Tectonic Inheritance Controls Nappe Detachment, Transport and Stacking in the Helvetic Nappe System, Switzerland: Insights from Thermomechanical Simulations
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Solid Earth, 11, 287–305, 2020 https://doi.org/10.5194/se-11-287-2020 © Author(s) 2020. This work is distributed under the Creative Commons Attribution 4.0 License. Tectonic inheritance controls nappe detachment, transport and stacking in the Helvetic nappe system, Switzerland: insights from thermomechanical simulations Dániel Kiss1, Thibault Duretz2,1, and Stefan Markus Schmalholz1 1Institute of Earth Sciences, University of Lausanne, 1015 Lausanne, Switzerland 2Université Rennes, CNRS, Géosciences Rennes – UMR6118, 35000 Rennes, France Correspondence: Dániel Kiss ([email protected]) Received: 14 August 2019 – Discussion started: 26 August 2019 Revised: 10 January 2020 – Accepted: 21 January 2020 – Published: 4 March 2020 Abstract. Tectonic nappes have been investigated for more to model nappe structures similar to the ones of the Helvetic than a hundred years. Although geological studies often re- nappe system. Our results indicate that the structural evolu- fer to a “nappe theory”, the physical mechanisms of nappe tion of the Helvetic nappe system was controlled by tectonic formation are still disputed. We apply two-dimensional nu- inheritance and that material softening mechanisms are not merical simulations of shortening of a passive margin to essential to reproduce the first-order nappe structures. 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 1 Introduction laws, Drucker–Prager and von Mises yield criteria. We con- sider tectonic inheritance with two initial mechanical hetero- Tectonic nappes were discovered more than a hundred years geneities: (1) lateral heterogeneity of the basement–cover in- ago and are considered typical tectonic features of orogenic terface due to half-grabens and horsts and (2) vertical hetero- belts (e.g., Price and McClay, 1981), particularly in the Alps geneities due to layering of mechanically strong and weak (e.g., Lugeon, 1902; Termier, 1906; Argand, 1916; Tollmann, sedimentary units. The model shows detachment and hori- 1973; Trümpy, 1980; Escher et al., 1993; Pfiffner, 2014). zontal transport of a thrust nappe that gets stacked on a fold Several definitions of a nappe have been proposed (see dis- nappe. The detachment of the thrust sheet is triggered by cussion in Price and McClay, 1981), for example, after Ter- stress concentrations around the sediment–basement contact mier(1922): “A nappe is a rock packet not in its place, resting and the resulting brittle–plastic shear band that shears off the on a substratum that is not its original one”. Two end-member sedimentary units from the sediment–basement contact. Hor- types of nappes are commonly distinguished, namely fold izontal transport is facilitated by a basal shear zone just above nappes and thrust nappes, or thrust sheets (e.g., Termier, the basement–cover contact, composed of thin, weak sedi- 1906; Price and McClay, 1981; Epard and Escher, 1996). ments that act as a décollement. Fold nappe formation occurs Fold nappes are recumbent folds exhibiting large-scale strati- by a dominantly ductile closure of a half-graben and the asso- graphic inversion, typically with amplitudes that exceed sev- ciated extrusion of the half-graben fill. We apply our model to eral kilometers. In contrast, thrust sheets are allochthonous the Helvetic nappe system in western Switzerland, which is sheets with a prominent shear zone or thrust at their base but characterized by stacking of the Wildhorn thrust nappe above without a prominent overturned limb. The importance of tec- the Morcles fold nappe. The modeled structures, the defor- tonic nappes for orogeny, especially for collisional orogens, mation rates and the temperature field agree with data from is nowadays well established; however, the physical mecha- the Helvetic nappe system. Mechanical heterogeneities must nisms of nappe detachment (or shearing off), transport and locally generate effective viscosity (i.e., ratio of stress to vis- stacking are still disputed. coplastic strain rate) contrast of about 3 orders of magnitude Published by Copernicus Publications on behalf of the European Geosciences Union. 