HR: 09:45h
AN: T51F-08    [Abstracts]
TI: Reanalysis of Geophysical Data With IODP Constraints at Atlantis Massif
AU: * Blackman, D K
EM: dblackman@ucsd.edu
AF: Scripps Institution of Oceanography, UCSD, 0225, La Jolla, CA 92093,
AU: Collins, J A
EM: jcollins@whoi.edu
AF: WHOI, MGG, Woods Hole, MA 02543,
AU: Searle, R C
EM: R.C.Searle@durham.ac.uk
AF: Durham University, Dept Earth Sciences, Durham, DH1 3LE, United Kingdom
AU: Karner, G D
EM: garry@ldeo.columbia.edu
AF: Exxon Mobil, Upstream Research Co, Houston, TX 77252-2189,
AB: The recovery of a dominantly gabbroic sequence during IODP Expeditions 304/305 at the oceanic core complex on the Mid Atlantic Ridge 30°N (Atlantis Massif, AM) motivated re-analysis of pre-existing geophysical data together with new borehole logging data. Re-assessment of prior interpretations of the domal high of AM as an ultramafic, flexurally uplifted footwall was warranted. Residual gravity anomalies corrected for seafloor topography and lithospheric temperature structure can be accounted for by a 3D gabbroic core, consisting of the central dome and southern ridge of AM, and a basaltic hanging wall. Mantle Bouguer anomalies are best fit by a model where the core terminates in the north at ~30°15'N where the corrugated detachment surface ends and a series of steep normal faults characterizes the median valley wall to the north. Prior OBS refraction analyses implied the existence of a 7.5 km/s layer (appropriate for fresh peridotite) at 600 m sub-seafloor and some along- strike variability in the shallower, lower velocity layers. This was consistent with known outcrops of serpentinized harzburgite on the southern ridge. A new analysis of the OBS data suggests somewhat greater along-strike variability in layer thicknesses but with velocities in the 500 m deep section constrained by the ray tracing generally less than 6 km/s, a mafic rather than ultra-mafic rock. This model is similar to a prior best-fit model of data along another NOBEL refraction profile to the east on the dome. Unfortunately, the shot spacing and short (2 km) NOBEL line length do not allow discrimination between the various refraction models. Analysis of refracted arrivals recorded on the MCS streamer using a new method (see Harding et al., Session S12, Fall AGU07) will provide the basis of an inversion for the shallow velocity structure of the central dome near IODP Hole U1309D. This tomographic model will be reported and, together with drilling results, the implications in terms of variability within the core of Atlantis massif will be discussed.
DE: 3035 Midocean ridge processes
DE: 3036 Ocean drilling
DE: 3045 Seafloor morphology, geology, and geophysics
SC: Tectonophysics [T]
MN: 2007 Fall Meeting