Jean-Claude Guezou

Distinguishing lateral folds in thrust systems: examples from Corbières (SW France) and Betic Cordillera (SE Spain): Reply

Abstract Folds with axes almost parallel to the mean tectonic transport direction are frequent in fold-thrust belts, particularly along the borders of arcuate belts. Such structures have been frequently explained in terms of superimposed tectonic events but, in some cases, they are most realistically understood in terms of lateral folds inherited from lateral footwall ramps and initiated subparallel to the tectonic transport direction. In order to distinguish frontal and lateral folds, we have established criteria based on field examples from two regions where folds occur parallel to the regional transport direction: the Betic Cordilleras (Southern Spain) and the Corbieres thrust-front (northeast Pyrenees, France). The main differences between frontal and lateral folds concern the geometry, the evolution of the structures and the modes of strain accommodation in the roof strata enveloping imbricate stacks. Recalling that, above a duplex or even on individual horse, the roof layers are thinned along sections normal to transport, we examine the precise location where the extension is expressed, illustrated by examples from the two regions. This leads us to address some tectonic problems of these erogenic zones, such as the thrust-sequence and development of thrust-related cleavage.

Microstructural evolution of metabasalts within a contact metamorphic aureole : a preliminary quantitative bi-dimensional approach

The emplacement of the Saint-Brieuc diorite (533 ± 12 Ma ; Brittany, France) developed a narrow contact aureole in metabasaltic host-rocks which were previously submitted to a regional-scale deformation and metamorphism at around 590-570 Ma. This study aims to qualify and quantify the microstructural changes of rocks that occur within the contact aureole and that are the result of the static recrystallization due to the thermal effect of the diorite. In order to quantify the textural evolution of rocks, an image analysis has been performed on thin sections. It focused on the measurements of morphological features of hornblende and opaque phases. They are anisotropy shape factors (stretching and elongation) of minerals, the preferred orientation of minerals and the distribution size (areas) of minerals. The quantification of these different parameters shows that the static recrystallization, which increases when approaching the contact with the intrusive, (i) is responsible of a reduction of the anisotropy shape and elongation parameters of grains, (ii) causes coarsening and (iii) is responsible for the disappearance of preferred orientation of minerals. Both together, the qualitative description and quantitative measurements show that the solid-state transformations due to contact metamorphism tend to make the rocks isotropic and equigranular.

Elastic Strain Study in Quartzites Using Neutron Diffraction

Abstract The residual elastic strains determination by neutron diffraction has been performed on quartzite samples from a former texture study. The measured complete residual strain tensor has been considered as representative of the “natural” finite plastic deformation. Such tensors fitted to plastic tensors have been introduced in texture modelings. The results demonstrate the potential of complete and meaningful strain tensor determinations in rocks.

Étude des déformations élastiques résiduelles dans les quartzites

Residual elastic strain determination by neutron diffraction has been performed on quartzite samples from an earlier texture study. The main objective of this study is to derive a complete strain tensor of a natural deformation for a later introduction in the texture modelling. The results demonstrate the potential of complete and meaningful strain tensor calculations, with the exception of largely recrystallized samples.