methods that will help the reader understand the basis for our preliminary interpretations and help the interested investigator select samples for further analysis. Authorship of site chaptersThe separate sections of the site chapters were written by the following shipboard scientists (authors are listed in alphabetical order; no seniority is implied):Principal results: Expedition 315 Scientists X-ray computed tomography: Lewis Lithology: Calves, Guo, Hashimoto, Underwood Structural geology: Behrmann, Byrne, Kanagawa, Lewis Biostratigraphy: Boeckel, Hayashi Paleomagnetism: Kanamatsu, Pares Inorganic geochemistry: Hulme, Tomaru Organic geochemistry: Saito Microbiology: Kaksonen Physical properties: Famin, Henry, Hirono, Kopf, Likos, Schmidt-Schierhorn, Zhu Core-log-seismic integration: Henry Reference depthsSeafloor depths and cored intervals below seafloor (core depth below seafloor [CSF]) are determined by drill pipe measurement. When using Integrated Ocean Drilling Program (IODP) Method A, core expansion lengths overlap and are not scaled. We sometimes had more core recovery than coring advance (usually 9.5 m), especially in shallow sediments. These core lengths are linearly compressed to adjust to the coring advance and are labeled CSF-B to indicate core depth below seafloor calculated using IODP Method B (compression). Drilling engineers prefer to use pipe length to present the depth as drillers depth below rig floor (DRF). This measurement can be converted to CSF by subtracting water depth and the height of the rig floor from the sea surface. In some contexts referring to logging-while-drilling (LWD) results (as in "Core-log-seismic integration"), logging depths are presented as LWD depth below seafloor (LSF), which is also based on drill pipe measurement below seafloor. Numbering of sites, holes, cores, sections, and samplesSites drilled by the D/V Chikyu are numbered consecutively from the first site with a prefix "C." A site refers to one or more holes drilled while the ship was positioned within 300 m of the first hole. The first hole drilled at a given site is assigned the site number modified by the suffix "A," the second hole takes the site number and suffix "B," and so forth. These suffixes are assigned regardless of recovery, as long as penetration takes place. During IODP Expedition 315, we drilled at Sites C0001 and C0002, both of which had been previously drilled during IODP Expedition 314. Therefore, hole names start with "E" for Site C0001 and "B" for Site C0002.Each cored interval is generally 9.5 m long, which is the length of a core barrel. Coring intervals may be shorter and may not necessarily be adjacent if separated by intervals that were drilled but not cored. The core depth interval assigned to an individual core begins with the depth below seafloor at which the coring operation began and extends to the depth at which the coring operation ended for that core.A recovered core is typically divided into 1.5 m long sections that ...
No abstract
No abstract available. <br><br> doi:<a href="http://dx.doi.org/10.2204/iodp.sd.8.01.2009" target="_blank">10.2204/iodp.sd.8.01.2009</a>
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