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doi:10.2204/iodp.pr.340T.2012

Abstract

During Integrated Ocean Drilling Program (IODP) Expedition 340T we conducted borehole logging in IODP Hole U1309D on the domal core of Atlantis Massif just west of the spreading axis of the Mid-Atlantic Ridge, 30°N. Prior seismic imaging showed considerable reflectivity within the footwall of this oceanic core complex, and our new results document the geologic explanation for at least some of the impedance contrast. The dominantly gabbroic section, cored to 1415 meters below seafloor (mbsf) during IODP Expeditions 304 and 305, would not inherently contain density/seismic contrasts sufficient to reflect seismic energy. Expedition 340T aimed to test the hypothesis that highly altered intervals and/or fluid-bearing fault zones at depth might be responsible for these contrasts, thus allowing interpretation of the reflectivity patterns in terms of hydration pathways within young oceanic crust. Our results confirm that borehole velocity of altered olivine-rich troctolite intervals at Site U1309 is sufficiently distinct from surrounding rock (V_P ~0.5 km/s slower) to produce a multichannel seismic reflection given their several tens of meters thickness. Small dips in temperature (0.3°–0.5°C) were measured in borehole fluid adjacent to known faults at 750 and 1100 mbsf. These suggest that percolation of seawater along the fault zone is still active, not just a past process that produced the alteration documented in Expedition 305 core from these intervals. In addition to obtaining the first seismic coverage of the 800–1400 mbsf portion of Hole U1309D and reliable downhole temperatures, we acquired other standard logging data that are in excellent agreement with Expedition 304/305 borehole measurements. Vertical seismic profile station coverage at zero offset now extends the full length of the hole, including the uppermost 150 mbsf, where detachment processes are expected to have left their strongest imprint. Opportunistic sampling of a seafloor feature, now designated IODP Site U1392 and located a few meters from Hole U1309D, recovered fragments of possible cap rock that may provide information on processes within the exposed detachment.

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