| Popis: |
The Toledo mylonite in the central area of the Iberian Massif represents a major ductile shear zone, only a small part of which has been exposed to view. Mylonites derived largely from migmatite occupy a 300–350 m thick, gently S-dipping zone along the southern margin of the Toledo Crystalline Massif. The upper plate comprises Palaeozoic lower-grade metasedimentary rocks intruded by a late Hercynian pluton. The juxtaposition of the upper and lower plates with different structural and metamorphic conditions indicates that the shear zone is a significant structure with a large displacement. Progressive simple shear under conditions of a clearly decreasing confining pressure and temperature is supported by a microbrecciated mylonite, cataclasite and ultracataclasite sequence overlying the mylonites. The top of the ledge of cataclasites corresponds to the detachment-fault part of the shear zone. Structural relations, combined with meso- and micro-structural evidence (stretching lineations and asymmetric shear sense indicators), support the conclusion that the mylonites and cataclasites associated with the detachment faulting formed during a continuous extensional deformation. Detachment-fault models developed for the formation of metamorphic core complexes can be applied in the Toledo Massif. We propose a model that implies a transition from ductile to brittle deformation associated with extension at greater structural depths, during lower Permian time. |
