J. Brandão Silva

Genesis of the Ibero-Armorican arc

AbstractThe Ibero-Armorican arc is continuous between Iberia and Armorica; its curvature increased with time due to subduction followed by continental collision; indentation produced left lateral transpression in Iberia and right-lateral transpression in Armorica. It is argued that whereas the antithetic shear is predominant in Iberia, in Armorica a synthetic shear prevailed because the identer rotated anticlockwise between the opposed forelands of the Variscan Fold Belt. It is proposed that the major Rheic ocean, closed by subduction towards the inner part of the arc, solving the space problem of centripetal vergences.

Development of local orthorhombic fabrics within a simple-shear dominated sinistral transpression zone: the Arronches sheared gneisses (Iberian Massif, Portugal)

Abstract The Coimbra-Cordoba shear zone (Iberian Massif), characterized by simple-shear dominated sinistral transpression, exposes several outcrops of strongly sheared peralkaline gneisses surrounded by mica schists and amphibolites. These gneisses are included in the Arronches Tectonic Unit, a thick unit of mylonitic rocks with a steep foliation and an associated gently plunging stretching lineation parallel to the fold axes. Strain partitioning is testified by widely spaced anastomosing shear bands around less-strained domains and by the existence of different shearing domains ranging from relatively ‘less-strained’ and coarse-grained mylonites to highly strained and fine-grained ultramylonites. Three shearing domains defined by textural and structural changes resulted from progressive deformation and increasing strain, which leads to increased mylonitization of gneisses. This is revealed by the increased modal percentage of the matrix and the decreased percentage of porphyroclasts, accompanied by evolution from orthorhombic to monoclinic fabrics: Conjugate Shearing Domain (CSD), Intermediate Sinistral Domain (ISD), and Sinistral Domain (SD). This contribution shows that in a simple-shear sinistral dominated transpression zone with a well-developed and widespread monoclinic fabric, it is possible to find mechanical conditions to produce local orthorhombic fabrics. In the Arronches gneisses a local strain regime exists in apparent contradiction with the bulk deformation regime.

The role of strain localization in magma injection into a transtensional shear zone (Variscan belt, SW Iberia)

This study deals with the interaction between deformation and magmatism in mid- to deep-crustal domains. The relation is analysed between migmatites and shear zones and the spatial distribution of leucogranitoid veins and dykes running through a footwall migmatite system, and reaching a transtensional shear zone operated under amphibolite- to greenschist-facies metamorphic conditions (Boa Fé shear zone, Variscan belt, SW Iberia). Statistical results show that the frequency of width and spacing of the leucogranitoid dykes conform to power-law distributions comparable with observations in volcanic systems. The fractal geometry of the distribution of leucogranitoid dykes highlights the development of a dense framework of thinner weakly or non-mineralized veins and dykes formed at higher nucleation/growth ratios in the footwall migmatite system that contrasts with the emplacement of thicker dykes associated with strongly mineralized thinner veins within the shear zone. The volume of injected leucogranitoid dykes in the shear zone is lower as compared with the footwall and is comparable with an expanding footwall shear zone with non-coaxial flow and volume increase. The Boa Fé shear zone seems to form a physical barrier to the transport of magma to the hanging wall.