Tectonic Inheritance and Kinematic Strain Localization As Trigger for the Formation of the Helvetic Nappes, Switzerland
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Swiss J Geosci (2017) 110:523–534 DOI 10.1007/s00015-017-0260-9 Tectonic inheritance and kinematic strain localization as trigger for the formation of the Helvetic nappes, Switzerland 1,2 3 Arthur Bauville • Stefan M. Schmalholz Received: 30 May 2016 / Accepted: 13 January 2017 / Published online: 2 March 2017 Ó Swiss Geological Society 2017 Abstract The Helvetic nappes in Switzerland consist of depth and 25 km width the localization generates (a) low- sediments, which have been sheared off and thrust over the angle shear zones at the basement-sediment interface, but also crystalline basement of the European passive continental entirely within the sediments, (b) horizontal transport[10 km margin during Alpine orogeny. Their basal shear zones usu- associated with the shear zones, (c) shear zones with thickness ally root above the external crystalline massifs. However, the in the order of 100 m, (d) an ordered stacking of model nappes mechanisms that initiated the shear zones and the associated and (e) shear zones that root above the basement. The results nappe formation are still debated. We perform two-dimen- suggest that tectonic inheritance in the form of half-grabens sional numerical simulations of the shearing of linear viscous and associated kinematic strain localization could have been fluids above a linear viscous fluid with considerably higher the triggering mechanism for Helvetic nappe formation, and viscosity (quasi-undeformable). The boundary between the not rheological softening mechanisms, which might, how- fluid, mimicking the sediments, and the quasi-undeformable ever, have subsequently further intensified shear localization fluid, mimicking the basement, exhibits geometrical pertur- significantly. bations, mimicking half-grabens. These geometrical pertur- bations can trigger significant strain localization and the Keywords Tectonic inheritance Á Kinematic strain formation of shear zones within the linear viscous fluid localization Á Helvetic nappes Á Wedge although no rheological softening mechanism is active. This kinematic, ductile strain localization is caused by the half- grabens and the viscosity ratio between basement and sedi- 1 Introduction ments. The viscosity ratio has a strong control on the kine- matics of strain localization, whereas the depth of the half- During Alpine orogeny, parts of the sedimentary cover of grabens has a weak control. For sediment viscosities in the the European passive margin have been sheared off and order of 1021 Pas and typical half-graben geometries of 5 km thrust over the crystalline basement whereby the pre- Alpine basement topography was characterized by half- grabens and horsts (e.g. Tru¨mpy 1960). The half-grabens Editorial handling: C. Sue and S. Schmid. and horsts represent a tectonic inheritance from an earlier & Arthur Bauville rifting event. At present the sheared and displaced sedi- [email protected] mentary units form sedimentary nappes within the Helvetic 1 nappe system of Switzerland (Fig. 1). Parts of the crys- Department of Mathematical Science and Advanced talline basement, including autochthonous cover sediments, Technology (MAT), Japan Agency for Marine-Earth Sciences and Technology (JAMSTEC), Yokohama, Japan have also been deformed and form the external crystalline massifs (e.g. Me´not et al. 1994). The sedimentary nappes 2 Geophysics and Geodynamics Group, Institute of Geosciences, Johannes Gutenberg University, Mainz, have been displaced over more than 10 km along low angle Germany shear zones. However, the mechanisms responsible for 3 Institute of Earth Sciences, University of Lausanne, triggering and forming these shear zones are still incom- Lausanne, Switzerland pletely understood. 524 A. Bauville, S. M. Schmalholz Fig. 1 Simplified geological cross sections across the Helvetic nappe Molasse and Flysch sediments (a). The cross sections are parallel to system (a–c) and conceptual model of its evolution (e, f). The the displacement direction of the nappes. e Conceptual palinspastic simplified tectonic map in d shows the location of the cross sections. reconstruction of the European margin in Western and Central a–c Cross sections across the Helvetic nappe system in Eastern, Switzerland. f Conceptual model of nappe emplacement: sediments central and western Switzerland, respectively (after Pfiffner 1993; initially filling distal basins are displaced tens of kilometers over more Herwegh and Pfiffner 2005; Escher et al. 1993, respectively). The proximal basement and sediment units while acquiring their typical Helvetic nappes are thrusted along a prominent basal shear zone nappe geometry (modified after Steck 1984; Bugnon 1986) (thick red line) over the Infra-Helvetic