Helvetic Nappes Research Articles

Priabonian (upper Eocene) larger foraminifera from the Helvetic Nappes of the Alps (Western Switzerland): new markers for Shallow Benthic zones 19-20

Here, we revise and update the biostratigraphy of larger foraminiferal assemblages in three sections of the Priabonian Sanetsch Formation in the Helvetic Nappes of theWestern Swiss Alps: The SexRouge (SE) and the Sanetsch Buvette (SA) sections in the Wildhorn Nappe Complex, and the Col des Essets (ETS) section in the most external Morcles Nappe. In the SE and SA sections, the Tsanfleuron and most of the Pierredar Limestone members of the formation are assigned to SBZ 19 (early Priabonian), while the uppermost part of the formation is assigned to SBZ 20 (late Priabonian). In the external ETS section the entire Sanetsch Formation contains assemblages characteristic of SBZ 19, suggesting an earlier, middle-late Priabonian onset of the hemipelagic Stad Formation (Globigerina Marls). Since it was established in 1998, the Shallow Benthic Zones (SBZ), a biozonation based on larger foraminifera, has been a useful tool in the biostratigraphy of the Paleogene. Biozonation proposals for the late middle-late Eocene are based mainly in biometrical subdivision of lineages of nummulitids and orthophragmines,which requiresmeasurements in oriented sections of isolated specimens. Here, we define previously unreported taxa from the Sanetsch Formation, which are considered characteristic for the Priabonian. They are easy to identify in random sections and thus useful biostratigraphical markers.We also describe a new orthophragminid genus, Virgasterocylina n. gen. (Orbitoclypeidae) characterized by the presence of rods, radial thickenings of calcite along ribs; a new species of Rotorbinella, R. epardi n. sp., and a new genus and new species of difficult suprageneric attribution, Sanetschella indeprensa n. gen., n. sp. We add the new taxa to the larger foraminiferal association characterizing the Priabonian (SBZ 19-20). The revision of the literature, together with our own sample collections revealed that these new taxa occur in Priabonian rocks from different basins in the western Tethys. Virgasterocylina n. gen. also occurs in the Caribbean bioprovince in the middle and upper Eocene. In the western Tethys, Virgasterocylina ferrandezi is subdivided into two subspecies, V. f. ferrandezi (Ozcan and Less) and V. f. lessi n. ssp., which characterize the SBZ 19 and 20 biozones respectively.

Large thrusting and late faulting shape the Aiguilles Rouges crystalline massif (Western Alps), structural implications

Although the Helvetic nappes of the Western Alps have been widely studied, their style, amount, and timing of deformation during the Alpine orogeny remain a subject of controversy. The discussion partly arises from various interpretations of the external crystalline massifs geometry and of the cover/basement relationships. Here, we apply structural analysis based on new field data, microtectonics, and 3D geological modeling on the Aiguilles Rouges and Mont-Blanc massifs, to better quantify their tectonic evolution and their relationship with the nappe stacking. Our results show large thrusting of the Helvetic Mesozoic sedimentary cover above the Aiguilles Rouges basement towards the WNW. The basal contact of the Helvetic nappes, which we call the Helvetic Basal thrust (HBT), often corresponds to a décollement level localized above the Triassic unconformity, but that locally also affects the basement. We suggest that most of the subalpine ranges are klippes above the HBT, and estimate the shortening of the thrust from previously published cross-sections to be between 13 and 32 km. According to published ages, we argue that the HBT was active in the Oligo-Miocene, between ∼30 and ∼14 Ma. After ∼14 Ma and until ∼6 Ma, the HBT is offset by late steep out-of-sequence faults, including back-thrusts and the Mont-Blanc shear zone that was previously a ramp of the HBT. The transition of the major thrust system from the HBT rooting above (east) of the Mont-Blanc to the Alpine sole thrust rooting below (west) of the Aiguilles Rouges occurred between 18 and 14 Ma.

