Strain and crystallographic fabric pattern in a folded calcite vein: the dependence on initial fabric

Abstract

Buckle folding of single competent rock layers is typically accommodated by tangential longitudinal strain (TLS), which involves layerparallel stretching in the outer arc of a fold and layer-parallel shortening in the inner arc, with a neutral surface separating the two. We describe a folded fibrous calcite vein in which the primary crystallographic fabric, with c-axes perpendicular to the vein walls, allows only for layer shortening by twinning on 0112; twinning cannot lead to layer stretching. As there are no other deformation mechanisms that allowed for stretching in the outer arc, the folding must have occurred without the development of a neutral surface in the layer. Intensity of twinning increases from the outer edge of the folded vein to the inner edge, suggesting that a neutral surface existed at the outer edge. This is supported by approximate measurements of twinning strain that show a strain gradient consistent with TLS, and that indicate also about 10% layer-parallel shortening prior to fold growth.