Recumbent folding in the Upper Cretaceous Eaux‐Chaudes massif: A Helvetic‐type nappe in the Pyrenees?

We describe a singular structure in Upper Cretaceous rocks of the Eaux‐Chaudes massif of the western Pyrenees, consisting of a kilometre‐scale fold nappe with a sheared overturned limb. High ductile strain attests a deformation style rarely reported for the alpine Pyrenees, and peak temperature in Upper Cretaceous carbonates is estimated by Raman spectroscopy of carbonaceous material palaeothermometry in the lower greenschist facies (>300°C). The normal fold limb retains the original sedimentary textures, while the overturned limb shows calcite crystal‐plastic deformation and dynamic recrystallization, with crystallographic preferred orientation. The observed ductility and metamorphic temperature bear similarities with the lower Helvetic nappes of the Alps, suggesting deep burial and/or possibly high geothermal gradient in this part of the Pyrenees.