HR: 11:05h
AN: T12B-04 INVITED    [Abstracts]
TI: Viscous and elastic anisotropy in partially molten rocks I: Experimental, field, and seismic observations
AU: * Holtzman, B K
EM: benh@ldeo.columbia.edu
AF: LDEO, Columbia University, 61 Rt 9W, Palisades, NY 10964, United States
AU: Takei, Y
EM: ytakei@eri.u-tokyo.ac.jp
AF: Earthquake Research Institute, Univ. of Tokyo, 1-1, Yayoi 1-chome, Bunkyo-ku, Tokyo, 113- 0032, Japan
AB: We present two papers on the causes and consequences of structural anisotropy in partially molten rocks. We show that viscous anisotropy can have significant effects on melt migration dynamics, with predictions that may be tested seismically by mapping the relationship between viscous and elastic anisotropy. This paper focuses on the observational motivations for the theory presented in the companion paper. End-member rheological behaviors of rocks in the Earth, elasticity and viscosity, are often approximated as isotropic. However, observations in the field, laboratory experiments, and seismic experiments all implicate significant anisotropy in rock properties. Nonethless, in geodynamics, definitive observational tests for viscous anisotropy have been difficult, so the approximation of isotropic viscosity is the norm. Structural anisotropy in mantle rocks comes from two main causes, 1) Lattice preferred orientation (LPO), or 2) Distribution of solid and/or fluid phases. 1) LPO is associated with dislocation creep. At high P-T, olivine a-axes generally align in the shear direction, but experiments at elevated pressure, stress and water fugacity show that each may affect LPO, presumably by affecting dislocation dynamics. Strain partitioning may also affect LPO. 2) Anisotropy due to phase distribution depends on the contrast in material properties between the two (or more) phases and their spatial distribution. Field observations of melt traces and dunites in ophiolites imply that melt is often aligned and segregated. In deformation experiments, spontaneous melt alignment and segregation during deformation is observed in a range of systems and deformation geometries. Basic theoretical aspects of this process are becoming understood. In experiments, a key observation is that melt appears to align before it segregates, which will cause a viscous anisotropy at the grain scale. In our companion paper, we show that this grain-scale anisotropy may be the cause of the instability that leads to longer-wavelength melt segregation. If such segregation and organization occur in the Earth, the viscous anisotropy (and reduction!) over a wide range of length scales should have macroscopic consequences for plate boundary dynamics. This hypothesis is becoming testable, using the granular model discussed in our companion paper to map between elastic and viscous properties using the same geometric description of melt. These models show that viscous properties are much more sensitive to anisotropy in melt than are seismic properties. This approach, combined with new seismic inversion methods and increased resolution, will enable new tests for the presence or absence of viscous anisotropy in the Earth.
DE: 0774 Dynamics
DE: 0798 Modeling
DE: 8120 Dynamics of lithosphere and mantle: general (1213)
DE: 8138 Lithospheric flexure
DE: 8160 Rheology: general (1236, 8032)
SC: Tectonophysics [T]
MN: 2007 Fall Meeting