HR: 0800h
AN: T41B-0570    [Abstracts]
TI: Annealing Versus Deformation: How can we Track Down the Spatial Evolution of Localization Structures?
AU: * Herwegh, M
EM: herwegh@geo.unibe.ch
AF: Institute of Geological Sciences, Baltzerstrasse 1 + 3, Bern, 3012,
AU: Brodhag, S
EM: brodhag@geo.unibe.ch
AF: Institute of Geological Sciences, Baltzerstrasse 1 + 3, Bern, 3012,
AU: Ebert, A
EM: ebert@geo.unibe.ch
AF: Institute of Geological Sciences, Baltzerstrasse 1 + 3, Bern, 3012,
AU: Berger, A
EM: berger@geo.unibe.ch
AF: Institute of Geological Sciences, Baltzerstrasse 1 + 3, Bern, 3012,
AB: Shear zones often show long lasting deformation histories incorporating variations in the spatial and temporal distribution of strain. Based on seismic activity of shear zones, we know that localized deformation can occur episodic and may also vary spatially. In case of exhumed natural shear zones, the preservation of the corresponding microstructures might be problematic, since deformation under retrograde conditions and/or post- tectonic annealing often overprint the prior deformation stages. This is particularly the case for monomineralic fabrics, where only slight changes in the physical conditions, allow diminishing older structures. In this study, we present an approach using polymineralic carbonate mylonites with various contents of second phase minerals. Since, in second-phase controlled microstructures the calcite grain size hardly changes during annealing due to pinning, the microstructures in pure calcite aggregates are able to adapt their fabrics to the new conditions. In this way the Zener relation, i.e. the relation between matrix grain size and second phase size and volume fraction, allows an unambiguous discrimination between annealing and deformation structures. Based on the different microstructural evolution within a vertical drill core across one of the major thrust systems in the Helvetic nappes (Doldenhorn nappe, Swiss Alps), we present the variation in localization structures. It becomes evident, that with decreasing temperature, strain continuously localizes into very narrow zones focusing more and more at the thrust contact. At the same time, the older high temperature deformation fabrics evolved at farther distances from the active more localized zone are affected by annealing. In these annealed deformation zones, the calcite grain size and consequently the degree of grain growth does not increase linearly with distance to the thrust contact but rather shows abrupt changes. Particularly this lateral occurrence of finer grained but annealed thrust parallel zones suggest that deformation did localize episodically at different levels within the shear zone before becoming overprinted by annealing.
DE: 3902 Creep and deformation
DE: 5112 Microstructure
DE: 8011 Kinematics of crustal and mantle deformation
DE: 8030 Microstructures
SC: Tectonophysics [T]
MN: 2007 Fall Meeting