15. Lineation

Any fabric or orientation in a rock developed in a linear fashion as a result of tectonic deformation is called lineation or linear structure. Though lineation is commonly developed on the surface of rocks, it may also penetrate to a small extent, up to a few millimetres only. As such, lineation is broadly grouped as non-penetrative lineation such as slickensides, slickenlines and slickenfibres and penetrative lineation such as mineral lineation, crenulation lineation, intersection lineation, boudins, mullions, rods, pencil structure and pressure shadows. Lineation can form by processes that can be metamorphism dominated, deformation dominated or geometrically controlled, though it is difficult to draw any specific line of demarcation between these groups of processes. Study of lineation is important in structural geology as some lineation types constitute structural markers to relate smaller structures with the larger ones. Some other types, such as intersection lineation, given by a folded layer and an axial-plane foliation indicate the orientation of fold axis. This chapter describes the various types of non-penetrative and penetrative lineation, lineation as a tectonic fabric, genesis and significance of lineation.