Ductile deformation of the Pyrenean hinterland during the Alpine collision: The case of the proximal Iberian margin in the Eaux-Chaudes massif (west-central Pyrenees)

Abstract

Alpine deformation in the hinterland of the Pyrenean belt is poorly constrained due to the strong erosion of the post-Paleozoic rocks in the topographic axis of the Pyrenees. The upper Cretaceous limestones of the Eaux-Chaudes massif, enclosed by Paleozoic rocks in the interior of the west-central Pyrenees, show evidence of complex ductile deformation and a large-scale structural style not reported hitherto for the Alpine collisional stage. This thesis proposes an innovative reinterpretation of the structure and evolution for the Eaux-Chaudes massif, addressing the collisional deformation of the upper Iberian margin previously developed during the Mesozoic rifting. Paleothermal and microstructural inspection of the upper Cretaceous limestones emphasize the ductility occurred in the main deformation event. The Pyrenean orogeny started by the flexure of the Iberian margin deepening to the north, evidenced by the sedimentation of a Campanian-Maastrichtian flysch. As the collision of the Iberian and European margins progressed, the flexural effect increased by stronger deepening of the Iberian plate as deduced for the Eaux-Chaudes massif. The climax of the main deformation event occurred under moderately high temperatures accounting for ductile structures and penetrative foliation and lineation. The strong Eaux-Chaudes granite in the Paleozoic basement near the Ossau valley controlled the development of a large-scale recumbent fold nappe in conjunction with two effective detachment levels, one in the upper part of the Paleozoic basement (in the Silurian), and the other in an allochthonous Keuper salt sheet emplaced by the major structure inverting the margin (the Lakora thrust system). Structural inheritance and rheological heterogeneity of the basement induced strong changes in the lateral evolution of this fold, turning into a simpler, ductile thrust imbricate fan. The novel scheme proposed in this work warns about the difficulties of unravelling this type of deformation in the interior of the Pyrenees and other orogens due to the strong erosion and the structural superposition in the ancient rocks which dominate in outcrop.

Date of Award	21 Nov 2022
Original language	English
Supervisor	Albert Griera Artigas (Director) & Antonio Teixell Cacharo (Director)