New insights into the Rhône–Simplon fault system (Swiss Alps) from a consistent earthquake catalogue covering 35 yr

SUMMARYSeismotectonic interpretations in regions characterized by low to moderate seismicity require consistent earthquake catalogues covering periods of several decades. Inevitable changes in network configuration and analysing procedures, however, introduce significant bias to the hypocentre parameters and uncertainty estimates reported in such catalogues. To overcome these limitations, we developed a procedure using coupled hypocentre-velocity inversions to compute consistent hypocentre locations covering time periods of several decades while accounting for changes in network geometry. We apply these procedures to 35 yr of instrumentally recorded seismicity along the Rhône–Simplon fault system in southwest Switzerland, which is at the transition between the Central and Western Alps. The entire catalogue is relocated using a probabilistic location algorithm in combination with the derived minimum 1-D velocity models. A combination of location parameters is used to define consistent location-quality classes allowing for reliable interpretation of epicentres and focal depths. The relocated catalogue is interpreted together with a recent 3-D P-wave tomographic model and available 2-D reflection seismic profiles. The relocated hypocentres indicate that the major band of seismicity north of the Rhône valley is associated with a 30–40 km long, steeply north-dipping shear zone, which roots in the crystalline basement of the Aar Massif and extends to the shallowest levels of the sedimentary cover of the Helvetic nappes in the Rawil Depression. Seismicity towards the southwest indicates the existence of a similar shear zone within the Aiguille Rouge Massif. This zone possibly extends to the northeast and joins the Rawil fault zone. To the south of the Rhône valley, seismicity is scattered within the Penninic nappes, but limited to the hanging wall of the Pennine Basal Thrust (PBT). The Penninic nappes are characterized by a relatively higher VP of about 5 per cent compared to the Aar Massif, indicating differences in composition or metamorphic grade across the PBT.

Complex shape of the Palaeoproterozoic Serpovidny refolded mega-sheath fold in northern Fennoscandia revealed by magnetic and structural data

The Serpovidny large-scale synform located in the Arctic zone of the north-eastern Fennoscandian Shield was studied with geological mapping and structural analysis, and its core with a ground detailed magnetic survey. This is a tight to isoclinal mega-sheath fold with the strongly thinned lower limb, almost undeformed upper limb, and the ՙcat's eye՚ core. The innermost rift-related sequence of the fold core is a synformal anticline, whilst limbs are composed of Palaeoproterozoic Keivy Group (metamorphosed redeposited products of mainly deep chemical weathering) and form a syncline. The Serpovidny mega-sheath fold formed during progressive non-coaxial, high strain deformation in simple shear/general shear regimes and a northward thrusting under mid-crustal conditions. This mega-sheath fold evolved from a NNE-verging anticline originated at the base of a nappe transported north-northeastwards from the Palaeoproterozoic Imandra-Varzuga Rift-Belt during the 1.9 Ga Lapland-Kola collisional orogeny. After a thrusting of this nappe onto the Keivy Group, one anticline plunged into these rocks and was transformed into the NNE-closing Serpovidny mega-sheath fold. Its geometric parameters and/or kinematic evolution are close to those of mega-sheath folds in Cretaceous rocks in the Oman Mountains, large-scale synformal anticlines in the Helvetic nappes in the Alps, and a Caledonian nappe in northwest Ireland.

Recumbent folding in the Upper Cretaceous Eaux‐Chaudes massif: A Helvetic‐type nappe in the Pyrenees?

AbstractWe describe a singular structure in Upper Cretaceous rocks of the Eaux‐Chaudes massif of the western Pyrenees, consisting of a kilometre‐scale fold nappe with a sheared overturned limb. High ductile strain attests a deformation style rarely reported for the alpine Pyrenees, and peak temperature in Upper Cretaceous carbonates is estimated by Raman spectroscopy of carbonaceous material palaeothermometry in the lower greenschist facies (>300°C). The normal fold limb retains the original sedimentary textures, while the overturned limb shows calcite crystal‐plastic deformation and dynamic recrystallization, with crystallographic preferred orientation. The observed ductility and metamorphic temperature bear similarities with the lower Helvetic nappes of the Alps, suggesting deep burial and/or possibly high geothermal gradient in this part of the Pyrenees.

