HR: 0800h
AN: T41B-0574    [Abstracts]
TI: Viscosity Contrast between Gabbro and Peridotite: A Case Study From the Oman Ophiolite
AU: * Homburg, J M
EM: jmh2166@columbia.edu
AF: LDEO, 61 Route 9W, Palisades, NY 10964,
AU: Hirth, G
AF: Brown Univeristy, 324 Brook Street Box 1846, Providence, RI 02912,
AU: Kelemen, P B
AF: LDEO, 61 Route 9W, Palisades, NY 10964,
AB: The viscosity contrast between the lower crust and upper mantle controls key tectonic processes such as lithospheric coupling between the crust and mantle and the origin and transmission of plate driving forces. While experimental data provide a basis to predict viscosity contrast, pertinent geologic observations of synchronously deformed gabbro and peridotite are not well documented. We are investigating this problem by analyzing microstructures in deformed gabbronorite dikes (~ 10-50 cm) and host harzburgite from the Oman ophiolite. Outcrop scale observations demonstrate that deformation is highly localized in 30-50 mm wide mylonitic shear bands within the gabbronorite. We constrained the conditions of deformation using pyroxene thermometry, petrology, grain size piezometry and EBSD analyses of lattice preferred orientation (LPO). Syn-deformation temperature ranged from 800-850 °C, based on Fe-Mg-Ca exchange between recrystallized orthopyroxene- clinopyroxene pairs using QUILF (Andersen et al., 1993), and pressure was ~ 300 MPa based on structural thickness of the section. Interpretation of plagioclase grain size indicates stress was ~ 100 MPa during localized, low T deformation in gabbronorite shear bands. In contrast, olivine grain size in peridotite corresponds to a stress of ~ 10 MPa, probably during earlier, high T, distributed deformation. Plagioclase exhibits a weak LPO throughout the analyzed samples when compared to fabrics observed in mylonitized gabbros from the Southwest Indian Ridge that deformed at similar temperatures (Mehl and Hirth, in press). Plagioclase fabric strength does not vary significantly with variations in grain size (from ~ 20–50 μm) or presence of second phases (e.g. amphibole). These observations suggest that deformation of the gabbronorite involved a significant component of diffusion creep. Olivine fabrics in the harzburgite, indicative of deformation by dislocation creep, are stronger than the associated plagioclase fabrics, and are consistently oriented throughout the field area with the [100] axis at ~ 45° to the shear plane/dike orientation. Several observations indicate that deformation occurred at a relatively high water content. In the finest grained regions, amphibole appears to be a participant in deformation and plagioclase has a high anorthite content (An97) indicative of high water content in the magma. Our observations indicate that gabbroic rocks were weaker than mantle peridotite during low T hydrous deformation.
DE: 8120 Dynamics of lithosphere and mantle: general (1213)
DE: 8140 Ophiolites (3042)
DE: 8159 Rheology: crust and lithosphere (8031)
DE: 8163 Rheology and friction of fault zones (8034)
SC: Tectonophysics [T]
MN: 2007 Fall Meeting