Numerical modelling of rock deformation: conglomerates and mechanical anisotropy

Abstract

Deformation of conglomerates has received a range of attention in structural geology. Of particular interest is the study of deformation processes, the rock rheology and the tectonic evolution. A range of studies based on field observations, analogue and rock experiments and on numerical modeling have revealed that a variety of parameters, such as the pebble shape, the material properties, the concentration and interaction of pebbles, can affect the deformation of the conglomerate. Despite these efforts it is not yet well understood how the concentration of pebbles and the interaction of neighbouring pebbles affect the deformation of conglomerates. Internal structures of pebbles in deformed conglomerates, such as folds, have been used to recognize the deformation process. Folds in pebbles can either originate in deformation processes of the source rock, prior to the formation of conglomerates, or during the deformation of the conglomerate. It is not clear how and when folds within pebbles develop during conglomerate deformation. Although mechanical anisotropy is a factor that can affect the development of structures, such as folds, only a few studies addressed its influence in numerical modelling. In this study we coupled the Viscoplastic Fast Fourier Transform Method (VPFFT) with the numerical platform ELLE and used it to simulate the deformation of conglomerates, and the development of folds and other structures in an anisotropic matrix. Our results suggest that pebbles in deformed conglomerates can behave as rigid, deformable and passive inclusions depending on both the viscosity ratio and their concentration. Changing the pebble concentration also changes the transition viscosity ratio between the deformation regimes. The effect of increasing pebble concentration is similar to a decrease of viscosity ratio between pebbles and matrix, and vice versa. Clusters of closely spaced pebbles can behave as single objects. A mean Rf-phi plot is suggested in order to gain an estimate of the pebble deformation behaviour and the amount of strain in case of permanently stretching pebbles. Deforming layered pebbles may develop internal folds. Internal folding is facilitated by a layering initially at a narrow range of steep angles relative to the shear plane, sufficiently thin internal layers to achieve fold wavelengths smaller than the diameter of the pebble, and a large area fraction of pebbles. It furthermore requires a narrow range of viscosity contrasts between pebble layers and matrix to allow enough strain to develop folds, but still keep the pebble recognisable as such. Using the mean Rf-phi plot, it is suggested that the deformed conglomerates of the Hutuo Group in the Wutai mountains, North China Craton had a viscosity ratio of 5 to 8 in case of a linear rheology (n=1) and of 2 to 5 in case of a power-law rheology (n=3) and underwent a simple shear strain of about six. The difficulty in achieving internal folds within pebbles may explain the scarcity of internally folded BIF-pebbles in deformed conglomerates at the base of the Hutuo Group. Few pebbles with folds do not necessarily indicate a previous deformation event, but may have been formed during deformation of the conglomerate itself. This may change the tectonic interpretation of the rock significantly, as it removes the need for a whole cycle of burial, metamorphism, deformation and exhumation preceding the deposition of the conglomerates. The results of our numerical simulations indicate that mechanical anisotropy can play a key role on the development of folds, mantled clasts and C' shear bands. Folding in an anisotropic matrix develops in similar-type folds or crenulations that do not decay away from the competent layer. Fold hinges align to form an axial-planar crenulation cleavage. In case of mantled clasts embedded in an anisotropic matrix, rotation of the clast is inhibited and thus a σ-clast forms. C' shear bands forms in all models of anisotropic composite material. Mechanical anisotropy leads to a distinct strain and strain-rate localisation in homogenous, anisotropic materials. The shear rate localizes in narrow shear bands, depending on the magnitude of anisotropy and the stress exponent.