Contrasting fabrics in deformed dykes and host rocks: natural examples and a simplified model

Abstract

The contrasting fabrics of metabasic dykes and their gneissic host in the Precambrian basement of the Schirmacher Hills, E. Antarctica, are interpreted in terms of a mathematical model and qualitative experiments. The field studies and experiments indicate that oblique contacts between host rocks and competent or incompetent dykes may be preferred sites for localization of ductile shear zones. The shear zones may develop within the matrix near the contacts of very competent dykes as well as within narrow incompetent dykes. The mathematical model and the qualitative experiments also suggest the development of normal and ‘abnormal’ patterns of cleavage refraction. If the dykeparallel longitudinal strain is not the same in the dyke and the host rock, there will be strain gradients in both the dyke and the host rock near the dyke—matrix contact. The experimental results suggest that the cleavage within thick dykes would show a sigmoidal pattern. If the bulk deformation is coaxial, a clockwise rotating dyke will show an S-shaped pattern of internal cleavage, and an anticlockwise rotating dyke will show a Z-shaped pattern.

Acknowledgements: I thank the Department of Ocean Development for facilities of field-work in Antarctica and the Council of Scientific and Industrial Research, India, for financial support of laboratory studies. I am grateful to Sue Treagus and D. Sokoutis for their comments on an earlier version of this manuscript. I dedicate this paper to Subir Ghosh for his inspiring contributions to many of the general concepts which have been utilized here.