2-D rotation behavior of a rigid ellipse in confined viscous simple shear: numerical experiments using FEM R. Taborda a, * , J. Antunes b , F.O. Marques c a Departamento Geologia and LATTEX, Faculdade Cie ˆncias, Universidade de Lisboa, Edificio C2, Piso 5, 1749-016, Lisboa, Portugal b Instituto Tecnolo ´gico e Nuclear, Applied Dynamics Laboratory, Estrada Nacional 10, 2686, Sacave ´m, Portugal c Departamento Geologia and CGUL, Faculdade Cie ˆncias, Universidade de Lisboa, Edificio C8, Piso 6, 1749-016, Lisboa, Portugal Received 14 May 2003; accepted 20 October 2003 Abstract The rotation behavior of rigid elliptical inclusions adherent to the viscous matrix in simple shear flow is investigated using a 2- D finite element numerical model. Several simulations were performed using different ratios (S) between shear zone width and inclusion’s least principal axis. A computational strategy was devised to calculate pressure and viscous forces exerted on the inclusion and deduce its angular velocity. For large S values, results agree remarkably well with theoretical predictions, while for small S values results deviate significantly from theory but are in agreement with previous analogue experiments. The numerical model provided detailed and coherent information about the physical parameters involved in the process (e.g., pressure, strain rate and vorticity distributions within the model). D 2004 Elsevier B.V. All rights reserved. Keywords: Numerical modeling; Elliptical inclusions; Non-slipping interface; Rotation; Viscous simple shear; Confined flow 1. Introduction The rotation behavior of rigid inclusions in mylo- nitic rocks has received a great deal of attention from structural geologists in the last decades, in order to understand the kinematics and mechanics within duc- tile shear zones. Theoretical and experimental studies on the rotation of rigid inclusions under simple shear flow have typically considered infinite width shear zones and perfect attachment of inclusion to the matrix (e.g., Jeffery, 1922; Muskhelishvili, 1953; Bretherton, 1962; Ghosh and Ramberg, 1976; Freeman, 1985; Passchier, 1987; Marques and Coelho, 2003). Howev- er, if flow inside ductile shear zones were simple shear and if Jeffery’s (1922) theory were directly applicable, then all rigid porphyroclasts should show signs of rotation. Because this is not the case in many natural ductile shear zones, recent studies have shown that there are important factors controlling the rotation behaviour of rigid inclusions besides the viscous sim- ple shear drag: (1) the flow inside the shear zone is a combination of pure shear and simple shear, or even general flow (Ghosh and Ramberg, 1976; Marques and Coelho, 2003) (see also Bretherton, 1962; Gay, 1968; Freeman, 1985; Jez `ek et al., 1994; Jez `ek et al., 1996). (2) The flow inside the shear zone is confined, because the ratio between shear zone width and inclusion’s least axis is small (Marques and Cobbold, 1995; Marques 0040-1951/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.tecto.2003.10.015 * Corresponding author. Tel.: +351-217500000; fax: +351- 217500064. E-mail address: rui.taborda@fc.ul.pt (R. Taborda). www.elsevier.com/locate/tecto Tectonophysics 379 (2004) 127– 137