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 fol iations. The sheer zones developed under retrograde metamorphic conditions, and they are preferentially located in crystalline schists. Lower grade me tasedimentary rocks accommodated deformation by folding. The shear zones ar e linked in an anastomosing framework with self-similar properties. They ar e interpreted to have formed as the result of a progressive wrench-dominate d deformation affecting anisotropic rocks. A progressive non-coaxial deform ation regime is considered responsible for the development of the complex k inematic pattern, wherein shear zones with opposite senses of shear may com e to lie in close parallelism. In such a framework, individual sheer 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 sheer zone marginal domains, indicate that these structures developed w ith a component of shortening across the sheer zones. The shear zones are i nterpreted to have nucleated as buckling instabilities that gradually evolv ed to become shear zones approaching the simple shear model. Microstructura l and quartz c-axis fabric analyses indicate that shear zone formation is a ssociated with strong strain partitioning on all scales. (C) 2001 Elsevier Science Ltd. All rights reserved.