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

Lineations composed of similarly oriented elongate mineral aggregates or gr ains are a common feature in deformed rocks, but it is unclear which factor s control the development of such lineations. Field observations and micros tructural analysis of samples, which were taken from discrete greenschist t o 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 deve lopment of lineations. The most important of these is dynamic recrystalliza tion. The way in which dynamic recrystallization influences lineation devel opment is largely a function of the initial fabric in a specific rock type. Different lithologies with different initial fabrics produce distinctly di fferent types and strengths of lineations even if deformed to the same fini te strain in the same shear zone. An initial fine grain size and a monomine ralic composition of the parent rock type commonly hinder the development o f lineations. In contrast, in initially coarse-grained polymineralic rocks, well-developed lineations commonly develop. Therefore, the ratio of initia l to dynamically recrystallized grain size determines to a large extend the development of such lineations. (C) 2001 Elsevier Science Ltd. All rights reserved.