Modifications of early lineations during later folding in simple shear

A series of experiments was carried out with analogue models to study deformation of early lineations on layers undergoing simultaneous buckling and flattening under a bulk simple shear. The results show that when buckling is accompanied by layer-parallel strain, the angle between the lineation and the fold axis changes differently in different parts of the fold. A U-shaped pattern of deformed lineation on the unrolled fold surface develops in bulk simple shear when (1) the initial orientation of the layer is in the field of shortening, (2) the layer is inclined to a plane that is perpendicular to the vorticity vector of simple shear (i.e., the plane perpendicular to the Y-axis of bulk strain), and (3) the early lineation is initially perpendicular to the later fold axis. With progressive simple shear, an initially symmetric U pattern of lineation may become asymmetric. The nature of the U pattern is modified over parasitic folds that grow on limbs of larger folds. When the initial angle between the early lineation and the later fold axis is moderate, a sigmoidal pattern of deformed lineation develops, the angle remaining a minimum at the hinge and progressively increasing on the limbs. If the curvature of the hinge region is large in a relatively thick layer, the pattern of deformed lineation is considerably modified by tangential longitudinal strain.