Numerical modelling of porphyroclast and porphyroblast rotation in anisotropic rocks

The rotational behaviour of rigid objects in a weaker rock matrix during deformation has been the subject of many field, experimental and numerical modelling studies, often centred on the question whether objects rotate or not in non-coaxial deformation. With numerical studies gaining increasing popularity and importance we here provide an overview of the results published so far and provide new simulations. Originally, shape and orientation were investigated, while the emphasis shifted to rheology and slip between object and matrix in the nineties of the last century. Due to improved numerical techniques, anisotropic rheology has become the focus of most recent studies, indicating that it is a primary factor in the rotation behaviour of objects. We present new simulations investigating the role of anisotropy on different scales relative to the object, and show how this influences the rotation rate, as well as the inclusion patterns in case of syntectonically growing porphyroblasts. These simulations show that a variety of factors play a role to determine the rate and sense of rotation of objects. The variability of the inclusion patterns that can develop necessitates extreme caution in the kinematic interpretation of these structures when observed in the field.

► A review of numerical modelling of porphyroblast/-clast behaviour.
► New simulations of syntectonic porphyroblast growth in anisotropic media.
► Anisotropy can produce significant deviations of rotation rate compared to theoretical solutions.
► Deformation partitioning produces the development of shear bands and folds.
► Rotation behaviour and inclusion patterns are controlled by strain localisation.