Quantity and distribution of methane entrapped in sediments of calcareous, Alpine glacier forefields

Abstract. Aside from many well-known sources, the greenhouse gas methane (CH4) was recently discovered entrapped in the sediments of Swiss Alpine glacier forefields derived from calcareous bedrock. A first study performed in one glacial catchment indicated that CH4 was ubiquitous in sediments and rocks and was largely of thermogenic origin. Here, we present the results of a follow-up study that aimed at (1) determining the occurrence and origin of sediment-entrapped CH4 in other calcareous glacier forefields across Switzerland and (2) providing an inventory of this sediment-entrapped CH4, i.e., determining the contents and total mass of CH4 present, and its spatial distribution within and between five different Swiss glacier forefields situated on calcareous formations of the Helvetic nappes in the Central Alps. Sediment and bedrock samples were collected at high spatial resolution from the forefields of Im Griess, Griessfirn, Griessen, Wildstrubel, and Tsanfleuron glaciers, representing different geographic and geologic regions of the Helvetic nappes. We performed geochemical analyses on gas extracted from sediments and rocks, including the determination of CH4 contents, stable carbon-isotope analyses (δ13CCH4), and the determination of gas-wetness ratios (ratio of CH4 to ethane and propane contents). To estimate the total mass of CH4 entrapped in glacier-forefield sediments, the total volume of sediment was determined based on the measured forefield area and either literature values of mean sediment thickness or direct depth measurements using electrical resistivity tomography. Methane was found in all sediments (0.08–73.81 µg CH4 g−1 dry weight) and most rocks (0.06–108.58 µg CH4 g−1) collected from the five glacier forefields, confirming that entrapped CH4 is ubiquitous in these calcareous formations. Geochemical analyses further confirmed a thermogenic origin of the entrapped CH4 (average δ13CCH4 of sediment of −28.23 (± 3.42) ‰; average gas-wetness ratio of 75.2 (± 48.4)). Whereas sediment-entrapped CH4 contents varied moderately within individual forefields, we noted a large, significant difference in the CH4 content and total CH4 mass (range of 200–3881 t CH4) between glacier forefields at the regional scale. The lithology and tectonic setting within the Helvetic nappes appeared to be dominant factors determining rock and sediment CH4 contents. Overall, a substantial quantity of CH4 was found to be entrapped in Swiss calcareous glacier forefields. Its potential release and subsequent fate in this environment is the subject of ongoing studies.

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

Abstract. We apply three-dimensional (3-D) thermo-mechanical numerical simulations of the shortening of the upper crustal region of a passive margin in order to investigate the control of 3-D laterally variable inherited structures on fold-and-thrust belt evolution and associated nappe formation. We consider tectonic inheritance by employing an initial model configuration with basement horst and graben structures having laterally variable geometry and with sedimentary layers having different mechanical strength. We use a visco-plastic rheology with a temperature-dependent flow law and a Drucker–Prager yield criterion. The models show the folding, detachment (shearing off) and horizontal transport of sedimentary units, which resemble structures of fold and thrust nappes. The models further show the stacking of nappes. The detachment of nappe-like structures is controlled by the initial basement and sedimentary layer geometry. Significant horizontal transport is facilitated by weak sedimentary units below these nappes. The initial half-graben geometry has a strong impact on the basement and sediment deformation. Generally, deeper half-grabens generate thicker nappes and stronger deformation of the neighbouring horst, while shallower half-grabens generate thinner nappes and less deformation in the horst. Horizontally continuous strong sediment layers, which are not restricted to initial graben structures, cause detachment (décollement) folding and not overthrusting. The amplitude of the detachment folds is controlled by the underlying graben geometry. A mechanically weaker basement favours the formation of fold nappes, while stronger basement favours thrust sheets. The model configuration is motivated by applying the 3-D model to the Helvetic nappe system of the Central Alps of France and Switzerland. Our model reproduces several first-order features of this nappe system, namely (1) closure of a half-graben and associated formation of the Morcles and Doldenhorn nappes, (2) overthrusting of a nappe resembling the Wildhorn and Glarus nappes, and (3) formation of a nappe pile resembling the Helvetic nappes resting above the Infrahelvetic complex. Furthermore, the finite strain pattern, temperature distribution and timing of the 3-D model is in broad agreement with data from the Helvetic nappe system. Our model, hence, provides a 3-D reconstruction of the first-order tectonic evolution of the Helvetic nappe system. Moreover, we do not apply any strain softening mechanisms. Strain localization, folding and nappe transport are controlled by initial geometrical and mechanical heterogeneities showing the fundamental importance of tectonic inheritance on fold-and-thrust belt evolution.

