Time constraints and evolution of the Eaux-Chaudes fold nappes (Pyrenees): a study combining 2D numerical simulations, U-Pb geochronology and zircon (U-Th)/He thermochronology

The Eaux-Chaudes massif (ECM) of the French Pyrenees consists of a nappe stack located in the western Axial Zone formed during Alpine times. It features a basement-cored recumbent fold nappe with a large overturned limb in Upper Cretaceous carbonates ductily deformed. Paleotemperatures of ~350°C for an autochthonous succession and for the overturned limb, and ~310°C for the normal limb were recorded during the main deformational event, which is equivalent to burial depths of 8-10 km. During the main deformation, syn- and post-deformation calcite veins formed, which could be dated by calcite LA-ICP-MS U-Pb. The whole nappe stack was eventually affected by late backthrusting on top of the Gavarnie thrust. 

The rare occurrence of such a fold nappe in the Alpine Pyrenees and the observed ductile strain makes necessary to understand under which conditions it was developed, and to put an age constraint on the ductile event within the history of the massif. Here, we present the results of 2D parametric simulations to address changes between thrust nappes (plastic/brittle-localisation) and recumbent fold nappes (viscous/ductile-distributed) using the thermomechanical staggered finite-difference code LaMEM. The simulations were carried out using a linear viscoelastoplastic rheology with the Drucker-Prager criterion for plasticity. We also present a systematic study of syn- and post-tectonic calcite veins as well as the deformation and exhumation history for the Eaux-Chaudes massif, constrained by means of U-Pb geochronology on veins, low-temperature zircon (U-Th)/He thermochronology and QTQt time-temperature simulations. 

Modelling results show that in all cases a footwall backstop causing stress concentration in the stiff Upper Cretaceous (key because allows to identify an alpine recumbent fold) layer (an underlying granite massif in the Eaux-Chaudes case) was necessary to induce recumbent folding. Deep burial and the combination of a thick, weak upper decoupling unit and a lower detachment level are essential features favouring viscous behaviour and spatially distributed deformation, enabling the formation of fold nappes by progressive hinge migration (material particles travel from the normal to the overturned fold limb). On the other hand, shallower conditions, shorter lengths of the stiff layer and lower friction angles of the key layer reduces hinge migration, enhancing instead reverse limb stretching and shearing, which eventually results in strain localisation and thrusting.

Geochronology results indicate that the ductile folding and thrusting event occurred between 48.87±5.66 Ma and 38.14±5.99 Ma (mid Eocene). Cooling ages indicate that the exhumation of the Eaux-Chaudes massif occurred later between ~40-20 Ma, coinciding with the known activity of the Gavarnie and Guarga basement thrusts that raised the Axial Zone of the Pyrenees.