288 D. Kiss et al.: Tectonic inheritance controls nappe detachment, transport and stacking We focus here on the Helvetic nappe system in western crist et al., 1987; Ebert et al., 2008; Bauville et al., 2013). Switzerland (see next section for a geological overview), However, there are also interpretations arguing for localized which is one of the birthplaces of the concept of tectonic thrusting and dominantly brittle–plastic deformation mecha- nappes. In 1841, Arnold Escher mentioned a nappe (he used nisms, such as fracturing related to fluid pressure (e.g., Boyer “Decke” in German) and a colossal overthrust (“colossale and Elliott, 1982; Granado and Ruh, 2019). Furthermore, the Überschiebung”) during the presentation of a geological map presumed driving forces of nappe transport are either exter- of the canton of Glarus, eastern Switzerland (Escher von der nal surface forces, due to tectonic compression, or internal Linth, 1841). Escher did not dare to publish his interpreta- body forces, due to gravity. Heterogeneous shearing due to tion but explained it in the field to Roderick Murchinson, buttressing in a general compressional regime is an exam- who published the overthrust interpretation in 1849 (Murchi- ple of deformation driven by external forces (e.g., Ramsay son, 1849), crediting Escher for the original observation. et al., 1983; Epard, 1990; Bauville et al., 2013; Boutoux Bertrand(1884) argued also convincingly for an overthrust et al., 2014). Gravity gliding and spreading are an example nappe (he used “masse de recouvrement” and “lambeaux de of deformation driven by body forces (e.g., Durney, 1982; recouvrement” instead of nappe) in the Glarus region, so that Merle, 1989; Merle and Guillier, 1989). For thrust sheets, finally also Heim(1906) accepted the overthrust interpreta- the prominent low-angle thrust planes are likely controlled tion instead of the earlier preferred double-fold interpreta- by mechanical heterogeneities, such as the orientation of the tion (“Überschiebungsfalte” instead of “Doppelfalte”). Al- basement–cover interface and of mechanically weak shale- though the important controversies and observations support- rich or evaporite layers, as has been suggested for the Hel- ing tectonic nappes are related to the Glarus region, which is vetic nappe system (e.g., Pfiffner, 1993; Steck, 1999; Pfiffner part of the Helvetic nappe system, the true birth date of the et al., 2011; Bauville and Schmalholz, 2017). Several soft- nappe concept in the Alps, according to Trümpy (1991), is ening mechanisms have been proposed to localize deforma- the publication by Schardt(1893) who worked in the Pre- tion at the base of the thrust sheet, such as reduction of the alps, belonging to the Penninic domain (e.g., Escher et al., effective stress due to pore fluid pressure causing a reduc- 1993). Schardt(1893) realized that Jurassic breccias have tion of the effective friction angle (e.g., King Hubbert and been thrust over Tertiary flysch and that large regions of the Rubey, 1959; Rubey and King Hubbert, 1959) or a domi- Prealps have been actually emplaced as a major overthrust. nantly ductile deformation mechanism (e.g., Smoluchowski, After decades of controversy, the existence of nappes was 1909; Goguel, 1948; Voight, 1976), presumably in combi- generally accepted approximately a century ago, revolution- nation with thermally, chemically or mechanically activated izing tectonics, Alpine geology and orogeny in general (for softening mechanisms (e.g., Poirier, 1980; Ebert et al., 2008; historical reviews, see Bailey, 1935; Masson, 1976; Merle, Poulet et al., 2014). 1998; Trümpy, 1991; Dal Piaz, 2001; Schaer, 2010). To make another step towards understanding the physical Since then, a considerable effort has been made in map- process of nappe formation, we investigate the detachment, ping the present-day structure of the Helvetic nappe system transport and stacking of nappes with two-dimensional (2- (e.g., Steck, 1999; Pfiffner et al., 2011). Structural and pale- D) numerical simulations based on continuum mechanics. ogeographic reconstructions have provided valuable insight To keep the model relatively simple, we focus here on ther- into the kinematics of nappe formation (e.g., Gillcrist et al., momechanical processes on the macro scale, larger than the 1987; Epard and Escher, 1996; Herwegh and Pfiffner, 2005; typical size of mineral grains. Hence, we do not consider Bellahsen et al., 2012; Boutoux et al., 2014). Therefore, the hydrochemical couplings, such as fluid release by carbon- geometrical structure and kinematic evolution of the Helvetic ate decomposition (e.g., Poulet et al., 2014), and microscale nappe system is reasonably well understood. There are also processes, such as microstructural grain size evolution with theoretical and analogue modeling studies investigating the secondary phases (e.g., Herwegh et al., 2011). The numerical formation of fold-and-thrust belts and nappes (e.g., Bucher, algorithm is based on the finite-difference method. We con- 1956; Rubey and King Hubbert, 1959; Dietrich and Casey, sider a standard visco-elasto-plastic deformation behavior, 1989; Merle, 1989; Casey and Dietrich, 1997; Wissing and heat transfer and thermomechanical coupling by shear heat- Pfiffner, 2003; Bauville et al., 2013; Poulet et al., 2014; Erdos˝ ing and temperature-dependent