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doi:10.2204/iodp.sp.340T.2011 AbstractIntegrated Ocean Drilling Program (IODP) Expedition 340T will conduct borehole logging in Hole U1309D, on the domal core of Atlantis Massif just west of the spreading axis of the Mid-Atlantic Ridge, 30°N. Seismic imaging shows considerable reflectivity within the footwall of this oceanic core complex. Results from IODP Expeditions 304 and 305 suggest two geologic reasons for such impedance contrasts: (1) variable alteration between lithologic units or (2) narrow fault zones with significant porosity and possibly pore fluids. We will obtain seismic and temperature logs throughout the hole. Any change in seismic velocity associated with altered olivine-rich troctolite intervals would favor the former hypothesis, whereas any temperature deviation within three previously mapped fault zones would favor the latter hypothesis. The new borehole data will guide design of a vertical seismic profile (VSP) experiment. If the hole is clear, the first phase of the VSP program (zero-offset shooting) will be carried out during this expedition. Ideally, wall rock magnetic susceptibility will also be logged to further document the characteristics and distribution of serpentinization in various depth intervals. During Expedition 305, borehole seismic data were obtained in the 30–800 mbsf interval, but instrument and weather problems precluded velocity measurements at the end of the expedition, when the hole had been deepened to 1415 mbsf. Postdrilling research has addressed many of the initial questions about average velocity structure and magmatic accretion during core complex formation. In the process, new interest in hydration and localized deformation processes within slow-spread lithosphere has arisen, as reflected at the May 2010 Chapman Conference on Oceanic Detachments. Although Expedition 340T work alone cannot answer all questions about what is responsible for the reflectivity seen, particularly at wide-angle source-receiver offsets, it does represent a crucial initial step that is required in order to plan optimal seismic experiments (e.g., walk-away VSP and new large-offset or three-dimensional multichannel seismic tests) that can fully document the potential pattern of lithospheric hydration. |
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