complex (b, c) and over Geologic reconstructions suggest that the sediments in a fold nappe as in Western Switzerland (Fig. 1; Siegenthaler, structurally higher nappes have been deposited in more distal 1974). The basal shear zones of the tectonic nappes are often (SE direction) regions of the passive margin (Fig. 1;e.g. (a) continuous over more than10 km, (b) subhorizontal, and Masson et al. 1980). The sedimentary units forming indi- (c) root in the external crystalline massifs (Fig. 1, see also vidual nappes have been attributed to different depositional Pfiffner et al. 2011). domains related to the paleo-topography of the pre-Alpine Many studies argue that softening mechanisms are European basement (Fig. 1). The lowermost nappes in required to generate ductile shear zones and that basal Western Switzerland are fold nappes (Morcles and Dolden- shear zones during nappe displacement are weaker than the horn nappes; parts of the so-called Infrahelvetic complex; less deformed rocks forming the nappe (e.g. Poirier 1980; Epard 1990; Pfiffner 1993) and consist of sediments that have Austin et al. 2008). Therefore, different thermo-mechanical been deposited in the North-Helvetic basin which is inter- processes have been suggested for the softening of basal preted as a half-graben (Epard 1990). The uppermost nappes shear zones, such as grain size reduction in combination are more similar to thrust sheets (so-called Helvetic nappes, with grain size sensitive diffusion creep (e.g. Ebert et al. e.g. Wildhorn nappe) and have been deposited in the more 2008; Austin et al. 2008), higher temperatures at the base distal Helvetic domain (basin) which was separated from the of the nappe in combination with a temperature-dependent North-Helvetic basin by a horst (e.g. Epard 1990). In Eastern viscosity (e.g. Bauville et al. 2013) or combined shear Switzerland, the Glarus thrust sheet has been sheared over heating and fluid release by carbonate decomposition (e.g. more proximal basement, autochthonous Mesozoic sediments Poulet et al. 2014). In contrast, we show in this study that and Flysch units whereby the Infrahelvetic units did not form basal shear zones can also be initiated and maintained Tectonic inheritance and kinematic strain localization as trigger for the formation of the… 525 during finite strain deformation in a linear viscous fluid (mimicking sediments) which is sheared over a rigid mate- without any softening mechanism. rial with laterally-varying topography (mimicking basement Structural observations, such as the buttressing of sedi- with half grabens). Following earlier nomenclature, we refer ments against the crystalline massif in the Ecrins and Bourg to such a strain localization process in a linear viscous fluid d’Oisan massifs in France (Gillcrist et al. 1987,Dumont as kinematic strain localization (e.g. Brun, 2002; Braun et al. et al. 2008, Bellanger et al. 2014), indicate that the basement 2010). Kinematic strain localization can be caused by the was more competent (i.e. having a higher mechanical applied boundary conditions (e.g. a velocity discontinuity; strength or greater resistance to deformation) than the sed- Brun 2002), the geometrical configuration (e.g. flow around imentary cover. The increase of strain towards the base of a rigid obstacle or particular shear zone geometry; Braun spreading-gliding nappes is another example for such com- et al. 2010) or internal variations in mechanical strength, petence contrast (e.g. Brun and Merle 1985;Merle1986). such as the viscosity variation (due to thermal gradient) in The impact of the basement-cover competence contrast on channel flow models (e.g. Bauville and Schmalholz 2015). nappe formation has been studied with analogue experi- The aims of this study are (a) to show that significant ments and numerical simulations. For example, experiments shear localization can occur within a linear viscous fluid if showed that the indentation of a viscous material by a rigid it is sheared above a rigid material with topography mim- object leads to the formation of a shear zone above the rigid icking half-grabens, (b) to quantify the shear localization indenter and the subsequent formation of a viscous fold and shear zone thickness inside the linear viscous fluid, nappe (e.g. Bucher 1956; Merle and Guillier 1989). Similar (c) to study the impact of half-graben depth and fluid vis- results have been obtained with numerical simulations cosity on shear localization and (d) to argue that the pre- (Duretz et al. 2011). Also, Wissing and Pfiffner (2003) Alpine basement topography, which represents the tectonic showed with viscoplastic numerical simulations that the inheritance from earlier rifting, could have triggered shear formation of a nappe with a prominent basal shear zone can zone formation within sediments and hence played an be triggered by a rigid obstacle