Calcite Fabric Development During the Spatial and Temporal Evolution of a High-Strain Zone
Steve Reddy & Craig Buchan
Tectonics Special Research Centre, Department of Applied Geology, Curtin University of Technology, Perth, WA6845, Australia
Microstructural analysis provides important information regarding naturally occurring deformation processes within high-strain zones. However, 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 history 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 (>50km) related to crustal extension at T » 400 °C and P » 9 kbar. Rb-Sr dating of micas within different GSZ fabrics, which dynamically recrystallised below their blocking temperature, record the time of deformation and show that the GSZ developed over a period of c. 9 million years (Ma) between c. 45 - 36 Ma ago. This temporal and kinematic framework of the Gressoney Shear Zone provides an excellent opportunity to investigate the microstructural evolution of high strain rocks that developed over different stages of a progressive deformation history.
Electron Backscatter Diffraction (EBSD) analysis of calcite-dominated high strain rocks collected from the GSZ has been used to: 1) characterise the effects of grain size (10-400mm) on crystallographic preferred orientations; 2) establish the relationship of known calcite deformation mechanisms to misorientations analysis; and 3) compare deformation processes in naturally-deformed samples with previously published experimental data from x-ray diffraction data of bulk samples deformed by pure, simple and general shear. In most cases, samples record a similar crystallographic preferred orientation (CPO) with (0001) lying parallel to the shear zone boundary. Coarser grains (>200mm) record deformation by e-twinning but also show the development of low-angle boundaries and core/mantle structures indicative of sub-grain rotation recrystallisation. Zones of higher strain within coarser grains are also commonly deformed by r-twinning. Smaller grains (10-200mm) show no evidence of twinning and generally record similar (0001) orientations to coarser grains. However, in older samples there is more variability and this leads to significant differences in CPOs in samples that record different Rb-Sr mica ages. Within all samples, r- and f- planes show no preferred orientation and slip directions associated with these known calcite slip planes are also randomly distributed. These observations are not consistent with previously recognised CPOs in experimentally deformed calcite and our data indicates considerably more complexity. Our data are not readily reconciled with existing data from experimental work on high strain zones. In part, this may reflect the effects of recovery and recrystallisation in naturally deformed samples.