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

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.

The 3‐D Thermal Structure of the Helvetic Nappes of the European Alps: Implications for Collisional Processes

AbstractUnderstanding the rheology of orogenic wedges requires the knowledge of the structural and thermal evolution of collisional units. In this study, we document the maximum temperature reached by the sedimentary cover nappes of the External Crystalline Massif (Western and central Alps) by Raman spectroscopy of carbonaceous material, between the Belledonne (France) and the Aar (Switzerland) Massifs. These cover units form the Helvetic/Dauphinois nappe complex. Maximum temperatures reached by the Upper Helvetic nappes lie in a range spanning from below 220 and 350 °C ± 50 °C. For the Lower Helvetic nappes, the temperatures spread between 226 and 358 °C ± 50 °C. These temperatures were projected on two structural cross sections in order to constrain the 3‐D thermal structure. From these data, we propose that the Helvetic nappes were deformed and emplaced before and/or during the thermal peak, which supports recent findings that shortening in the External Crystalline Massif was mainly accommodated during a 5‐ to 10‐Myr‐long thermal peak before deformation localized along crustal thrusts, which exhumed and cooled down the wedge. During this late exhumation, the isotherms corresponding to the thermal peak were passively folded.

Thermochronometry Across the Austroalpine‐Pennine Boundary, Central Alps, Switzerland: Orogen‐Perpendicular Normal Fault Slip on a Major “Overthrust” and Its Implications for Orogenesis

AbstractFifty‐one new and 309 published thermochronometric ages (nine systems with closure temperatures ranging from ~450 to 70°C) from the Graubünden region of the Central Alps demonstrate that a pronounced thermal mismatch between the Austroalpine allochthon (Alpine “orogenic lid”) and the Pennine zone persisted until at least 29 Ma and, allowably, until circa 18 Ma. The observed mismatch supports previous suggestions that the famous “overthrust” between the Austroalpine allochthon and the Pennine zone, historically regarded as primarily an Eocene top‐north thrust fault, is in fact primarily an Oligocene‐Miocene normal fault that has a minimum of 60 km of displacement with top‐south or top‐southeast sense of shear. Two hallmarks of Alpine geology, deposition of the foredeep Molasse and emplacement of the Helvetic nappes, appear to be coeval, peripheral manifestations of crustal thickening via the interposition of the Pennine zone as a northward intruding wedge between the Austroalpine “lid” and the European cratonic margin, with the Helvetic system (European margin) acting as the “floor” of the wedge. We presume the Penninic wedge is driven by the buoyant rise of subducted crust no longer able to remain attached to the descending slab. If so, emplacement of the Pennine wedge could have occurred mainly after Adria was juxtaposed against cratonic Europe.

Lithosphere strain rate and stress field orientations near the Alpine arc in Switzerland

In this study we test whether principal components of the strain rate and stress tensors align within Switzerland. We find that 1) Helvetic Nappes line (HNL) is the relevant tectonic boundary to define different domains of crustal stress/surface strain rates orientations and 2) orientations of T- axes (of moment tensor solutions) and long-term asthenosphere cumulative finite strain (from SKS shear wave splitting) are consistent at the scale of the Alpine arc in Switzerland. At a more local scale, we find that seismic activity and surface deformation are in agreement but in three regions (Basel, Swiss Jura and Ticino); possibly because of the low levels of deformation and/or seismicity. In the Basel area, deep seismicity exists while surface deformation is absent. In the Ticino and the Swiss Jura, where seismic activity is close to absent, surface deformation is detected at a level of ~2 10−8/yr (~6.3 10−16/s).

