Evolution of polymineralic upper mantle shear zones

• Applicant: Jolien Linckens, Ph.D.
• Subject Area: Palaeontology
• Term: from 2017 to 2022
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 373601599

Strain in the upper mantle is inhomogeneously distributed. The deformation is localized in zones of high strain, i.e. shear zones. Shear zones control the deformation of the upper mantle and are essential for plate tectonics. Therefore, it is crucial to know how shear zones form and evolve. Due to microstructural and chemical variation in the upper mantle there is a viscosity variation, and strain localization can initiate in domains with a low viscosity. However, in order for strain to remain localized in shear zones a strain softening process has to occur. Strain softening can occur due to reactions (fluid, metamorphic or melt), grain size reduction, strengthening of CPO and shear heating. During the evolution of a shear zone different strain softening processes can occur simultaneously and/or sequentially. An important aspect during shear zone development is grain size reduction combined with phase mixing as ubiquitously observed in upper mantle ultramylonites. When phases are mixed, grain growth is suppressed, which helps to preserve shear zones. The present proposal is designed to characterize the feedback processes between phase mixing, reactions, grain size reduction and deformation and their importance for rheology and shear zone evolution in the upper mantle. Three upper mantle shear zones (Ronda, Southern Spain, Lanzo, North-West Italy and Erro-Tobbio, Italy) will be analysed in detail. Previous studies have suggested that the three shear zones are dominated by one of three possible phase mixing process; grain boundary sliding (Ronda), melt-rock reactions (Lanzo) and metamorphic reactions (Erro-Tobbio). A detailed microfabric (nano-CT, secondary electron microscopy (SEM) combined with electron backscatter diffraction (EBSD)) and chemical study (Electronprobe Microanalyzer (EPMA)) of samples of the three shear zones will be combined to quantify the feedback between phase mixing process, reactions, olivine grain size and strain softening in the upper mantle. The results will be combined to develop a model for the microstructural and rheological evolution of polymineralic upper mantle shear zones. The research works towards a more realistic viscosity model of the polymineralic upper mantle. The results will be of interest for geophysicists, experimentalists and structural geologists.

• DFG Programme: Research Grants