Applications of inclusion behaviour models to a major shear zone system: The Nordfjord-Sogn Detachment Zone in western Norway

Abstract

Rigid inclusion models have reached a stage where one should be able to use them to obtain quantitative values from ductile shear zones. We used natural data collected in three sites and combined analogue and theoretical modelling to assess vorticity, strain, nature of rigid inclusion/matrix interface and confinement in the large-scale, ductile Nordfjord-Sogn Detachment Zone (NSDZ) of the Caledonides of western Norway. Our study shows that: (1) the observed shape preferred orientation (SPO) at higher structural levels of the NSDZ at Site 1, Gjervika, can be explained by simple shear (pure shear/simple shear ratio S r = 0) associated with a slipping inclusion/matrix interface. (2) The observed SPO at deeper structural levels of the NSDZ at Site 2, near Sandane can be produced by simple shear associated with a significant amount of shortening across the shear zone ( S r ≈ 1), acting upon rigid inclusions in slipping contact with the enclosing matrix. (3) Observed back rotated boudins deeper in the NSDZ at Site 3, Biskjelneset, can form in confined flow associated with a considerable amount of shortening across the shear zone ( S r ≥ 0.4). (4) The observed tails of porphyroclasts indicate a minimum (at least local) strain of ca. γ ≈ 20. (5) The clasts in the studied shear zones strongly depart from Jeffery's model [Jeffery, G. B., 1922. The motion of ellipsoidal particles immersed in a viscous fluid. Proceedings of the Royal Society of London A102, 161–179]. The large scale extensional NSDZ under investigation shows evidence of strain partitioning: rocks vary from protomylonites to ultramylonites, and the simple shear and pure shear components are heterogeneously distributed. Therefore, we conclude that flow in the NSDZ was very heterogeneous both at the kilometre and the metre scale. However, the present study suggests that the amount of shortening across the shear plane throughout the NSDZ increases with depth, and the flattening component contributes to exhumation of the eclogite facies rocks in its footwall.