Anisotropy of magnetic susceptibility as strain indicator in a fold-and-thrust belt sandbox model above décollements with frictional contrast

Magnetic fabric is used as strain indicator to provide further insights into different tectonic settings. Applying anisotropy of magnetic susceptibility (AMS) analysis on analogue models has shown to be a useful approach to understand details of deformation. Here we use this technique on shortened sandbox models to illustrate the relationship between rotation of grains and the influence of décollement friction in fold-and-thrust belts. Layers of sand were scraped to a thickness of 2.5 cm on top of high-friction sandpaper on one side and on low-friction fibreglass on the other side of the sandbox model. After shortening the model by 26%, samples were taken at the surface and at depth for measuring AMS. During shortening, above the high-friction décollement, a stack of imbricates was formed, which shows distinct clustering of the main principal magnetic susceptibility axes (k1 ≥ k2 ≥ k3) around the dip of the forethrusts. In contrast, AMS data above the low-friction décollement show a more heterogeneous AMS pattern due to complex structure development with box folds and fault bending. In general, the magnetic fabric can be differentiated between the initial model fabric in the foreland and a tectonic overprint within the hinterland. The AMS analysis show that strain increases with the development of structures towards the hinterland and additionally with depth, but differs between the two frictional décollements. At the transition zone between the two different frictional environments, a deflection zone developed where the trace of thrusts change trend causing additional rotation of sand grains within this zone perpendicular to main shortening direction, as reflected by the orientation of the k1 and k3 axes. Overall, the orientation of the AMS axes and shape of anisotropy depend on the structure geometry and movement, which are determined by the friction of the individual décollement beneath. Consequently, AMS in models indicates and describes the development of structures and reflects strain above different basal friction.