Western Swiss Alps Research Articles

Present-day and future tectonic underplating in the western Swiss Alps: reconciliation of basement/wrench-faulting and décollement folding of the Jura and Molasse basin in the Alpine foreland

The western Alps form a geodynamically active mountain belt showing the typical features of an evolving orogenic wedge with its pro-wedge geometry to the NNW and its retro-wedge structures to the SSE. Renewed tectonic underplating of European continental crust occurred after the orogenic wedge underwent major dynamic disequilibrium following the break-off of the southward subducting slab of the European passive margin. The most important of these basement imbricates are the Mont-Blanc–Aiguilles Rouges and Gastern–Aar crystalline massifs, also forming the Alps' highest mountains. The upper plate–present-day orogenic wedge of the western Alps includes the Molasse basin and the Jura fold-and-thrust belt, both decoupled from the basement over a basal décollement surface. The overall geometry of this wedge appears to be strongly unstable according to simple wedge models. In its attempt to regain stability, out-of-sequence thrusts form in the existing basement nappes; but also new basement nappes should develop beneath the southern portion of the Molasse basin. New out-of-sequence thrusts in the cover, trigger higher than average uplift rates concentrated around the newly forming structures and are accompanied by a concentration of earthquakes. Tectonic underplating is further corroborated by neotectonics and the tectonic structures observed in the Préalpes, Molasse basin and Jura. Similarly, uplift rates, and earthquakes along the southern edge of the Jura mountains seem to witness the development of a new/incipient basement nappe at depth (partial inversion of former Permo–Carboniferous grabens in the basement). A possible spatial coincidence of areas with strong earthquake activity and zones with uplift rates above surrounding values, suggest a common mechanism for their origin in the western Swiss Alpine foreland. Combined with information from basement geometry and wedge dynamics it is proposed that the common mechanism is the development of basement imbricates by tectonic underplating. The proposed model for ongoing and possible future tectonic underplating beneath an active Alpine orogenic wedge also allows to reconcile the models of basement/wrench-faulting in the Molasse basin and Jura with the distant push theory, where the Molasse basin and Jura develop over a basal décollement horizon.

Fluid migration through thrust faults in the Helvetic nappes (Western Swiss Alps)

The δ18O, δ13C and 87Sr/86Sr values of calcite and organic matter were measured for carbonate mylonites from numerous thrusts in the Helvetic Alps. Carbonate mylonites in most of the thrusts retain essentially unaltered protolith δ18O and δ13C values, consistent with there having been little to no advection of isotopically distinct fluid through these faults. Only carbonate mylonites from the basal thrusts of the largest nappes have δ18O and/or δ13C values that differ from those of their protoliths. The zone of isotopic alteration/exchange is confined to c. 10 to 20 meters of these fault contacts. We propose the fluids that migrated through these faults contained variable amounts of organically derived carbon and radiogenic strontium, and were probably derived from dewatering of the sedimentary rocks and prograde metamorphic reactions in the nappes' root zones. Apart from the basal thrusts of the largest nappes that behaved as narrow, laterally extensive conduits for fluids, there is little isotopic evidence that large quantities of fluids passed through most of the carbonate-hosted thrusts in the Helvetic Alps.

Evidence for ongoing extensional deformation in the western Swiss Alps and thrust-faulting in the southwestern Alpine foreland

To verify the discordant orientations of P- and T-axes found by earlier studies in the Penninic domain of the southern Valais, Switzerland, and in the surrounding regions of France and Italy, we have evaluated the focal mechanisms of 11 of the best-recorded earthquakes that occurred in this area between 1985 and 1990. By employing two-dimensional ray-tracing techniques, we have made use of what is known about the lateral variations of the crustal structure to obtain constraints on the possible focal-depth range of the hypocenters and on the take-off angles at the source. In addition, we have been able to identify one of the two nodal planes as the actual fault plane of one of the events, based on high-resolution relative locations of its aftershocks. The resulting normal faulting and oblique-slip focal mechanisms show that, down to depths of about 10 km, the compressional structures of the Penninic nappes, which were formed during the Alpine orogeny, are presently undergoing extensional deformation and that a significant component of this extension is perpendicular to the Alpine arc. Thrust faulting focal mechanisms from events at the northwestern margin of the Po plain, however, indicate that the southern Alpine foreland is still subject to compressional deformation consistent with the large-scale stress field expected from the convergence of the African and European plates. Thus, our results lend support to geodynamic models that predict extensional deformation across the crest of a mountain range, while the flanks and lowlands continue to undergo crustal shortening.

