Tectonic inheritance controls nappe detachment, transport and stacking in the Helvetic nappe system, Switzerland: insights from thermomechanical simulations

Abstract

Abstract. Tectonic nappes have been investigated for more than a hundred years. Although geological studies often refer to a “nappe theory”, the physical mechanisms of nappe formation are still disputed. We apply two-dimensional numerical simulations of shortening of a passive margin to investigate the thermomechanical processes of detachment (or shearing off), transport and stacking of nappes. We use a visco-elasto-plastic model with standard creep flow laws, Drucker–Prager and von Mises yield criteria. We consider tectonic inheritance with two initial mechanical heterogeneities: (1) lateral heterogeneity of the basement–cover interface due to half-grabens and horsts and (2) vertical heterogeneities due to layering of mechanically strong and weak sedimentary units. The model shows detachment and horizontal transport of a thrust nappe that gets stacked on a fold nappe. The detachment of the thrust sheet is triggered by stress concentrations around the sediment–basement contact and the resulting brittle–plastic shear band that shears off the sedimentary units from the sediment–basement contact. Horizontal transport is facilitated by a basal shear zone just above the basement–cover contact, composed of thin, weak sediments that act as a décollement. Fold nappe formation occurs by a dominantly ductile closure of a half-graben and the associated extrusion of the half-graben fill. We apply our model to the Helvetic nappe system in western Switzerland, which is characterized by stacking of the Wildhorn thrust nappe above the Morcles fold nappe. The modeled structures, the deformation rates and the temperature field agree with data from the Helvetic nappe system. Mechanical heterogeneities must locally generate effective viscosity (i.e., ratio of stress to viscoplastic strain rate) contrast of about 3 orders of magnitude to model nappe structures similar to the ones of the Helvetic nappe system. Our results indicate that the structural evolution of the Helvetic nappe system was controlled by tectonic inheritance and that material softening mechanisms are not essential to reproduce the first-order nappe structures.

Control of 3-D tectonic inheritance on fold-and-thrust belts: insights from 3-D numerical models and application to the Helvetic nappe system

“…In cross-sectional view our results are similar to results by Kiss et al (2020), who performed 2-D high-resolution numerical simulations for a model configuration close to the one of our reference model at the lateral side with the deep HG. The evolution, timing and final structures of the nappe stack are similar in both studies.…”

“…Some additional parameters are constant: the thermal expansion coefficient α = 3 × 10 −5 K −1 , the heat capacity C p = 1050 J K −1 and the radiogenic heat production Q r = 10 −6 W m −3 . We use creep flow laws from Schmid et al (1977), 1 Hansen and Carter (1983) 2 and Kronenberg et al (1990). 3 We apply free slip boundary conditions on all sides of the model except on the top where we model a free surface boundary condition with the sticky air method (Fig.…”

“…Adding a circular inclusion with a smaller viscosity to the viscous material will not generate a significant shear zone inside the linear viscous material for this deformation configuration. The only possibility to generate a shear zone in the viscous material is to add a softening mechanism, such as thermal softening (Jaquet et al, 2015;Kiss et al, 2019) or grain size reduction with grain-size-sensitive flow laws (Bercovici and Ricard, 2003;Austin et al, 2008). In contrast, if the linear viscous material is sheared over a non-planar interface, resembling a half-graben, then a shear zone can develop inside the linear viscous material even without any softening mechanism (Bauville and Schmalholz, 2017).…”

Extrusion of subducted crust explains the emplacement of far-travelled ophiolites

“…In agreement with previous studies 22,31 , results show that shear heating is an important mechanism that facilitates strain localization and nappe formation. The precise reproduction of smaller scale nappes (nappe thickness of several kilometers) which is often observed in case of continental subduction 7,10 would require very high-resolution numerical modeling and built-in heterogeneities inside the upper crust to localize shear zones at multiple horizons 32,33 . Our results also support that crustal decoupling and exhumation may take place with different timing and position in the subducted continent depending on the rheology of the continental crust 34,35 .…”

Extrusion of subducted crust explains the emplacement of far-travelled ophiolites

“…In agreement with previous studies 26,30 , the results show that shear heating is an important mechanism that facilitates strain localization and nappe formation. The precise reproduction of smaller scale nappes (nappe thickness of several kilometers) which is often observed in case of continental subduction 9,13 would require very high-resolution numerical modeling and built-in heterogeneities inside the upper crust to localize shear zones at multiple horizons [37][38][39] . Our results also indicate that crustal decoupling and exhumation may take place with different timing and position in the subducted continent depending on the rheology of the continental crust 40,41 .…”