HR: 0800h
AN: T41D-1328    [Abstracts]
TI: Mechanisms of Uplift of Deep Gabbro Bodies at Slow-Spreading Ridges
AU: * Badham, S J
EM: sjbadham@uwyo.edu
AF: Department of Geology and Geophysics, University of Wyoming, Laramie, WY 82071 United States
AU: Cheadle, M J
EM: cheadle@uwyo.edu
AF: Department of Geology and Geophysics, University of Wyoming, Laramie, WY 82071 United States
AU: John, B E
EM: bjohn@uwyo.edu
AF: Department of Geology and Geophysics, University of Wyoming, Laramie, WY 82071 United States
AU: Swapp, S M
EM: swapp@uwyo.edu
AF: Department of Geology and Geophysics, University of Wyoming, Laramie, WY 82071 United States
AU: Dick, H J
EM: hdick@whoi.edu
AF: Department of Geology and Geophysics, Woods Hole Oceanographic Institution, Woods Hole, MA 02543 United States
AB: It is becoming accepted that the lithosphere may be anomalously thick beneath slow-spreading ridges and that gabbro bodies may be intruded into the mantle lithosphere at 5-20km depth. Less is understood about the mechanisms by which these bodies are subsequently uplifted and denuded at the sea floor. To address this problem, we examined a suite of 34 roughly evenly-spaced, oriented oxide gabbro samples from ODP Hole 1270B, located beneath an exposed fault surface on the eastern flank of the Mid-Atlantic Ridge (MAR) south of the 15°20' fracture zone. These rocks reveal a complicated deformation history within a now gently-dipping, approximately 50m thick, ductile shear zone cutting a gabbroic intrusion within mantle peridotite. The earliest stage of this history is revealed by the presence of thin screens of high-temperature mylonitic peridotite. These screens are intruded by oxide gabbro that subsequently underwent crystal plastic deformation. Lastly, the shear zone has been cut by a later fault that facilitated the exposure of the shear zone at the surface. Analysis of these samples via electron back-scatter diffraction (EBSD) has allowed the crystallographic orientation of individual mineral phases to be determined for the purpose of identifying the dominant deformation mechanisms (e.g., diffusion creep vs. dislocation creep) active in each phase. In addition to silicate phases (plagioclase, clinopyroxene, orthopyroxene, and olivine), EBSD analysis permits determination of the crystallographic orientation of opaque Fe-Ti oxides (ilmenite and magnetite). Crystallographic analysis of all phases present in this suite of samples suggests the dominance of dislocation creep in accommodating strain (for example, plagioclase (An45) exhibits (010)[100] slip, and ilmenite exhibits slip on the basal plane perpendicular to [0001]). When evaluated with the aid of the two-pyroxene and Fe-Ti oxide geothermometers and deformation mechanism maps of the individual phases, this information constrains the temperature and stress conditions attendant to deformation within the shear zone. The absence of a low-temperature, low-pressure overprint in the samples suggests that deformation of the gabbroic intrusion occurred at moderate depths in the lithosphere before being transported to the near surface (<50 meters below seafloor) by displacement on a later fault. Additionally, data from this study supports the suggestion that a relationship exists between the modal occurrence of rheologically weak Fe-Ti oxides and deformation intensity within oceanic gabbros. Recent paleomagnetic studies have revealed large (up to 90°) tectonic rotations at the MAR at 15° N. This rotation may be accommodated by domino-style normal faulting, whereby a normal fault that has ceased to be active after significant rotation is later cut by a new fault that more efficiently accommodates the local strain field. Subsequent generations of such progressively rotating faults may facilitate the transport of rocks from significant depths within the lithosphere to be emplaced near or at the seafloor, thus preserving the original high temperature deformation fabric developed within a deep shear zone. The rocks drilled at Hole 1270B represent one of these deep shear zones.
DE: 3040 Plate tectonics (8150, 8155, 8157, 8158)
DE: 3614 Mid-oceanic ridge processes (1032, 8416)
DE: 8118 Dynamics and mechanics of faulting (8004)
DE: 8120 Dynamics of lithosphere and mantle: general (1213)
DE: 8178 Tectonics and magmatism
SC: Tectonophysics [T]
MN: Fall Meeting 2005