Alpine deformation and 40Ar/39Ar geochronology of synkinematic white mica in the Siviez‐Mischabel Nappe, western Pennine Alps, Switzerland

We explore the timing of deformation and exhumation of the Siviez‐Mischabel Nappe (western Swiss Alps), which has been considered a classic example of a midcrustal crystalline nappe since the studies of Argand [1916]. This study presents 40Ar/39Ar ages obtained on both synkinematic white mica from Permo‐Triassic cover sediments and more complex white mica populations from basement gneisses of the Siviez‐Mischabel and middle Pennine Nappes. Primary foliation developed in cover units by nucleation, growth, and rigid rotation of mica grains during noncoaxial Alpine deformation. Although some samples show a crenulation of this primary foliation, mica growth appears to have occurred only during the development of primary foliation, the main phase of greenschist facies deformation related to imbrication of the Siviez‐Mischabel Nappe and other middle Pennine Nappes. Good agreement exists between independent estimates of the timing of deformation and reported 40Ar/39Ar white mica ages from cover units of the central and southern Siviez‐Mischabel Nappe. In cover units from the central and southern Siviez‐Mischabel regions of the study area, 40Ar/39Ar ages appear to date synkinematic white mica growth. Results suggest that the Siviez‐Mischabel Nappe was emplaced and developed foliation during a 5 m.y. period from 41 to 36 Ma. In cover units from the eastern Siviez‐Mischabel, however, 40Ar/39Ar white mica ages appear to date postkinematic thermal events. These thermal events may be related to Oligocene magmatic activity in the lower Pennine Nappes or to Miocene development of the Simplon fault zone. Variations in the relation between Alpine age and grain size for cover samples from the central, eastern, and southern Siviez‐Mischabel correlate well with the regional variations in temperature inferred from quartz microfabrics and the pattern of regional metamorphism. When considered in concert with other recent isotopic studies on the timing of major tectonic and thermal events in the western Swiss Alps, these data support arguments that the relative timing of events such as thrusting and back thrusting of crystalline nappes in hinterland units and exhumation of high‐pressure units in the suture zone of the western Alps are intimately related and synchronous on the scale of a few million years.

Crustal thickening and crustal extension as controls on the evolution of the drainage network of the central Swiss Alps between 30 Ma and the present: constraints from the stratigraphy of the North Alpine Foreland Basin and the structural evolution of the Alps

The combined information about sedimentary petrography from the North Alpine Foreland Basin and structural geology from the Alps allows a qualitative reconstruction of the drainage network of the central Swiss Alps between 30 Ma and the present. This study suggests that crustal thickening and crustal thinning significantly controlled the location of the drainage divide. It also reveals the possible controls of crustal thickening/thinning on the change of the orientation of the drainage network from across‐strike between 30 and 14 Ma to along‐strike thereafter.Initial crustal thickening in the rear of the wedge is considered to have formed the drainage divide between north and south at 30 Ma. Because the location of crustal thickening shifted from east to west between ≈30–20 Ma, the catchment areas of the eastern dispersal systems reached further south than those of the western Alpine palaeorivers for the same time slice. Similarly, the same crustal dynamics appear to have controlled two phases of denudation that are reflected in the Molasse Basin by petrographic trends. Uplift in the rear of the wedge caused the Alpine palaeorivers to expand further southward. This is reflected in the foreland basin by increasing admixture of detritus from structurally higher units. However, tectonic quiescence in the rear of the wedge allowed the Alpine palaeorivers to cut down into the Alpine edifice, resulting in an increase of detritus from structurally lower units. Whereas uplift in the rear of the wedge was responsible for initiation of the Alpine drainage systems, underplating of the external massifs some 50 km further north is thought to have caused along‐strike deviation of the major Alpine palaeorivers.Besides crustal thickening, extension in the rear of the wedge appears to have significantly controlled the evolution of the drainage network of the western Swiss Alps. Slip along the Simplon detachment fault exposed the core of the Lepontine dome, and caused a 50‐km‐northward shift of the drainage divide.

Active tectonism in the central Alps: contrasting stress regimes north and south of the Rhone Valley

ABSTRACTAn integrated interpretation of seismicity, fault plane solutions and deep seismic reflection data suggests that the NE–SW to NW–SE trending Rhone–Simplon fault zone and the gently S‐dipping basal Penninic thrust separate fundamentally different stress regimes in the western Swiss Alps. North of the Rhone‐Simplon fault zone, strike‐slip earthquakes on steep‐dipping faults within the Helvetic nappes are a consequence of regional NW–SE compression and NE–SW extension. To the south, vertical maximum stress and N–S extension are responsible for normal mechanism earthquakes that occur entirely within the Penninic nappes above the basal Penninic thrust. Such normal faulting likely results from extension associated with southward movements (collapse) of the Penninic nappes and/or continued uplift and relative northward displacements of the underlying Alpine massifs. Geological mapping and fission‐track dating suggest that the two distinct stress regimes have controlled tectonism in the western Swiss Alps since at least the Neogene.

