Details

• Lecturer: Dra. María Gema Llorens,  Geociencias Barcelona-CSIC
• Date: 16 February 2021; 12:00 h
• Venue: Online http://bit.ly/37hW0o9
• Further information:Dr.Juan Alcalde (GEO3BCN-CSIC)

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Abstract

Deformation of anisotropic rocks, ice and metals mainly occurs by crystal-plastic mechanisms, where crystals dominantly deform by glide of dislocations in crystal slip planes. However, understanding and predicting the deformation of anisotropic rocks is not straightforward, especially because their behaviour under stress is very complex. This is mainly due to:

1. the variety of deformation mechanisms that operate in polycrystalline materials,
2. the range of deformation conditions (pressure, temperature, kinematics of deformation, etc.) that affect them, which in fact change the balance between deformation processes,
3. the variety of resulting anisotropy and microstructures, which in turn alter the mechanical properties of the deforming rock

Two fundamental scientific problems nowadays are:

1. how do different deformation mechanisms operate and interact in rocks?
2. how are the rheological properties at the micro- scale control the large-scale rock behaviour?

Ice is one of the most common minerals found on the Earth’s surface. It has been widely used as an analogue for quartz- or olivine-rich rocks, as all these minerals deform following similar mechanisms resulting in non-linear flow laws. The study of polar ice gives us valuable information as we can directly link the measured strain rate in ice sheets with the microstructures observed from ice drill cores and the seismic velocities estimated in ice sheets. By numerical simulations of ice microstructure evolution during deformation and recrystallisation, we demonstrate that the mechanical anisotropy at the crystal scale is transferred to the scale of the polycrystalline rock. Mechanical anisotropy has an enormous impact on understanding and predicting localisation of deformation at different scales (Jansen et al., 2016; Llorens et al., 2016), rock strength (Llorens et al., 2017) and seismic velocity estimations (Llorens et al., 2020). These are all key for forecasting ice-sheet flow to the oceans, and for understanding past climate changes and predicting future ones. Moreover, the lessons learned from the study of polar ice microstructures and how they control physical properties can be applied to other geological problems, such us understanding flow of mantle and lower crustal rocks as well as evaporites.

Further information

• Jansen, D., Llorens, M.G., Westhoff, J., Steinbach, F., Kipfstuhl, S., Bons, P.D., Griera, A. and Weikusat, I. 2016. Small-scale disturbances in the stratigraphy of the NEEM ice core: observations and numerical model simulations. The Cryosphere, 10, pp.359-370.
• Llorens, M.G., Griera, A., Bons, P.D., Lebensohn, R.A., Evans, L.A., Jansen, D. and Weikusat, I., 2016. Full-field predictions of ice dynamic recrystallisation under simple shear conditions. Earth and Planetary science letters, 450, pp.233-242.
• Llorens, M.G., Griera, A., Steinbach, F., Bons, P.D., Gomez-Rivas, E., Jansen, D., Roessiger, J., Lebensohn, R.A. and Weikusat, I., 2017. Dynamic recrystallization during deformation of polycrystalline ice: insights from numerical simulations. Philosophical Transactions of the Royal Society A: Mathematical, Physical and Engineering Sciences, 375(2086), p.20150346.
• Llorens, M.G., Griera, A., Bons, P.D., Gomez-Rivas, E., Weikusat, I., Prior, D.J., Kerch, J. and Lebensohn, R.A., 2020. Seismic anisotropy of temperate ice in polar ice sheets. Journal of Geophysical Research: Earth Surface, 125(11), p.e2020JF005714.