HR: 0800h
AN: T41D-1334    [Abstracts]
TI: Structural constraints on the evolution of Atlantis Massif based on results from IODP Expedition 304/305
AU: * Hirth, G
EM: ghirth@whoi.edu
AF: WHOI, Department of Geology and Geophysics, Woods Hole, MA 02543 United States
AU: Escartin, J
EM: escartin@ipgp.jussieu.fr
AF: Geosciences Marines (CNRS/IPGP), Case 89 - 4 Place Jussieu, Paris, 75252 France
AU: Grimes, C
EM: cgrimes@uwyo.edu
AF: University of Wyoming, Department of Geology and Geophysics, Laramie, WY 82071 United States
AU: Halfpenny, A
EM: a.halfpenny@liverpool.ac.uk
AF: University of Liverpool, Department of Earth and Ocean Sciences, Liverpool, L69 3GP United Kingdom
AU: Hayman, N
EM: hayman@duke.edu
AF: Duke University, Earth & Ocean Sciences, Duke University, Durham, NC 27704 United States
AU: Hirose, T
EM: hirose@erdw.ethz.ch
AF: ETHZ, Geologisches Institut, Zurich, 8092 Switzerland
AU: Michibayashi, K
EM: sekmich@ipc.shizuoka.ac.jp
AF: Shizuoka University, Institute of Geosciences, Shizuoka, 422-8529 Japan
AU: Suhr, G
EM: guenter.suhr@uni-koeln.de
AF: Universit„t K”ln, Mineralogisch-Petrographisches Institut, K”ln, 50674 Germany
AB: Structural observations made on the 1400 m section of core drilled at Site U1309 during IODP Expedition 304/305 provide new insights into deformation processes active during the formation and exhumation of Atlantis Massif (30o N Mid-Atlantic Ridge). In general, magmatic deformation fabrics are rather weak and were recorded in only 22 percent of the core. High strain crystal-plastic deformation was recorded in only 3 percent of the core. Crystal-plastic deformation is restricted to clearly defined, mostly granulite grade shear zones ranging in width from a few millimeters to a few meters. These shear zones are most prevalent in the upper 320 meters below seafloor (mbsf). Fiber-orientations on some late greenschist grade vein sets are subhorizontal (particularly below ~300 mbsf) indicating strike-slip movement during their formation. The upper 170 mbsf are marked by a high, but downhole-decreasing intensity of cataclasis, numerous nominally undeformed diabase intrusions, pervasive greenschist-grade alteration and a near-present day orientation of the paleomagnetic inclination. By contrast, below ~170 mbsf paleomagnetic data indicate significant rotation of the footwall at temperatures less than ~580oC (i.e., the Curie temperature). Models for the formation and exhumation of the Central Dome of Atlantis Massif need to account for the following observations. (1) There is little magmatic deformation associated with the igneous history. This observation suggests that the plutons were "shielded" from deformation owing to emplacement within the mantle lithosphere. (2) There is little evidence for deformation at amphibolite facies. Deformation fabrics in the shallow part of Holes U1309B and U1309D suggest that faulting responsible for exhumation initiated at lower amphibolite to greenschist grade conditions. These observations indicate that the footwall cooled considerably between pluton emplacement and exhumation. (3) At face value, the relative paucity of high temperature deformation fabrics indicates that models for oceanic core complexes that require broad viscous strain zones are not applicable to the formation of Atlantis Massif.
DE: 3035 Midocean ridge processes
DE: 3036 Ocean drilling
DE: 8004 Dynamics and mechanics of faulting (8118)
DE: 8012 High strain deformation zones
DE: 8030 Microstructures
SC: Tectonophysics [T]
MN: Fall Meeting 2005