“Deformation and anisotropy in crystalline materials”
– Montpellier, 14-20 March 2016 –
How do rocks deform at the micro- and at macro-scales? Can we find the deformation mechanism by analyzing crystal structures and orientations? What can this information be used for?
These were some of the basic questions addressed at the second CREEP short course. Similar to the first short course, lectures and practicals were spread over 5 days, followed by a weekend field trip to the Pyrenees where we could observe rock deformation in nature. Thanks to Andrea Tommasi, David Mainprice (both Geosciences Montpellier, FR) and Nicola De Paola (Durham, UK), we were led enthusiastically through lectures in which we learned about the principles behind crystal plasticity, brittle to ductile deformation and elastic and seismic properties of minerals and rocks.
Figure 1: Schematic illustration of the main processes acting during deformation in single crystal. These processes mainly depend on T, stress, strain-rate, fluid content and mineral structure. Courtesy by Andrea Tommasi.
The first two days of lectures focused on ductile deformation mechanisms at the crystal scale. A. Tommasi introduced a robust summary of crystal plasticity, which is the dominant process active in ductile deformation. The key point here is that in this regime, crystals can change their shape and orientation while maintaining continuity with their immediate neighbors. This is possible because dislocations (deformation mechanism: dislocation glide) and point defects (deformation mechanism: diffusion creep) always exist in the crystal lattice. Dislocations can be observed using transmission electron microscopes (TEM) and crystal orientations can be observed using scanning electron microscopes (SEM), the latter of which we able to see in action in Montpellier’s SEM laboratory. We learned how to calculate flow laws by manipulating experimental olivine data at different temperature conditions. Olivine is of particular interest since it is the most abundant mineral in Earth’s upper mantle. The resulting flow laws were then used to understand the dominant deformation mechanism(s).
Since deformation mechanisms depend on temperature, pressure, stress regime and strain rate (Fig. 1), minerals re-orient themselves when these conditions change and subsequently, the crystal preferred orientation (CPO)/lattice preferred orientation (LPO) change. The resulting orientation can be made visible using electron backscattered diffraction (EBSD) (Fig. 2a), which produces a map of individual crystal orientations and phases within a rock sample.
Figure 2: (a) Scanning Electron Microscope suitable for EBSD analysis. (b) MTEX processing and pole figure of CPO data in olivine and related seismic anisotropy calculation. AVs parameter quantifies shear waves splitting. Courtesy by Andrea Tommasi.
We learned how to treat this data using MTEX (a MatLab based software) with which we could calculate a trend in orientation of the various minerals in rock samples and produce stereogram pole figures (Fig. 2b). We then calculated the elastic properties and seismic velocities of samples. From the resulting velocity stereogram, it was also possible to compute shear wave splitting and anisotropy of rocks (Fig. 2b & Fig. 2c).
Figure 2c. MTEX tutorial with David Mainprice. Students work with example rock data to produce stereograms.
In a further exercise, we employed stereograms to interpret flow behavior beneath various rift settings (e.g. divergent or transform) and show how it relates to mantle convection. We also studied how seismic waves propagating through the deformed lithospheric mantle, at each rift setting, would reveal the anisotropy of crystals produced by mantle flow. Therefore, probing seismic anisotropy is an important tool for characterizing flow-patterns, deformation and the associated CPO development (Fig. 3).
Although this coursed focused predominantly on small-scale processes, Nicola De Paola introduced us to the basics of large-scale processes such as earthquakes. The link between these two fields may not be intuitive, but when rocks are no longer in the ductile regime, they can deform via brittle failure, producing faults. He also opened discussions on the ductile-brittle-plastic regime transitions and how faulting in the brittle regime controls the strength of the crust.
Overall, the second short course improved our understanding of different deformation mechanisms at the micro to macro-scale, the physical processes associated with them and the resulting microstructures and rheologies. The field trip was a wonderful opportunity to observe different stages of ductile processes in nature and to grasp the magnitude of deformation in certain shear zones.
Figure 3. Interpretation of seismic anisotropy in terms of mantle convection. The flow creates microscopic deformation in crystals and the averaged anisotropic properties of the media are seen by seismic waves passing through them. Courtesy by Andrea Tommasi. The picture on the right is taken and modified from Mainprice, Treatise on Geophysics, 2007.
Written by Danielle Brand, Angelo Pisconti, Jana Schierjott, Vivekendra Singh and Manuel Thieme
Please find below the courses given by Dr. Tommasi and Dr. Mainprice
Tommasi-Course1-3a , Tommasi-Course1-3b , Tommasi-Course1-3c , Tommasi-Course1-3d , Tommasi-Course1-3e , Tommasi-Course5a-CPO&Anis , Tommasi-Course5b-CPO&Anis , Tommasi-ShearZones-1 , Tommasi-ShearZones-2
D.Mainprice – MTEX Intro , D.Mainprice – Practical Rheology – Calcite , CREEP_DM_1_Intro_4thRankTensors_March2016 , D.Mainprice – Average Tensors
