Zooming on Northern Cap de Creus shear zones

The Northern Cap de Creus shear belt provides fine examples of shear zones developed at different scales in rocks bearing pre-existing penetrative foliations. The shear zones developed under retrograde metamorphic conditions, and they are preferentially located in crystalline schists. Lower grade metasedimentary rocks accommodated deformation by folding. The shear zones are linked in an anastomosing framework with self-similar properties. They are interpreted to have formed as the result of a progressive wrench-dominated deformation affecting anisotropic rocks. A progressive non-coaxial deformation regime is considered responsible for the development of the complex kinematic pattern, wherein shear zones with opposite senses of shear may come to lie in close parallelism. In such a framework, individual shear zones are characterised by marked strain gradients not only across the shear zone, but also along it. These features, together with the geometrical analysis of shear zone marginal domains, indicate that these structures developed with a component of shortening across the shear zones. The shear zones are interpreted to have nucleated as buckling instabilities that gradually evolved to become shear zones approaching the simple shear model. Microstructural and quartz c-axis fabric analyses indicate that shear zone formation is associated with strong strain partitioning on all scales.