Formation of orogenic wedges and crustal shear zones by thermal softening, associated topographic evolution and application to natural orogens

The model of an orogenic wedge has been applied to explain the tectonic evolution of many orogens worldwide. Orogenic wedges are characterized by (1) a first-order shear zone which underthrusts the mantle lithosphere and lower crust beneath the adjacent mantle lithosphere and (2) a sequence of second-order upper crustal shear zones which form tectonic nappes. Shear zone formation in the lithosphere is, however, incompletely understood. We perform two dimensional thermo-mechanical numerical simulations of lithospheric shortening to study shear zone formation, propagation and associated wedge formation. The only perturbation in the model lithosphere is a different temperature at the left (1300° C) and right (1400° C) half of the model bottom. Despite this smooth and weak perturbation, simulations show self-consistent and spontaneous formation of first- and second-order shear zones generating an orogenic wedge. The shear zones are caused by thermal softening and temperature-dependent rock viscosity. Lateral spacing of upper crustal shear zones spans between 30 and 50km and is controlled by the depth of the boundary between upper and lower crust which acts as mechanical detachment level. Modelled upper crustal shear zones are active for ∼1 to ∼4My. Surface processes such as sedimentation and erosion influence shear zone orientation, spacing and duration but do not impact fundamental processes of shear zone formation and propagation. Simulations produce both singly-vergent and doubly-vergent wedges. Topographic uplift rates are controlled by the applied bulk shortening rate. The modelled surface uplift and subsidence associated with crustal shear zones could explain major consecutive thrusting events and related sedimentation within flexural basins during the formation of the Helvetic nappe system in Western Switzerland. Furthermore, results for shear zone propagation along a mid-crustal detachment and associated uplift of crustal basement could potentially explain foreland basement-cored uplifts in natural orogens such as the Laramide orogen, Taiwan or the Shillong Plateau.

Tectonic inheritance and kinematic strain localization as trigger for the formation of the Helvetic nappes, Switzerland

The Helvetic nappes in Switzerland consist of sediments, which have been sheared off and thrust over the crystalline basement of the European passive continental margin during Alpine orogeny. Their basal shear zones usually root above the external crystalline massifs. However, the mechanisms that initiated the shear zones and the associated nappe formation are still debated. We perform two-dimensional numerical simulations of the shearing of linear viscous fluids above a linear viscous fluid with considerably higher viscosity (quasi-undeformable). The boundary between the fluid, mimicking the sediments, and the quasi-undeformable fluid, mimicking the basement, exhibits geometrical perturbations, mimicking half-grabens. These geometrical perturbations can trigger significant strain localization and the formation of shear zones within the linear viscous fluid 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 sediments. The viscosity ratio has a strong control on the kinematics of strain localization, whereas the depth of the half-grabens has a weak control. For sediment viscosities in the order of 1021 Pas and typical half-graben geometries of 5 km depth and 25 km width the localization generates (a) low-angle shear zones at the basement-sediment interface, but also entirely within the sediments, (b) horizontal transport >10 km associated with the shear zones, (c) shear zones with thickness in the order of 100 m, (d) an ordered stacking of model nappes and (e) shear zones that root above the basement. The results suggest that tectonic inheritance in the form of half-grabens and associated kinematic strain localization could have been the triggering mechanism for Helvetic nappe formation, and not rheological softening mechanisms, which might, however, have subsequently further intensified shear localization significantly.

