Résumé
The Saint-Beat basin is located in the Internal Metamorphic Zone of the French Pyrenees and is one of the numerous metamorphic sedimentary domains that are the witnesses of the Albo-Aptian extension. It is composed mainly by marbles formed by the transformation of previous limestones and dolomites dated from Lias to Early Albian (from South to North). Marbles are bounded to the north by Variscan in age migmatites, and to the south by the North Pyrenean Fault (NPF) and Paleozoic folded rocks. Field observations combined with microscopic analysis, textures analysis and fabrics determination using EBSD (Electron Back Scattered Diffraction) allow constructing a model of the structural evolution of the studied area in which characteristics of the two main tectonic events, i.e. Albo-Aptian extension and Pyrenean compression are determined. Deformation was analysed both within marbles and crystalline basement. Foliation in marbles, locally underlined by grey, black and white bands, presents a general high-dipping toward the south. Locally, we observe slightly marked horizontal planes, representative of the initial bedding. We can see many evidences of grain boundary migration (and bulging), undulose extinction and formation of subgrains under the effects of dynamic recrystallization. In crystalline rocks, the foliation trends N080 degrees E with very high dipping towards the South, and stretching lineation is NS. Close to the contact with marbles, quartz grains show evidences of recrystallisaztion that we attribute to the effects of Albo-Aptian extension. EBSD measurements demonstrate that metamorphism that formed the marbles was synchronous of a non coaxial deformation with north-verging reverse kinematics except close to the southern contact were kinematics is south-verging. Quartz grains of the crystalline basement show contrasting results with respect of their size. Large primary grains are deformed by the activation of the rhomb glide system whereas recrystallized little grains yield evidences of high temperature prism system glides. Taking into account all these results, we propose a geometrical and structural model for the formation of the St Beat basin in which Albo-Aptian extension developed both north and south-verging tectonics, controlled by a dome-shape extensional detachment. These structures were reactivated and inverted during the Pyrenean compression thus explaining the reverse north-verging motion actually shown by the specific geometry of the St Beat marble massif.