HR: 1340h
AN: MR33A-0159    [Abstracts]
TI: Modeling the Viscous to Plastic Transition in Mid-Ocean Ridge Gabbros: Implications for Crystallographic Preferred Orientation of Plagioclase and Seismic Anisotropy
AU: * Ildefonse, B
EM: benoit.ildefonse@dstu.univ-montp2.fr
AF: CNRS - Laboratoire de Tectonophysique, Université Montpellier 2, Montpellier, 34095 France
AU: Mainprice, D
EM: david.mainprice@dstu.univ-montp2.fr
AF: CNRS - Laboratoire de Tectonophysique, Université Montpellier 2, Montpellier, 34095 France
AB: Gabbroic rocks are a key component of our understanding of the physical properties (rheology, seismic velocities and anisotropy, ...) of the oceanic crust. We have established a database of over 50 samples of crystallographic preferred orientation (CPO) in gabbroic rocks from the oceanic crust using electron backscattered diffraction EBSD. The measured samples cover magmatic flow to high temperature plastic flow conditions. Plagioclase is the dominant mineral phase and forms the solid framework in most of the studied samples. The presence of strong CPO in magmatic rocks further complicates the interpretation of plasticity in deformed gabbroic rocks. On average, the magmatically deformed samples display higher fabric strength than the plastically deformed ones. The seismic anisotropy of plagioclase-rich rocks is generally weak. It depends on both the fabric strength and the plagioclase content of the rock. Most of our samples contains more than 50% plagioclase, and consequently exhibit Vp fast-propagation directions perpendicular to their foliations. The highest Vp anisotropies are found in magmatically deformed gabbro samples from the Oman ophiolite. We have undertaken a series of model calculations to predict the CPO of plagioclase during the viscous to plastic transition using the VPSC method using the slip systems documented by TEM 1/2[001](010), [100](001), [100](010), 1/2[201](11-2),1/2[111](10-1),1/2[1-11](011),1/2[101](10-1) plus some inferred slip systems from CPO studies, such as [100](011). We have taken into account the grain shape of euhedral magmatic crystals and varied the rheology from Newtonian linear viscous creep (stress exponent n=1) to power law creep with an n=3 or 5. The strength of CPO produced varies with n, being stronger for higher n. The type of CPO varies with critical resolved shear stress (CRSS) of the 1/2[001](010) and [100](0kl) slip systems. To activate 1/2[001](010) typical of plastic deformation CPO with [001] parallel to the lineation the [100](0kl) systems has to have a very high CRSS. Grain shape affects the strength of the (010) pole figures as the crystals tend to have long dimensions in the [100] and [001] directions and shortest dimension along [010]. The strength of the (010) pole figures directly correlates with Vp anisotropy.
DE: 5102 Acoustic properties
DE: 5112 Microstructure
DE: 5120 Plasticity, diffusion, and creep
DE: 8012 High strain deformation zones
DE: 8030 Microstructures
SC: Mineral and Rock Physics [MR]
MN: Fall Meeting 2005