HR: 0800h
AN: T21B-0515 INVITED     [Abstracts]
TI: Natural Examples of Olivine Lattice Preferred Orientation Patterns With a Flow-Normal a-Axis Maximum
AU: * Mizukami, T
EM: mizukami@eqchem.s.u-tokyo.ac.jp
AF: Laboratory for Earthquake Chemistry, Univ. of Tokyo, Bunkyo-ku, Tokyo, 113-0033 Japan
AU: Wallis, S
AF: Nagoya Univ., Chikusa-ku, Nagoya, 464-8602 Japan
AU: Yamamoto, J
AF: Institute for Geothermal Sciences, Kyoto univ., Noguchibaru, Beppu, 874-0903 Japan
AB: Olivine lattice preferred orientation (LPO) due to ductile deformation is one of the main causes of mechanical anisotropy in the upper mantle and the patterns are useful to infer the direction of mantle flow from the seismic anisotropy in various settings. In subduction zones the mantle anisotropy near subduction boundaries suggests that olivine a-axes are arranged roughly perpendicular to plate motion. This anisotropy has been attributed to localized subduction-normal flow, applying a common type of olivine LPO with a `flow-parallel' a-axis maximum to the mantle. However, a recent deformational experiment provides an alternative interpretation that the B-type LPO with a `flow-normal' a-axis maximum can be developed in water-rich mantle above subducting slab. We document the widespread occurrence of B-type LPO in the Higashi-akaishi peridotite body, SW Japan, and examine the physical conditions in which it was formed. Our structural studies define four deformational phases in the Higashi-akaishi body (D1-D4) that are related to the tectonic evolution in the Cretaceous subduction zone at the Eurasian margin. The main deformational stage, D2, is associated with dynamic recrystallization of olivine to form porphyroclastic microstructure consisting of clear olivine neoblasts and porphyroclasts with abundant micro-inclusions. Parallel alginment of olivine neoblasts defines a stretching lineation (L2) and tectonic foliation (S2) and the D2 olivine LPO is identified as the B-type fabric with a-axes normal to L2, b-axes normal to S2 and c-axes parallel to L2. Micro-Raman spectroscopic analyses reveal that the syn-D2 micro-inclusions include hydrous minerals such as serpentine, indicating water-rich conditions for the D2 deformation. Garnet-orthopyroxene geothermobarometry applied to the D2 garnet peridotite reveals that the D2 stage was associated with the almost isothermal burial (700-800C, 2-3GPa). These D2 physical conditions in which the B-type LPO was formed are expected for mantle wedge near subduction boundaries. These results confirm the suggestion from the experimental work and imply that mantle flow above subduction zones is parallel to plate motion.
DE: 8030 Microstructures
DE: 8045 Role of fluids
DE: 8120 Dynamics of lithosphere and mantle--general
SC: Tectonophysics [T]
MN: 2004 AGU Fall Meeting