An experimental approach to wide-necked pinch-and-swell structures

Abstract

In the granite gneiss of Purulia District of West Bengal, India, foliation-parallel or -subparallel quartz veins show exceptionally wide-necked pinch-and-swell structures. Tensile experiments with tubes of polyethylene, a viscoelastic plastic polymer, have been carried out at a constant load (just exceeding the yield stress, 0.68 × 10⁸ dynes cm^-2) in order to study such structures. The development of pinch-and-swell in the experimental models involves widening of neck zones by propagation of the neck fronts in the extension direction. Both the theoretical and experimental results enumerate a linear relationship between neck zone widening and bulk elongation when neck fronts propagate at a constant neck thickness (defined as a stable neck). The study predicts that propagation of neck zones in a stable state gives rise to the wide-necked pinch-and-swell structures. On the other hand, necks undergoing continuous thinning all along its course of development (defined as an unstable neck) may produce a pinch-and-swell structure with narrow torn neck zones. In the progressive deformation of experimental models a swell is reduced in size due to propagation of its adjacent two necks and subsequently the swell dies out when the two necks coalesee with each other.