Development of flanking structures in layered gneissic rock: Insights from numerical modelling

Abstract

Flanking structures defined by dragged planar fabrics alongside of quartzo-feldspathic veins in gneissic rocks are very common. Varieties of mesoscale flanking structures were observed in parts of the Chhotonagpur Granite Gneissic Complex (CGGC), Eastern India. Alternate thin layering of quartzo-feldspathic and mafic minerals in gneissic rock causes mechanical anisotropy in rock during deformation. This study aims to explore specially the influence of mechanical anisotropy on the development of flanking structures. With the help of two –dimensional Finite Element modelling, we attempt to understand the effect of initial orientation of cross-cutting element (θ), kinematic vorticity number (Wk) and the anisotropy factor (δ) on the development of flanking structures. Deformed shape of cross-cutting element (RN), amount (LN) and nature (antithetic or synthetic) of offset of layers and pattern of drag (reverse or normal) are considered as important parameters for their nomenclature. As the final geometry of flanking structure does not only depend on the instantaneous deformational condition, we analyze the progressive development of flanking structures by varying the above parameters. Study reveals that mechanical anisotropy has strong influence on the geometry of flanking structures in layered gneissic rock, which depends on the initial orientation of long axis of cross-cutting element and its final position with respect to the orientation of principal stress axes and pole of anisotropy. The study also reveals that the flanking structures can be used as important tool for estimating the mechanical anisotropy in layered rock. Shape of cross-cutting element, amount of offset and the shifting of the object-tip points are the most important parameters for determining the anisotropy factor.