HR: 0800h
AN: T41D-1330    [Abstracts]
TI: Structure and Deformation Conditions at the 15deg 45N Oceanic Core Complex, Mid-Atlantic Ridge
AU: * MacLeod, C J
EM: macleod@cardiff.ac.uk
AF: School of Earth, Ocean & Planetary Sciences, Cardiff University, Cardiff, CF10 3YE United Kingdom
AU: Escartin, J
EM: escartin@ipgp.jussieu.fr
AF: Labo. de Geosciences Marines, Institut de Physique du Globe, Paris, 75252 France
AU: McCaig, A M
EM: andrew@earth.leeds.ac.uk
AF: School of Earth Sciences, University of Leeds, Leeds, LS2 9JT United Kingdom
AB: We have surveyed and sampled in detail the corrugated massif exposed at 15deg 45N on the Mid-Atlantic Ridge (ODP Site 1275) in order to understand better the structure and deformation conditions that operate at oceanic core complexes. Supplemented by 23 dredge hauls, we obtained 63 drill cores from the surface and flanks of the 15deg 45N massif. The cores were taken using the BRIDGE seabed rock drill, which takes metre-length samples with full geographical orientation. This offers a significant advantage for structural geological studies over dredge, submersible or ODP samples, which are non- or at best partially-orientated. Reorientating the BRIDGE drill cores has allowed us to constrain the kinematics of deformation of fault rocks that we recovered everywhere from the corrugated surface of the massif. Fault rocks are dominated by talc-, chlorite- and/or tremolite-schists, predominantly derived from an ultramafic protolith, indicating that deformation of mantle peridotite occurred in the greenschist facies. Fluid pressures and silica activity must have been high. High temperature deformation is all but absent, and is not associated directly with the detachment fault that evidently forms the surface of the 15deg 45N massif. Synkinematic emplacement of diabase dykes into the fault zone from an immediately subjacent gabbro pluton in the footwall implies that the detachment must have been active as a low-angle fault surface at very shallow levels directly beneath the ridge axis. Our observations do not support 'rolling hinge' models for core complex formation. Instead, strain localisation occurred in response to the weakening of a range of hydrous secondary minerals at a very early stage, possibly along some kind of alteration front, and was highly efficient. Deformation at deeper levels within the lithosphere, below the inferred alteration front, must have been accommodated by a different mechanism, one spatially unrelated to the observed detachment fault zone.
DE: 3035 Midocean ridge processes
DE: 3036 Ocean drilling
DE: 8011 Kinematics of crustal and mantle deformation
DE: 8012 High strain deformation zones
DE: 8031 Rheology: crust and lithosphere (8159)
SC: Tectonophysics [T]
MN: Fall Meeting 2005