Abstract Title: A micromechanics based numerical modelling investigation of flanking structures and its application on determining quantitative kinematic information from ductile shear zones

Abstract Submitted to: TECTONOPHYSICS

Abstract Text:

Flanking structures are deflections in linear or planar fabric elements around any cross-cutting element such as a fracture, vein or dyke and can serve as potential tools to infer flow kinematics of ductile shear zones. Previous numerical modeling investigations are limited to special cases where the cross-cutting element is either a frictionless free slip surface, or a rigid material. Works with more variable cross-cutting element’s rheology are limited to low finite strains, cross-cutting element’s high aspect ratios and limited cases of cross-cutting element’s initial orientations. In natural setting, the rheological properties of the cross-cutting element mainly its strength, shape and orientation can be highly variable. Common examples include flanking structures associated with a cross-cutting dyke or a strong inclusion. Here, we use Eshelby formalism to simulate flanking structures around a cross-cutting element of varying rheological properties, shape and orientation. Specifically, we regard a cross-cutting element as a 3D Eshelby inclusion embedded in a viscous medium. The numerical exterior Eshelby solutions provide velocity fields in the vicinity around the element. These velocity fields are then used to simulate the deflection of marker elements surrounding the cross-cutting element, under a macroscale general plane strain flow ranging from simple to pure shear. We reproduced all observed flanking structure types recognized from natural shear zones. In contrast to the previous models, our modeling results show that all three types of flanking structures with antithetic (a-type), no- (n-type) and synthetic (s-type) displacement along the cross-cutting element can be formed around any cross-cutting element stronger than the embedding medium such as a dyke or a strong mineral inclusion. The a-type flanking structure may transition into an s-type depending on cross-cutting element’s viscosity and macroscale finite strain. We have developed a reverse-dynamic modelling tool that can provide a quantitative estimate of flow vorticity, finite strain and cross-cutting element’s viscosity relative to the embedding medium from an observed natural flanking structure.