HR: 0800h
AN: V21A-0593    [Abstracts]
TI: Exhumed Analogue of Crustal Ductile Deformation and Numerical Models of Shear Zone Deformation
AU: * Casey, M
EM: m.casey@earth.leeds.ac.uk
AF: School of Earth and Environment, Leeds University, Leeds, LS2 9JT United Kingdom
AU: Tatham, D
EM: d.tatham@earth.leeds.ac.uk
AF: School of Earth and Environment, Leeds University, Leeds, LS2 9JT United Kingdom
AB: The Laxfordian deformation of NW Scotland took place under amphibolite facies conditions with ductile crystal plastic deformation. The deformation is often localized into zones of intense shear with modification of rock microstructure and crystallographic preferred orientation. A previous study has demonstrated that the zones of intense deformation are zones of simple shear with variable movement directions. Field observations indicate that the deformation is thus highly heterogeneous and hence the strength of the ductile part of the crust cannot be determined by an assumed homogeneous deformation with a simple rheology. Instead it can be considered in terms of the bulk effects of strongly localized shear strain. Moreover, the rheology within the localized deformation zone will be highly variable in time and space due to strong microstructural and lattice preferred orientation development during strain. A quantitative estimate of the shear zone displacements and the occurrence frequency of shear zones within a unit crustal block will allow an approximation of the bulk finite strain for a given deformation event to be calculated. A lower bound micromechanical model of deformation temperature dependence, grain-size variation and the development of preferred crystallographic orientation has been developed and tested on observations from experimentally deformed calcite rocks in uniaxial compression and extension as well as in torsion. The model treats deformation from a combination of power law dislocation creep and a diffusion accommodated grain boundary sliding. The dislocation strain-rate is assumed to be proportional to the third power of the shear stress resolved onto the slip plane in the direction of the Burger's vector divided by a reference shear which determines the ease of operation of the slip system. The temperature dependence of dislocation slip-rate is the usual activation energy term. The diffusion accommodated mechanism is grain-size dependent and can be made anisotropic to account for the effects of oriented grain boundaries. The model is applied to the development of the rheology of deformed Scourie dykes in the Laxfordian shear zone at Badcall, Sutherland. Lattice preferred orientation measurements of dyke constituent minerals are analysed along a strain gradient from the wall rock into the zone of highest deformation. The development of an increasingly symmetric and ordered fabric and deviation from a random distribution indicate an increasing lattice preferred orientation development, and hence strain, into the shear zone. This is particularly well developed in the hornblende phase, and less so in quartz and plagioclase. Grain size reduction into the shear zone, relative to the original dyke material, support predictions from experimental results of lattice preferred orientations. These observations match the predictions of the micromechanical model in terms of progressive rock weakening with increasing shear strain.
DE: 3652 Pressure-temperature-time paths
DE: 8110 Continental tectonics: general (0905)
DE: 8159 Rheology: crust and lithosphere (8031)
SC: Volcanology, Geochemistry, Petrology [V]
MN: Fall Meeting 2005