The Eaux-Chaudes massif provides keys to unravel the deep-seated
deformation of the Iberian rifted margin during the Alpine orogeny in
the Pyrenees. The massif conforms to an inlier of upper Cretaceous
carbonate rocks within the Paleozoic basement of the western Axial Zone,
originally deposited in the upper margin shelf before the Cenozoic
collision. New geological mapping and cross-section construction lead to
the description of the lateral structural variation from a km-scale fold
nappe in the west to a ductile, imbricate fold-thrust fan in the east.
The transition from a Variscan pluton to Devonian metasediments
underlying the autochthonous Cretaceous induced this structural change.
Recumbent folding, which involved upper Paleozoic rocks, was facilitated
by a lower detachment in Silurian slates and an upper detachment in an
overlying Keuper shale and evaporite thrust sheet. Remains of this
allochthonous sheet form shale and ophite bodies pinched within the
upper Cretaceous carbonates, conforming unusual tertiary welds. Ductile
shear in the overturned limb of the Eaux-Chaudes fold nappe imparted
strong mylonitic foliation in carbonate rocks, often accompanied by N-S
stretching lineation and top-to-the-south kinematic indicators. The
burial of the massif by basement-involved thrust sheets and the Keuper
sheet, along with their Mesozoic-Cenozoic cover, account for ductile
deformation conditions and a structural style not reported hitherto for
the Alpine Pyrenees. Two hypotheses for the tectonic restoration of this
part of the Pyrenean hinterland are proposed in this work.