NUMERICAL MODELLING OF ASYMMETRIC BOUDINAGE

Abstract

Asymmetric boudinage structures are commonly used as shear sense indicators but their development is incompletely understood. This paper describes the influence of initial shape and kinematic parameters on the evolution of boudin trains using a numerical approach based on the finite difference code FLAC. Boudin trains are simulated as a series of competent objects embedded in a soft matrix subjected to general monoclinic ductile flow.Deformation of boudin trains includes heterogeneous stretching, rotation of boudins and offset along the neck regions. The sense of relative boudin offset is mainly influenced by the initial orientation of the interboudin plane in the boudinaging layer, while kinematic vorticity number of the flow and the orientation of the boudin train with respect to the flow extensional eigenvector, usually the shear zone boundary, also play a role. Viscosity ratio and aspect ratio influence the magnitude of offset along the neck regions and the amount of rotation of boudins but not the sense of slip and rotation on the interboudin plane. Knowledge of the orientation of the interboudin plane itself is insufficient to use asymmetric boudins as independent shear sense indicators. Details of the boudin geometry such as the sense of deflection of marker horizons along the interboudin plane must be used for this purpose.