Strain localisation in the continental crust
Strain localization in shear zones is a fundamental process controlling deformation at plate boundaries and strength evolution of the lithosphere. However, the initiation of shear zones and the mechanisms of strain localisation are still unclear, and integration of field studies with experimental rock deformation and numerical modelling and has generated different models.
A common observation is that shear zone initiation and localization is pre-determined by the availability of geometric and mechanical discontinuities such as compositional boundaries and brittle precursors. Shear zones preferentially exploit such heterogeneities instead of spontaneously nucleate in an intact host rock. Regardless of the actual nucleation process, the development of localized shear zones requires some form of strain weakening as the deformation progresses (the material within the shear zone must be weaker than outside of it).
This project will study examples of shear zones that exploit the boundary between aplite dykes and granodiorite. This case study is enigmatic, in that numerical models fail to explain this particular type of nucleation and localisation. The outcrops are located in the Eastern Italian Alps on polished, de-glaciated surfaces at 2600 m of altitude. Hand specimens and thin sections are already available, but fieldwork is recommended to understand the geological context of shear zone development.
The goal is to understand the mineralogical, geochemical and microstructural changes responsible for strain weakening and localisation. To do so, thin sections will be examined with light- and scanning electron microscopy, with the electron microprobe and with image analysis software.