HR: 12:05h
AN: T51H-07    [PDF]
TI: Localized Deformation Beginning more than 15 km Beneath the Mid-Atlantic Ridge, 14 to 16 N
AU: * Kelemen, P
EM: peterk@whoi.edu
AF: WHOI, MS 8, Woods Hole, MA 02543 United States
AB: ODP Leg 209 drilled 19 holes at 8 sites along the Mid-Atlantic Ridge from $14\deg$43 to $15\deg$ N. All sites were surveyed by submersible, and chosen to be $<$ 200 m from peridotite or dunite exposed on the seafloor; outcrops of gabbroic rock were also close to some sites. One of our primary goals was to constrain the mechanism of mantle upwelling, corner flow and exhumation of shallow mantle rocks. Drilling at Sites 1268, 1270-72, 1274 and 1275 penetrated 1075 meters, and recovered 354 m of core. At Sites 1268 and 1270-72 we recovered 25% gabbroic rocks and 75% residual mantle peridotite. Core from Site 1274 was mainly residual peridotite, while core from Site 1275 was mainly gabbroic. Most of the residual peridotites have nearly undeformed, protogranular textures. Orthopyroxenes are interstitial to olivine or even poikilitic. Rare, isolated clinopyroxene grains are also interstitial. Skeletal spinel grains have mm-scale extensions in three dimensions, with no discernable shape fabric. These textures are clearly different from porphyroclastic textures typical in ophiolites and fracture zone dredges. As described elsewhere at this meeting, impregnated peridotites contain olivine, 2 pyroxenes, plagioclase and spinel, and equilibrated at 0.54 GPa ($\pm$0.14 GPa, 2$\sigma$) and $1220\deg$C ($\pm$$16\deg$C, 2$\sigma$) [Kinzler \& Grove, JGR 92]. Melts entered the thermal boundary layer beneath the Mid-Atlantic Ridge at about 20 km [e.g., Sleep, JGR 75; Reid \& Jackson, MGR 82; Grove et al JGR 92; Cannat JGR 96; Michael \& Chase CMP 97; Braun et al., EPSL 00], and began to crystallize within impregnated peridotites and as discrete plutons intruding peridotite. Gabbroic rocks and peridotites from most sites underwent large tectonic rotations since aquiring remanent magnetization. At some sites, rotations may have exceeded $60\deg$ around near-horizontal axes parallel to the Mid-Atlantic Ridge. Such large rotations are unlikely to have been accomodated along a single fault, and instead blocks were progressively rotated along a series of fault systems. Sites 1270 (25% gabbroic) and 1275 (75% gabbroic) were drilled into large, low angle fault surfaces previously identified as oceanic core complexes. Deformation at Site 1270 is similar to most other Sites, while core from Site 1275 is the most weakly deformed. However, high pressure igneous assemblages indicate that some Site 1275 rocks were exhumed from depths of 20 km or more, as were residual peridotites at the other Sites. At all sites except 1269, 1273 and 1275, we recovered high temperature mylonitic shear zones (mainly with impregnated peridotite mineralogy, but also mylonitic gabbros), and intervals of low temperature fault gouge. Shear zones and faults are not all parallel; numerous, cross-cutting planes of localized deformation formed at $>$ $1000\deg$C to $<$ $100\deg$C, from $>$ 15 km depth to near the seafloor. These accommodated nearly all subsolidus deformation during corner flow and exhumation of residual peridotites (plus high pressure igneous rocks), in keeping with the inference that the thermal boundary layer in this region extends to at least 20 km. Penetrative, viscous deformation of blocks between shear zones and faults was minor. If this is a general process at slow-spreading ridges, then one would predict that shallow mantle anisotropy in the Atlantic would be less pronounced than in the Pacific, consistent with recent seismic data [FAIM Experiment, Gaherty, Collins et al, this session].
DE: 1020 Composition of the crust
DE: 1025 Composition of the mantle
DE: 3035 Midocean ridge processes
DE: 3040 Plate tectonics (8150, 8155, 8157, 8158)
DE: 3640 Igneous petrology
SC: Tectonophysics [T]
MN: 2003 Fall Meeting