HR: 0800h
AN: T41B-0571    [Abstracts]
TI: Strain Localization on Different Scales and their Related Microstructures - Comparison of Microfabrics of Calcite Mylonites from Naxos (Greece) and Helvetic Nappes (Switzerland)
AU: * Ebert, A
EM: ebert@geo.unibe.ch
AF: Institute of Geological Sciences, Baltzerstr. 3, Bern, 3012, Switzerland
AU: Herwegh, M
AF: Institute of Geological Sciences, Baltzerstr. 3, Bern, 3012, Switzerland
AU: Karl, R
AF: Institute of Geological Sciences, Baltzerstr. 3, Bern, 3012, Switzerland
AU: Edwin, G
AF: Muséum d'Histoire naturelle, 1 route de Malagnou, Genève 6, 1211, Switzerland
AU: Decrouez, D
AF: Muséum d'Histoire naturelle, 1 route de Malagnou, Genève 6, 1211, Switzerland
AB: In the upper crust, shear zones are widespread and appear at different scales. Although deformation conditions, shear zone history, and displacements vary in time and space between shear zones and also within them, in all shear zones similar trends in the evolution of large- to micro-scale fabrics can be observed. The microstructural analyses of calcite mylonites from Naxos and various Helvetic nappes show that microstructures from different metamorphic zones vary considerably on the outcrop- and even on the sample- scale. However, grain sizes tend to increase with metamorphic degree in case of Naxos and the Helvetic nappes. Although deformation conditions (e.g. deformation temperature, strain rate, and shear zone geometry, i.e. shear zone width and rock type above/below thrust) vary between the different tectonic settings, microstructural trends (e.g. grain size) correlate with each other. This is in contrast to many previous studies, where no corrections for second phase contents have been applied. In an Arrhenius-type diagram, the grain growth trends of calcite of all studied shear zones fit on a single trend, independent of the dimensions of localized large-scale structures, which is in the dm to m- and km-range in case of the Helvetic thrusts and the marble suite of Naxos, respectively. The calcite grain size increases continuously from few μm to >2mm with a temperature increase from <300°C to >700°C. In a field geologist's point of view, this is an important observation because it shows that natural dynamically stabilized steady state microfabrics can be used to estimate temperature conditions during deformation, although the tectonic settings are different (e.g. strain rate, fluid flow). The reason for this agreement might be related to a scale-dependence of the shear zone dimensions, where the widths increase with increasing metamorphic conditions. In this sense, the deformation volumes affected by localization must closely be linked to the strength of the affected rocks. In comparison to experiments, similar microstructural trends are observed. Here, however, shifts of these trends occur due to the higher strain rates.
DE: 3625 Petrography, microstructures, and textures
DE: 8000 STRUCTURAL GEOLOGY
DE: 8012 High strain deformation zones
DE: 8030 Microstructures
DE: 8031 Rheology: crust and lithosphere (8159)
SC: Tectonophysics [T]
MN: 2007 Fall Meeting