HR: 1340h
AN: T33A-1329    [Abstracts]
TI: DEVELOPMENT AND APPLICATION OF A NEW METHOD FOR CALCULATING THE STRENGTH OF LATTICE-PREFERRED ORIENTATION (LPO)
AU: * Skemer, P A
EM: philip.skemer@yale.edu
AF: Department of Geology and Geophysics, Yale University, 210 Whitney Ave., New Haven, CT 06511 United States
AU: Katayama, I
EM: ikuo.katayama@yale.edu
AF: Department of Geology and Geophysics, Yale University, 210 Whitney Ave., New Haven, CT 06511 United States
AU: Jiang, Z
EM: zhenting.jiang@yale.edu
AF: Department of Geology and Geophysics, Yale University, 210 Whitney Ave., New Haven, CT 06511 United States
AU: Karato, S
EM: shun-ichiro.karato@yale.edu
AF: Department of Geology and Geophysics, Yale University, 210 Whitney Ave., New Haven, CT 06511 United States
AB: Quantifying the strength of lattice-preferred orientation (LPO) has many practical applications in the study of deformation microstructures. We show that a commonly used indicator of fabric strength, the J-index (Bunge, 1982), which is calculated from the distribution of orientation data in Euler-space, is highly sensitive to numerical procedures. Thus, we conclude that the J-index is not a suitable indicator of fabric strength. We have developed a new method for determining the strength of LPO using the distribution of uncorrelated misorientation angles. This misorientation index (M-index) is defined as the difference between the observed distribution of uncorrelated misorientation angles and the theoretically determined distribution of uncorrelated misorientation angles for a random fabric. This method has been tested with orientation data from experimentally deformed olivine and is found to (1) correlate well with shear-strain and seismic anisotropy, (2) give robust measures of fabric strength for as few as 150 grains, and (3) exhibit minimal artifacts of its numerical calculation. We have applied the M-index to the study of recrystallized orthopyroxene bands in sheared lherzolite xenoliths from the Jagersfontein kimberlite, South Africa. Using the M-index, we can observe and quantify the progressive randomization of LPO inherited during dynamic recrystallization. This fabric randomization is used to infer a transition to grain-size sensitive deformation, indicating that strain-weakening of orthopyroxene may play an important role in the rheology of the upper mantle.
DE: 8030 Microstructures
DE: 8120 Dynamics of lithosphere and mantle--general
DE: 5112 Microstructure
DE: 3902 Creep and deformation
SC: Tectonophysics [T]
MN: 2004 AGU Fall Meeting