Behaviour of spherical rigid objects and passive markers during bulk inhomogeneous shortening of a fluid

Abstract

Quantitative analysis of the kinematics of an experimental, gravity-driven, inhomogeneous time-dependent flow showed that the kinematics of the flow varied both temporally and spatially. The variation in kinematics of the flow included vorticity rotation sense reversals and the results provide possible insight into the kinematics of gravity spreading flows. The variation of the kinematics reflected the influence of the boundary conditions upon the flow. Spherical inclusions and thin, columnar, passive markers placed within the fluid prior to deformation also allowed the fluid flow to be used to examine some of the possible relationships that may form between porphyroblasts and passive foliations in inhomogeneous time-dependent flows. Comparison of initial and final orientations of several spherical inclusions suggested an apparent lack of rotation during deformation. Conversely, comparison of the initial and final orientation of passive strain markers placed in close proximity to these inclusions suggested a non-coaxial deformation environment rather than a coaxial environment. The apparent lack of rotation of these inclusions resulted from a reversal in their rotation sense during deformation of the system. The reversal in rotation sense of these inclusions was the result of time-varying levels of spin and shear induced vorticity. During deformation, fold-like structures were produced in the passive markers. This allowed the flow to be used to examine possible relationships that may form between porphyroblasts and layers passively folded during inhomogeneous time-dependent flow. The inclusion/passive-marker relationships across the folds suggest that relationships between porphyroblasts (with inclusion trails) and foliations in rock layers, passively folded by an inhomogeneous time-dependent flow, may contain similar relationships to folds that formed by layer buckling. Knowledge of the level of passive behaviour of the layers and boundary conditions during this type of deformation is essential for utilizing porphyroblast/foliation geometry to evaluate deformation history.