Trip through the Wildhorn Nappe from Cretaceous to Neogene time (Helvetic Nappes, Switzerland)

trip through the Wildhorn Nappe from Cretaceous to Neogene time (Helvetic Nappes, Switzerland)

3D structural model and kinematic interpretation of the Panixer Pass Transverse Zone (Infrahelvetic Complex, eastern Switzerland)

The Panixer Pass Transverse Zone in the eastern Swiss Alps is oriented perpendicular to most alpine structures in the area. Its main element is the SSE-trending Crena-Martin Fold, a downward facing fold with Permian Verrucano in its core, which is cut by the Glarus Thrust. Hence Verrucano can be found below the Glarus Thrust in the Infrahelvetic Complex. Across the Panixer Pass Transverse Zone the structural buildup of the Infrahelvetic Complex changes considerably. Multiple published theories of the structural evolution are not satisfying particularly because traditional 2D geological cross-sections are insufficient to understand the 3D complexity. The main result and product of our study is a 3D structural model of the Panixer Pass Transverse Zone providing insight into its geometry. As modeling input, we produced a lithostratigraphic map and collected structural orientation data. The 3D structural model honors the observed surface geology and the expected 3D subsurface geometry. Our field data indicates that the shearing and transport direction was continuously NNW-directed, except for a phase of north-directed shearing during the early movement along the Glarus Thrust (late Calanda Phase) and related foliation development in the Helvetic Nappes. The Panixer Pass Transverse Zone developed prior to the penetrative foliation during a thrust-dominated deformation phase (Cavistrau Phase), for which we created a kinematic block model. According to this model, the Crena-Martin Fold is the result of multiple lateral ramps and related lateral fault-bend folds that all developed in a similar positon. In particular, we do not propose ENE-WSW-directed shortening to form the Crena-Martin Fold. The latter was finally cut at low angle by a dextral strike-slip fault to create the final geometry of the Panixer Pass Transverse Zone. Our kinematic model reproduces the main features of the 3D structural model and embeds well into previously proposed sequences of deformation phases.

3-D numerical models of viscous flow applied to fold nappes and the Rawil depression in the Helvetic nappe system (western Switzerland)

The Helvetic nappe system exhibits three-dimensional (3-D) features such as the lateral variation in geometry between the Morcles and Doldenhorn fold nappes or the Rawil depression. We perform 3-D finite element simulations of linear and power-law viscous flow to investigate fold nappe formation during shortening of a half graben with laterally varying thickness. 3-D ellipsoids and corresponding 2-D intersection ellipses are used to quantify finite strain. Fold nappes which formed above a thicker graben have (i) larger amplitudes, (ii) a less sheared and thinned overturned limb, and (iii) a larger thickness than fold nappes formed above a thinner graben. These results agree with observations for the Morcles and Doldenhorn nappes. We also perform 3-D simulations for a tectonic scenario suggested for the evolution of the Rawil depression. The basement is shortened and extended laterally and includes a graben which is oblique to the shortening direction and acts as mechanical weak zone. The graben causes laterally varying basement uplift generating a depression whose amplitude depends on the graben orientation and the stress exponent of basement and sediments. The axial plunge of the depression is smaller (approximately 10°) than the observed plunge (approximately 30°) indicating that additional processes are required to explain the geometry of the Rawil depression.

Basement-involved thin-skinned and thick-skinned tectonics in the Alps

AbstractThe deformation of continental crust during continental collision by folding and thrusting follows three types of structural styles: (1) in a true thin-skinned style only cover rocks are involved; (2) in the case of a thin-skinned basement-involved style, thin slabs of crystalline basement rocks are piled up into a nappe stack; (3) in a true thick-skinned style, the entire upper crust is involved in the deformation. In the Alps all three styles can be recognized. The Helvetic nappes and parts of the Penninic nappes exhibit true thin-skinned style tectonics. Triassic evaporites, Jurassic shales and Cretaceous marls acted as detachment horizons. Basement-involved thin-skinned tectonics is typical for the Penninic nappes in the core of the orogen. The thickness of the basement thrust sheets is controlled by the effects of Mesozoic rifting, by deep burial and heating of the subducting crust and by the presence of Late Palaeozoic structures. Thick-skinned style is observed in the more external parts of the orogen, the external massifs and the Southalpine nappe system. It occurred in the late phase of collision and involved the entire upper crust. The basal detachment occurred possibly along phyllonites generated by the breakdown of load-bearing feldspar. Considering the Alpine orogen as a whole, the lower crust deformed seemingly independently from the upper crust. The detachment of the cover units by thin-skinned tectonics occurred prior to thrusting related to basement-involved thin-skinned tectonics. Thrust faults of both types were overprinted by ‘post-nappe folding’.

