Transition from thin- to thick-skinned tectonics and consequences for nappe formation: Numerical simulations and applications to the Helvetic nappe system, Switzerland

•We quantify the transition from thin- to thick-skinned dominated deformation.•Effective viscosity ratios for basement and sediment control the transition.•Fold nappes are formed during thick-skinned dominated deformation.•Numerical results are applied to the Helvetic nappe system.

We use two-dimensional numerical simulations to study the deformation style and the formation of nappes during the shortening of basement-cover systems with half-grabens. We perform simulations with only linear and power-law viscous rheology, and also thermo-mechanical simulations with viscoelastoplastic rheology and temperature-dependent viscosities. The accumulated, area-averaged strain is calculated for the sediments, εs, and for the basement, εb, to quantify the deformation style. The results show that the transition from thin-skinned dominated (εs/εb > 1) to thick-skinned dominated (εs/εb ≤ 1) deformation is controlled by two effective viscosity ratios: the ratio of viscosity at the top of the basement to viscosity at the bottom of the basement, and the ratio of viscosity at the top of the basement to viscosity of the sedimentary cover. A higher basement-internal viscosity ratio favors thick-skinned deformation whereas a higher basement-cover viscosity ratio favors thin-skinned deformation. Simulations with only viscous rheology show the same first-order results as the thermo-mechanical simulations. The results further show that fold nappe formation is associated with thick-skinned dominated deformation and thrust nappe formation with thin-skinned dominated deformation. Fold nappes are formed by shearing and extrusion of sediments during the deformation and closure of half-grabens whereas thrust nappes are formed due to the shearing-off of sediments from half-grabens which are nearly undeformed. The results are applied to the Morcles fold nappe and the Helvetic nappe system, Switzerland.

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