Observaciones sobre las fracturas de las diaclasas plumosas

Abstract

Many joint surfaces show en échelon arrays of microfractures (F-joints) along one or both fringes of hackle marks. Our field observations indícate that, where both sets are present, they are stepped in oposite senses. So the F-joints can not be used as shear criteria for discontinuities. Good examples of hackle marks with F-joints have been reproduced in the laboratory by submitting silicone putty to tensile tests when these include sorne rotational component. Rotation induces shear in different senses if observed from an external point of view; so it can explain the development of two sets of Riedel fractures showing opposite stepping. In geological conditions, this rotation occurs around the tips of fractures undergoing vertical displacement. Thus, from this model, hackle marks and F-joints could be used for detecting the sense of small dip-slip movements on joints