HR: 0800h
AN: T41B-0591    [Abstracts]
TI: Effect of strain localization on sub-rift upper mantle rheology and anisotropy
AU: * PALASSE, L N
EM: palasse@geo.uu.nl
AF: structural Geology group,Utrecht University, Faculty of Geosciences Utrecht University PO Box 80.021, utrecht, 3508ta, Netherlands
AU: Drury, M
EM: martynd@geo.uu.nl
AF: structural Geology group,Utrecht University, Faculty of Geosciences Utrecht University PO Box 80.021, utrecht, 3508ta, Netherlands
AU: Vissers, R
EM: rvissers@geo.uu.nl
AF: structural Geology group,Utrecht University, Faculty of Geosciences Utrecht University PO Box 80.021, utrecht, 3508ta, Netherlands
AB: In order to explain a possible weak rheology of the uppermost mantle suggested by work on the focal mechanism distribution in the continental lithosphere, we have investigated microstructures, LPO and water contents using FTIR of peridotites from present-day (Baja California) and ancient (North Pyrenean) rift zones. The aim of this study is link microstructural analysis with the rheology and the seismic anisotropy from small scale to lithosphere-scale processes. Infrared studies reveal a significant amount of water in the San Quintin xenoliths minerals (Peslier et al., 2002) while our infrared analyses done on the Pyrenean peridotites show that coarse olivines and pyroxenes have low water contents. Furthermore, microstructures and LPO orientations suggest that the North Pyrenean rift mantle had overall low water content with higher but modest water content in shear zones. The inferred anisotropy from olivine LPO is crucial to understand the observed shear wave-splitting (anisotropic pattern). In detail, we address the effects of ultra fine-grained shear zones development on the mechanical and thermal evolution of the upper mantle underneath the North Pyrenean Mesozoic rift and the contribution of inferred anisotropy from olivine LPO in Baja California with the shear wave splitting direct observations. Electron Backscatter Diffraction work on Pyrenean mylonites shows that LPOs of fine-grained minerals are scattered in contrast to coarse olivine grains with strong LPOs. These LPOs suggest a dislocation creep mechanism in large grains and grain size sensitive creep in the mylonite. The fine-grained shear zones may initiate as fractures and will weaken the lithosphere (H2O+CO2 infiltration) depending on the spatial distribution of the zones. Shear zone development will significantly reduce the seismic anisotropy of the lithosphere, as higher strains produce more fine-material with weak LPO. Detectable seismic reflections may be produced between host rocks and mylonites if the mylonite zones are wide enough. Both the upper-mantle of Baja California and shear zone of the Pyrenean rifts have wet rather than dry rheology. The fine-grained hydrated shear zones result in a very weak uppermost mantle.
DE: 7218 Lithosphere (1236)
DE: 8012 High strain deformation zones
DE: 8030 Microstructures
DE: 8033 Rheology: mantle (8162)
DE: 8162 Rheology: mantle (8033)
SC: Tectonophysics [T]
MN: 2007 Fall Meeting