Calcite fabric development during the spatial and temporal evolution of a high-strain zone
Abstract
High-strain zones commonly have complex deformation histories because of the spatial and temporal localisation of deformation during their development. Linking microstructural development to particular stages of this progressive deformation may provide a significant advance in our understanding of how high-strain zones develop but such studies are difficult unless the temporal framework of deformation can be constrained. The Gressoney Shear Zone (GSZ) in the Italian Alps is a kilometre-wide, calcite-dominated high strain zone characterised by top-SE movement related to crustal extension. Rb-Sr dating of micas within different fabrics recrystallised below their blocking temperature thus recording the time of deformation, show that the GSZ developed between c. 45 -- 36 Ma ago. This well constrained temporal and kinematic framework provides an excellent opportunity to investigate the microstructural evolution of high strain rocks. Electron Backscatter Diffraction (EBSD) has been utilised to: 1) characterise the effects of grain size on crystallographic preferred orientations (CPO); 2) establish the relationship of calcite deformation mechanisms to misorientations; and 3) compare deformation processes in naturally-deformed samples with experimental data. In most cases, samples record a similar CPO with (0001) lying parallel to the shear zone boundary. Coarser grains (>200 μm) record e-twinning but also the development of low-angle boundaries and core/mantle structures indicative of sub-grain rotation. Smaller grains (10--200 μm) show no evidence of twinning and generally record similar (0001) CPO to coarser grains. The samples with older mica ages exhibit more variability with significant differences in CPO. Within all samples, r- and f- planes show no preferred orientation and slip directions associated with these calcite slip planes are randomly distributed. Our data indicates considerably more complexity than experimentally deformed calcite and are not readily reconciled with the existing data. This may reflect the effects of recovery and recrystallisation in naturally deformed samples. Dominant glide systems are those with their rotation axes parallel to the predicted vorticity vector inferred from field data. This property could provide a means of testing the non-correspondence of tectonic transport direction and mineral lineation orientation in high strain zones predicted by models for general shear.
- Publication:
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EGS - AGU - EUG Joint Assembly
- Pub Date:
- April 2003
- Bibcode:
- 2003EAEJA.....8168B