Ductile shear zones

Abstract

Two alternative pathways to generate shearband boudins in a HT shear zone have been observed at the Salgosa outcrop field (NW of Portugal, N4642140 E524687, WGS84) – Parautochthonous of Variscan Central Iberian Zone, Silurian age –: The classical one, associated with relatively thick veins (see final geometry and explanation in Figure 2.15), and a newly proposed model of fold-boudin evolution related to relatively thin veins: The boudinage genesis starts with the folding of tabular competent bodies of different thickness immersed in a less competent matrix. The evolution of the thin bodies (low t) follows the folding process until it culminates with the coalescence and stacking of the folds (Figure 2.36 and Figure 2.37 and respective captions). By this mechanism, a critical thickness is reached and a new phase of boudinage is triggered, similar to that followed by originally thicker veins (high t) – see final geometry and explanation in Figure 2.39.