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.
Highlights
Tectonic nappes were discovered more than a hundred years ago and are considered typical tectonic features of orogenic belts (e.g., Price and McClay, 1981), in the Alps (e.g., Lugeon, 1902; Termier, 1906; Argand, 1916; Tollmann, 1973; Trümpy, 1980; Escher et al, 1993; Pfiffner, 2014)
The main aim of this study is to show that a thermomechanical model based on the theory of continuum mechanics (i) with a well-established visco-elasto-plastic rheology based on standard flow laws, (ii) with mechanical heterogeneities mimicking pre-Alpine extensional heritage and stratigraphic layering and (iii) with a wedge-type compressional configuration can self-consistently explain the firstorder features of nappe detachment, transport and stacking in the Helvetic nappe system
In agreement with previous modeling studies (e.g., Beaumont et al, 2000; Wissing and Pfiffner, 2003; Bellahsen et al, 2012; Lafosse et al, 2016; Bauville and Schmalholz, 2017), our results suggest that tectonic inheritance in the form of half-grabens and horsts has a strong impact on the development of fold and thrust nappes during crustal deformation
Summary
Tectonic nappes were discovered more than a hundred years ago and are considered typical tectonic features of orogenic belts (e.g., Price and McClay, 1981), in the Alps (e.g., Lugeon, 1902; Termier, 1906; Argand, 1916; Tollmann, 1973; Trümpy, 1980; Escher et al, 1993; Pfiffner, 2014). Thrust sheets are allochthonous sheets with a prominent shear zone or thrust at their base but without a prominent overturned limb. The importance of tectonic nappes for orogeny, especially for collisional orogens, is nowadays well established; the physical mechanisms of nappe detachment (or shearing off), transport and stacking are still disputed
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