HR: 12:00h
AN: T41F-07    [PDF]
TI: Grain Size Reduction and Deformation Mechanisms in Ultra-Fine-Grained Shear Zones From the Lherz Peridotite
AU: Pennock, G M
EM: gpennock@geo.uu.nl
AF: Department of Earth Sciences, Utrecht University, PO Box 80.021, Utrecht, 3508TA Netherlands
AU: * Drury, M R
EM: martynd@geo.uu.nl
AF: Department of Earth Sciences, Utrecht University, PO Box 80.021, Utrecht, 3508TA Netherlands
AU: Ave Lallemant, H G
EM: ave@rice.edu
AF: Department of Geology and Geophysics,Rice University, PO Box 1892, Houston, 77251-1892
AB: We have used SEM and Electron backscattered diffraction to investigate the petrology, microstructure and lattice preferred orientation (LPO) of narrow (5-10 mm wide), high strain, ultramylonitic shear zones exposed in the Lherz peridotite body, N. Pyrenees, France. The wall rock is a protomylonite with a normal olivine LPO ([100] parallel to the stretching lineation). Fractures occur in the wall rock and terminate at the shear zone margin. There is a systematic step-like grain-size decrease (2 mm, 0.1 mm, 0.02 mm, 0.01 mm, 0.005 mm) at the margin of the shear zone. In the shear zone grains are elongated with a good alignment of grain boundaries. The mineral assemblage consists of olivine, pyroxene, spinel and amphibole clasts in a matrix of olivine, pyroxene, spinel, magnesite (MgCO3). An olivine LPO occurs in the matrix similar to the type-B LPO reported by Jung and Karato(Science, 293,1460, 2001). The narrow ultramylonite zones may initially have formed as fractures that were subsequently infiltrated by CO2 derived from adjacent limestones. If this hypothesis is correct then the initial grain-size reduction probably involved some cataclasis with shear zones forming during crustal emplacement of the peridotite. Microstructures and mineral assemblages indicate that recrystallization and the reaction (ol + CO2 = opx + magnesite) have also contributed to grain size reduction. The decrease of matrix grain size into the shear zone may reflect increasing cataclastic strain, decreasing temperature during reaction or increasing stress during dynamic recrystallisation. The ultra-fine grain size and aligned grain boundaries suggest an important role for sliding along interfaces. Granular flow is the most likely deformation mechanism in such fine-grained rocks, unless deformation occurred at very high stress and strain rate. The presence of LPO in these ultra-fine grained shear zones is unexpected. The type-B LPO may be (a) a relict from early dislocation creep (b) produced by high stress-high strain rate dislocation creep or (c) could be characteristic of granular flow in aggregates with an elongated grain shapes.
DE: 3902 Creep and deformation
DE: 5104 Fracture and flow
DE: 5112 Microstructure
SC: Tectonophysics [T]
MN: 2003 Fall Meeting