HR: 13:55h
AN: T43D-02 INVITED    [Abstracts]
TI: Mechanisms of Ductile Shear Localization From Observations of Naturally Deformed Peridotites
AU: * Warren, J M
EM: jmwarren@whoi.edu
AF: Geology and Geophysics, Woods Hole Oceanographic Institution, Woods Hole, MA 02543, United States
AU: Hirth, G
EM: Greg_Hirth@Brown.edu
AF: Geological Sciences, Brown University, Providence, RI 02912, United States
AU: Kelemen, P B
EM: peterk@ldeo.columbia.edu
AF: LDEO, Columbia University, Palisades, NY 10964, United States
AB: Deformation in the oceanic lithosphere is largely confined to plate boundaries, indicating the importance of strain localization. To examine the role of ductile processes during shear localization, we present results from microstructural analyses of naturally deformed peridotites. We examine the roles of dislocation accommodated grain boundary sliding (DisGBS) and melt in promoting strain localization. Peridotite mylonites have been collected at many oceanic transform faults. In mylonites from the Shaka Fracture Zone, our observations indicate that olivine deformation was accommodated by a combination of DisGBS in coarser (10-100 μm) grained regions and diffusion creep in finer (1-10 μm) grained regions. Within olivine-rich domains, the preferred orientation of misorientation axes for misorientation angles <10° is consistent with [010] tilt walls resulting from olivine slip on (001)[100]. By contrast, at higher misorientation angles, misorientation axes do not exhibit strong crystallographic control, suggesting the influence of grain boundary sliding. DisGBS also leads to the mixing of different phases, providing a mechanism to limit grain growth. While multiple phases are not necessary for the transition to DisGBS and diffusion creep, the presence of multiple phases appears to promote permanent strain localization as a result of grain size pinning. For example, we observe that deformed dunites often have a coarser grain size than adjacent harzburgites. We suggest that a correlation exists between the modal abundance of secondary phases (pyroxenes and spinel) and the degree of grain size reduction. Another factor leading to strain localization is the presence of melt, as observed in a series of shear zones from the Josephine Peridotite in southwest Oregon. In one shear zone, strain gradients indicate a > 100-fold variation in effective viscosity over a distance of 25 meters, as inferred from the strain localization. Grain size variation across the shear zone is minor and cannot explain the viscosity variation. Instead, we suggest that focused melt transport in the form of a syn-deformational dunite, as inferred from field observations (Kelemen and Dick, 1995), promoted the strain localization. Based on experimental constraints for the effect of melt fraction on effective viscosity, these observations require melt contents within the shear zone of 10-20% during deformation.
DE: 3902 Creep and deformation
DE: 7250 Transform faults
DE: 8011 Kinematics of crustal and mantle deformation
DE: 8012 High strain deformation zones
DE: 8033 Rheology: mantle (8162)
SC: Tectonophysics [T]
MN: 2007 Fall Meeting