Development of fabrics in experimental shear zones

PROJECT DESCRIPTION

The anisotropy of magnetic susceptibility (AMS) reflects the overall microstructural configuration and particular microstructural components of rocks (Machek et al., 2018). In this study, we try to identify what are the micro-scale processes that control the development of AMS during the formation of tectonic shear zones (e.g. deformation gradient and localization) and describe the relationship between AMS and applied strain. The apparatus for analogue modelling is designed as a large sandbox with variable width of the central zone of shearing (Kusbach et al., 2019). The movement of the box is provided by a stepper motor and the deformation rate can be adjusted by a gearbox. The experiments are created using colored plaster of Paris. Homogeneously admixed fine-grained magnetite is used to trace the material flow by measuring AMS fabrics. The final solidified model is cut in a regular grid, processed by image analysis and drilled in a 3D mesh of 1 cm wide samples, which are analyzed in terms of AMS. The used material displays a shear thinning thixotropic rheology and is capable of reproducing the strain localization in natural rocks.

Low-temperature shear zone within marble rocks in Portugal subjected to detailed study. The colored circles represent oriented drill holes (see  Kusbach et al., 2019 for detailed information).

Results of simple shear experiments using sheared and solidified plaster. (a) Experimental shear apparatus. (b) Overview of the model setup. (c) shows a plan view photograph of the experimental shear zone and (d) displays the patch of the drilled cores in detail. (e) An example of magnetic anisotropy data in the form of T parameter (related to shape of AMS ellipsoid).