Thrust vs. fold nappes: Mechanical and geometrical controls from 2D numerical simulations and applications to a recumbent fold of the Pyrenees

Thrust and fold nappes are found in the internal and external portion of orogenetic belts, and have been the subject of geometric and kinematic characterization during the last century. In spite the extensive studies, there is still not a full understanding of the processes and properties that favour thrusting over folding and vice versa. We address this issue by numerical modelling with application to a natural case of the Pyrenees, the Eaux-Chaudes massif, an Alpine fold-and-thrust structure in the western Axial Zone. The Eaux-Chaudes structure consists of a basement-cored recumbent fold nappe with a large reverse limb in ductilely-deformed Upper Cretaceous carbonates, transitioning laterally to the east to an imbricate thrust fan that also exhibits ductile deformation to the east. The spatio-temporal association of these structural styles at Eaux-Chaudes can be a consequence of the pre-orogenic configuration and highlights the need to investigate under which conditions and precursor geometries one or the other nappe style are favoured. Here, we present a systematic numerical modelling study of the variability in the initial mechanical and geometrical conditions, using the thermomechanical staggered finite-difference code LaMEM. We also investigate the mechanism that favour the potential migration of fold hinges and lead to the preservation of layer thickness in the reverse fold limbs, and quantify it with a new nondimensional parameter, the localisation index (Iloc). Our results demonstrate the need of a stiff forestop for nappe development. The absence of a forestop causes detachment buckle folds in the strong layers. Deep burial and the combination of a thick upper decoupling unit and a lower detachment level are essential features favouring viscous behaviour and spatially distributed deformation, enabling recumbent folding by progressive hinge migration, and characterized by low and stable values of Iloc. On the other hand, shallower conditions, short lengths of the stiff layer and lower friction angles inhibit hinge migration. Instead, they enhance instead reverse limb stretching and shearing, which eventually results in strain localisation and thrusting. These are characterized by a moderate-to-quick rises of Iloc. Our results may be applicable to other orogenic belts and to other parts of the Axial Pyrenean hinterland where the Mesozoic cover has been eroded and the Alpine deformation is obscure.