Fluid-assisted localized embrittlement in an overall ductile fold-and-shear belt: A case study from Cap de Creus

Abstract

The Cap de Creus massif in the eastern Pyrenees is a renowned late Variscan fold-and-shear zone belt characterized by partitioning of an overall ductile strike-slip-dominated dextral transpression into coeval folding, ductile shearing and localized embrittlement. The local partitioning of deformation mode (fracture vs. ductile flow) can be related to rheologically contrasting lithologies and syntectonic infiltration of hydrothermal fluids that were likely residual fractions of earlier pegmatite intrusion. In particular, the influx of hydrothermal fluids results in cm-to m-sized damage zones of localized fractures and veins with associated hydrothermal alteration across most lithological units. Such damage zones systematically formed at a low angle to the axial surfaces of contemporaneous folds, even in cases with minimal shortening. In higher strained areas, evident by tighter folding and shearing, cataclasites developed along these same damage zones. This study provides an example of contemporaneous ductile and brittle structures, in this case resulting from localized embrittlement due to fluid flow.