Conditions favouring fold vs. thrust nappes: insights from the modelling of a Pyrenean example and implications on hinge migration vs. limb stretching mechanisms

Fold nappes and thrust nappes are found either in the internal or the external parts of orogenic belts worldwide and are geometrically and kinematically well constrained after more than a century of studies. However, the mechanics favouring one vs. the other remain incompletely understood due to the uncertainty and variability of pre-contractional configurations.

Recent numerical modelling of the Helvetic nappes of the Alps highlighted the relevance of the competence contrast between stiff and weak layers in controlling the deformation style. Similarly, the Eaux-Chaudes fold nappe of the French Pyrenees (Caldera et al. 2021) appears governed by the mechanical stratigraphy. However, the ductile extrusion of a half graben with which the Helvetic nappes have been modelled cannot be invoked for the Pyrenean example, which appears formed by shearing of a stiff carbonate layer between two weak decoupling units and a backstop. The occurrence of the two structural styles in the Eaux-Chaudes region highlights the need to find out under which conditions or pre-orogenic configurations one or the other nappe style are favoured.

We employed the thermomechanical staggered finite difference code LaMEM (Kaus et al., 2016) to perform 2D parametric simulations to address changes between thrust nappes (plastic/brittle-localisation) and recumbent fold nappes (viscous/ductile-distributed). The simulations were carried out using a linear viscoelastoplastic rheology with the Drucker-Prager criterion for plasticity. We measured the hinge migration during folding by implementing passive tracer elements tracking the position of markers through time. Based on the Eaux-Chaudes fold nappe as a reference natural example, we tested the pre-orogenic geometry but also the intrinsic mechanical properties of the stiff key layer and the adjacent units. Our results demonstrate a strong control of the configuration of weak and strong units on the deformation style.

In all cases a backstop causing stress concentration in the stiff layer (an underlying granite massif in the Eaux-Chaudes case) was necessary to induce either recumbent folding or thrusting. The absence of a backstop causes detachment buckle folds in the stiff layers, hindering nappe development. 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 the formation of fold nappes by progressive fold hinge migration (material particles are travelling from the normal to the reverse limb of the nappe). 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. Our results may be applicable to other orogenic belts and also to other parts of the Axial Zone of the Pyrenees where the Mesozoic cover is eroded and the Alpine deformation is obscure.

Caldera, N., Teixell, A., Griera, A., Labaume, P. and Lahfid, A. (2021): https://doi.org/10.1111/ter.12517

Kaus, B.J.P., Popov, A., Baumann, T, Püsök, A., Bauville, A., Fernandez, N. and Collignon, M. (2016): Forward and Inverse Modelling of Lithospheric Deformation on Geological Timescales, NIC Symposium 2016–Proceedings, Germany, NIC Series, 48, 299-307.