Kasahara, Junzo; Stephen, Ralph A. (2006): Leg 200 synthesis; a broadband seismic station in oceanic crust at the Hawaii-2 Observatory and coring into the Nuuanu Landslide. Texas A&M University, Ocean Drilling Program, College Station, TX, United States, In: Kasahara, Junzo, Stephen, Ralph A., Acton, Gary D., Calhoun, R. Scott, Haraguchi, Satoru, Hoskins, Hartley, Kittredge, Steven, Lustrino, Michele, Manz, Werner, Nakamura, Mikako, Natland, James H., Nielsen, Ingun, Paul, Heather, Schumann-Kindel, Gabriela, Sherman, Sarah, Sun, Yue-Feng, Wilson, John, Proceedings of the Ocean Drilling Program; scientific results; drilling at the Hawaii-2 Observatory (H2O) and the Nuuanu Landslide; covering Leg 200 of the cruises of the drilling vessel JOIDES Resolution; Honolulu, Hawaii, to San Diego, California; Sites 1223 and 1224; 16 December 2001-27 January 2002, 200, georefid:2007-002116

Two sites were drilled during Ocean Drilling Program Leg 200: the Hawaii-2 Observatory (H2O) site (Site 1224) and the Nuuanu Landslide site (Site 1223). In Hole 1224D we drilled basaltic basement to 59 meters below seafloor (mbsf) and installed a reentry cone with cemented casing for future installation of a borehole broadband seismometer. During operations at the H2O site, broadband seismometer records obtained from the shallowly buried H2O seismometer were analyzed. In the midocean, the traditional "double-frequency" microseism peak splits into two peaks associated with different kinds of noise sources. Microseism levels in the band approximately 0.2-0.5 Hz correlated with sea state and wind speed observed from the drillship, indicating that this noise is generated locally at the sea surface. Microseism levels in the band 0.1- 0.2 Hz correlate with high sea states impacting distant shorelines. Large quasi-periodic noise generated by whales and drilling-related noise were also identified. Also during Leg 200, a 3.5-kHz source attached to the camera sled on the drill string was lowered to near the seafloor to improve lateral resolution and depth penetration of the echo-sounding records. Acoustic pings were acquired on the ship's hull-mounted echosounding system and processed onboard. Hole 1224F, 20 m away from Hole 1224D, was drilled to 173.5 mbsf. Drilling in Hole 1224F recovered >100 m of approximately 46-Ma basaltic basement that comprises of three units. Although these three units are geochemically distinct, they are more highly evolved than most mid-ocean-ridge basalt (i.e., they have experienced a protracted history of cooling and crystal fractionation within the crust). Most basalts are <5% altered to secondary mineral assemblages of Fe oxyhydroxides, celeadonite, saponite, Ca carbonate, trace pyrite, and rare phillipsite and quartz. Ca carbonate formed within 20 m.y. of crustal formation and at low temperatures (4 degrees -11 degrees C). Logging and physical property measurements correlate well with lithology, geochemistry, and alteration of minerals in basalt. Fractures and low-temperature alteration greatly reduce VP, suggesting a cause of VP approximately 4.5 km/s in oceanic Layer 2B. Microbiological analyses discovered fungi in calcite veins in abyssal basalts, evidence that eukaryotic life existed in this extreme environment below the seafloor. The Nuuanu Landslide site, approximately 260 km northeast of the island of Oahu near the crest of the 500-m-high Hawaiian Arch, was also drilled. We drilled to 41 mbsf and identified eight sandy layers. The layers contain fresh and angular glass fragments typical of Hawaiian shield volcanoes. Degassed glasses are estimated to be subaerially erupted. Seven of the sandy layers are thought to be associated with the Koolau Landslide prior to 1.77 Ma. Of the four thicker sandy layers, it is unclear which are related to the Nuuanu Landslide. Analysis of the Site 1223 cores suggests repeated occurrences of collapsing Hawaiian volcanoes and debris flows to approximately 260 km distance.
West: 141.0000 East: 156.0000 North: 28.0000 South: 22.0000
Expedition: 200
Site: 200-1223
Site: 200-1224
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Provider: SEDIS Publication Catalogue
Data set link: http://sedis.iodp.org/pub-catalogue/index.php?id=10.2973/odp.proc.sr.200.001.2006 (c.f. for more detailed metadata)
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