Thermal history of the westernmost Eastern Alps (Penninic Rhenodanubian Flysch nappes, Helvetic nappes, and Subalpine Molasse thrust sheets)

The frontal part of the westernmost Eastern Alps comprises from top to bottom of the Austroalpine and Penninic nappes, Ultrahelvetic slices, and two Helvetic thrust sheets, thrust upon the northern Alpine Molasse Basin. The thermal evolution of the Penninic Rhenodanubian Flysch nappes, the Helvetic nappes, and the allochthonous part of the Alpine Molasse Basin is constrained by vitrinite reflectance measurements and apatite fission track dating and implemented in a tectonic evolution scheme. Within the Helvetic nappes, vitrinite reflectance increases regionally from north to south and stratigraphically from the Campanian–Maastrichtian Wang Formation to the Toarcian Mols Member. Apatite fission track ages from Penninic and Subalpine Molasse units are consistently younger than the deposition age. They indicate therefore a post-depositional thermal overprint exceeding approximately 120 °C, the upper temperature limit of the apatite partial annealing zone. 1D thermal modelling suggests that the Penninic nappes attained deepest burial between the latest Cretaceous and Early Palaeocene with the Penninic basal thrust being located at approximately 8 km in the north compared to approximately 12 km in the south. Deepest burial of the upper Helvetic nappe occurred between the latest Eocene and Early Miocene. Its base was buried down to approximately 10.5 km in the north compared to 11.5 km in the south. Exhumation of the entire nappe stack started in the Early to Middle Miocene. For both Penninic and Helvetic models, a heatflow minimum during the Cenozoic deformation (max. 27–32 mW/m2), followed by an increase from the Middle Miocene onwards (up to 60 mW/m2), was assumed.

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

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.

Inherited tectonic structures controlling the deformation style: an example from the Helvetic nappes of the Eastern Alps

Based upon tectonic as well as facies arguments, two different Helvetic nappes can be distinguished in western Austria (Vorarlberg) and in southwestern Germany (Upper Allgau): the Hohenems nappe and the overlying Vorarlberg Santis nappe. Both encompass Middle Jurassic to Eocene strata deposited on the internal to external shelf of the southward deepening European margin of the Eurasian plate. Synsedimentary normal faults caused changes in thickness and facies of the various strata, which play a crucial role in deformation behavior. Arcuate fold axes in map view and an almost 3 km thick sequence of stacked Middle Jurassic shales and sandstones drilled below a Jurassic anticlinorium in the southern part of the Bregenzerwald are thought to be indicative of an inverted Jurassic basin. Inversion occurred during the Cenozoic Alpine nappe formation along synsedimentary normal faults, reactivated as ramps and tear faults. A lateral ramp, segmented by tear faults, running along the Iller Valley, and a supposed lateral ramp in the subsurface of the Rhine Valley mark the extension of the inverted former basin. Fault deformation style changes across the Rhine Valley. East of it, i.e. in Vorarlberg, the Vorarlberg Santis nappe comprises a coherent succession of Jurassic and Cretaceous strata detached along Middle Jurassic sediments. In the west, on the other hand, Cretaceous strata of the Swiss Santis nappe were largely detached from their Jurassic substrate (Gonzen-Walenstadt imbricates) along the Santis thrust. This allows to correlate the Helvetic nappe stack of eastern Switzerland, comprising the Swiss Santis nappe (together with the Gonzen-Walenstadt imbricates) and the underlying Murtschen nappe, with the Vorarlberg Santis nappe and the Hohenems nappe of Austria.