Controls on lineation development in low to medium grade shear zones: a study from the Cap de Creus peninsula, NE Spain

Abstract

Lineations composed of similarly oriented elongate mineral aggregates or grains are a common feature in deformed rocks, but it is unclear which factors control the development of such lineations. Field observations and microstructural analysis of samples, which were taken from discrete greenschist to lower amphibolite facies shear zones of the easternmost Variscan Pyrenees, show that strain is only one of several factors that control the strength and type of a lineation. Dynamic recrystallization, metamorphic reactions and rigid body rotation are also important controlling factors for the development of lineations. The most important of these is dynamic recrystallization. The way in which dynamic recrystallization influences lineation development is largely a function of the initial fabric in a specific rock type. Different lithologies with different initial fabrics produce distinctly different types and strengths of lineations even if deformed to the same finite strain in the same shear zone. An initial fine grain size and a monomineralic composition of the parent rock type commonly hinder the development of lineations. In contrast, in initially coarse-grained polymineralic rocks, well-developed lineations commonly develop. Therefore, the ratio of initial to dynamically recrystallized grain size determines to a large extend the development of such lineations.