HR: 0800h
AN: V21A-0585    [Abstracts]
TI: Grain Growth Pinning and Strain Localization: Implications for Plagioclase Flow Laws and Strength of the Lower Crust
AU: * Mehl, L
EM: lmehl@mit.edu
AF: WHOI/MIT Joint Program, 77 Massachusetts Ave., 54-710, MIT, Cambridge, MA 02139
AU: Hirth, G
EM: ghirth@whoi.edu
AF: Woods Hole Oceanographic Institute, MS #8, WHOI, Woods Hole, MA 02543
AB: The rheology of the lower crust remains poorly understood despite its importance for understanding the strength of plates, driving forces for plate motion, and how strain is translated up to and/or down from the brittle upper crust. In this study we evaluate the applicability of experimental flow laws using insights from the analysis of naturally deformed gabbros from lower oceanic crust (Hole 735B, Southwest Indian Ridge). For comparison with experimentally-derived flow laws, we evaluate stress, temperature, and deformation mechanism in the SWIR samples. Strain is localized in discrete shear zones, and we assume a strain rate of 10^-12 to 10^-14 s^-1. Differential stress ranges from ~20 to 150 MPa, estimated by grain size piezometry with the empirical relationship of Twiss (1977). Syn-deformation temperatures are 800-950 °C, calculated by Fe-Mg-Ca exchange between recrystallized orthopyroxene-clinopyroxene pairs using QUILF (Andersen et al., 1993). Finally, deformation mechanisms were analyzed by measuring the lattice preferred orientation (LPO) with an electron back scattered detector (EBSD) on a scanning electron microscope. Monophase layers within the shear zones have LPOs indicative of deformation by dislocation creep at all grain sizes (down to 12.5 μm). Polyphase layers have random or very weak fabrics implying dominance of diffusion creep, even in samples with a relatively coarse grain size (well within the dislocation creep field). Plagioclase grains in the polyphase layers are smaller than those in monophase layers indicating that grain growth is apparently inhibited by clinopyroxene at grain boundaries. The variation in plagioclase grain size fits models that account for the size and abundance of a pinning phase. We conclude: 1) Flow law parameters for dry An₁₀₀ (Rybacki and Dresen, 2000) agree well with natural polyphase gabbro shear zones. This is a relatively `strong' flow law that predicts viscosities higher than olivine flow laws for a wet upper mantle. 2) Phase mixing can inhibit grain growth and promote shear zone deformation by diffusion creep field. Diffusion creep is dominant at lower stresses than dislocation creep, and thus these weak zones will continue to be exploited. These shear zones make up ~3% of the lower SWIR gabbro, which we calculate is at least 10% weaker than an equivalent gabbroic section without fine-grained (pinned) shear zones.
DE: 3625 Petrography, microstructures, and textures
DE: 3902 Creep and deformation
DE: 5120 Plasticity, diffusion, and creep
DE: 8159 Rheology: crust and lithosphere (8031)
DE: 8164 Stresses: crust and lithosphere
SC: Volcanology, Geochemistry, Petrology [V]
MN: Fall Meeting 2005