Applications of inclusion behaviour models to a major shear zone system: The Nordfjord-Sogn Detachment Zone in western Norway Fernando O. Marques a, * , Daniel W. Schmid b , Torgeir B. Andersen b a Faculdade de Cie ˆncias, Departamento de Geologia and IDL, Universidade Lisboa, Edificio C6, Piso 2, 1749-016 Lisbon, Portugal b Physics of Geological Processes, University of Oslo, P.O. Box 1048, Blindern, 0316 Oslo, Norway Received 10 February 2007; received in revised form 3 May 2007; accepted 15 May 2007 Available online 30 June 2007 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 ¼ 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 short- ening across the shear zone (S r z 1), acting upon rigid inclusions in slipping contact with the enclosing matrix. (3) Observed back rotated bou- dins 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. g z 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. Proceed- ings of the Royal Society of London A102, 161e179]. 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 hetero- geneous 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. Ó 2007 Elsevier Ltd. All rights reserved. Keywords: Inclusion orientation and shape; Vorticity; Slipping inclusion/matrix interface; Confinement; Shape preferred orientation (SPO); Ductile shear zone 1. Introduction Studies of ductile shear zones are crucial to understand the tectonic evolution at depth and the exhumation of deep-seated rocks to shallow levels in the lithosphere. Much effort has therefore been put into experimental and theoretical modelling of rock behaviour, in particular the behaviour of effectively rigid inclusions embedded in mylonites (see discussion below). One of the main goals of this research is to comprehend the behaviour of such inclusions in a variety of far-field load set- tings and to identify transient or, ideally, stable shape preferred orientations (SPO) and to relate these results to field studies where SPOs are often observed. Natural SPOs, together with other field data, potentially yield measures for quantities of key interest such as far-field vorticity and total strain. The rea- son for the particular importance of SPO is that only systematic trends in the data may be interpreted to give meaningful inter- pretations; in cases where no strong SPO is developed this is not possible. The main problem with the interpretation of SPOs was, until recently, that they generally show inclusion stabilization at shallow positive angles (cf. Fig. 1), which could not be explained by theories that assume an isolated, * Corresponding author. Tel.: þ351 21 750 0000; fax: þ351 21 750 0064. E-mail address: fomarques@fc.ul.pt (F.O. Marques). 0191-8141/$ - see front matter Ó 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.jsg.2007.05.008 Journal of Structural Geology 29 (2007) 1622e1631 www.elsevier.com/locate/jsg