Gravity anomalies and spatial variations of flexural rigidity at mountain ranges

New continent‐wide compilations of gravity and topography data have been used to determine spatial variations in the long‐term mechanical properties of the foreland lithosphere at mountain ranges. In the southern Appalachians, a large elastic thickness Te (in the range 40–70 km) is required with variation only along the length of the mountain range. To explain the Appalachian Bouguer gravity anomaly “high”, a subsurface load and crustal thinning are necessary. For the Andes, Te varies in the range 5–85 km perpendicular and parallel to the length of the mountain range. In the Alps, Te is high in the French Alps and then decreases in the western Swiss Alps and increases toward the eastern Alps. In the Romanian Carpathians, Te variations in the range 5–20 km are found. Although the thermal state, curvature, and inelastic yielding may play some role, these variations in Te are interpreted as the result of inheritance of the thermal and mechanical properties of foreland lithosphere. Inheritance plays a role in influencing the development of the next plate tectonic event. At mountain ranges, preexisting strength variations offer some control on tectonic style, the geometry of fold and thrust belts, and the stratigraphic patterns that develop in the adjacent foreland basin. Foreland lithosphere is involved in a cycle of events that began with continental rifting and passive margin formation and ended in continental collision. Compilations of Te from continental rifts, passive margins, and foreland lithosphere show large variations. The lowest values are associated with rifts, margins, and foreland lithosphere which has recently been been involved in continental breakup. The highest values are associated with foreland lithosphere for which a long time has elapsed since the previous rifting event. We speculate that continental lithosphere undergoes some form of strength recovery.

Hypocentral parameters and velocity estimation in the western Swiss Alps by simultaneous inversion of P- and S-wave data

Abstract The extraction of tectonic information from seismicity data requires accurate hypocenter locations. Hypocenter and velocity-depth parameters may be obtained by simultaneously inverting P- and S-wave arrival times. The success and efficiency of such an inversion procedure depends on data quality and the suitability of the velocity model representation. To ensure high data quality, a robust outlier detection scheme based on Wadati diagrams has been adopted, and the effects of certain systematic errors and the inclusion of a priori constraints have been investigated. When the true velocity structure includes variable vp/vs ratios, incorporating S-wave data on the assumption of a constant vp/vs ratio may lead to more inaccurate hypocenter determinations than using P-wave data alone. By means of a synthetic example, the benefits of decoupled P- and S-wave models are demonstrated. The simultaneous inversion procedure has been applied to new earthquake travel-time data recorded in the western Swiss Alps. The derived velocity-depth functions provide a reasonable average of the upper crustal structure in the investigation area, and based on accurate hypocenter information, a nearly vertical active fault zone has been confirmed north of the Rhone valley. Markedly different epicenter distribution patterns are observed on either side of the Rhone valley.

Microearthquake cluster detection based on waveform similarities, with an application to the western Swiss Alps

SUMMARY Highly similar waveforms of different earthquakes are due to similar focal mechanisms and common propagation paths. The relative hypocentral locations of events in clusters of similar earthquakes can provide useful insights into geometry and style of faulting at depth within the crust. The detection of such earthquake clusters within large data sets can only be accomplished efficiently by means of an automatic procedure. Therefore, we developed an algorithm based on correlation analysis that detects and associates events with similar waveforms. The algorithm has been applied to a data set recorded in the western Swiss Alps: 619 out of a total of 1497 events exhibit similarities with other events. Based on a more detailed investigation of two selected clusters with known focal mechanisms, it could be shown that the active fault planes correspond to neotectonic structures mapped in the study area. Due to their oblique orientation relative to the larger-scale epicentral alignment, these faults have been interpreted as Riedel shears.

Initial isotopic heterogeneity and secondary disturbance of the SmNd system in fluorites and fluid inclusions: A study on mesothermal veins from the central and western Swiss Alps

This paper presents SmNd data on hydrothermal fluorites, fluid inclusions and leach experiments on samples from the Mont-Blanc and Gotthard units, Swiss Alps. Both areas have experienced Alpine greenschist-facies overprint (≤350° and ∼450°C, respectively). Mont-Blanc samples represent late Variscan fluorites from three individual veins whereas Gotthard samples were taken from one hand-specimen of probable late Variscan age. Nd concentrations of fluorites range from 3.6 to 23 ppm, 147 Sm 144 Nd ratios from 0.12 to 0.35. None of the samples sets yielded an isochron relationship. Imperfect correlations indicate apparent ages between late Variscan and Alpine, reflecting neither the formation nor the age of metamorphic overprint. Reasons are wall-rock influence during formation and post-formation disturbance. Secondary fluorite (Gotthard) shows the highest Nd concentration and a low Sm/Nd ratio, close to that of fluid inclusion fractions. A leach experiment showed the same trend even more pronounced: ∼7% of Nd and ∼2% of Sm were removed from fluorite by 0.18 N HCl at 100°C. Hence, secondary fluids are likely to leach and fractionate Sm and Nd, and may easily cause dissolution and reprecipitation of fluorite. Consequently, the system is highly suspicious if secondary fluid flow cannot totally be ruled out. This is in sharp contrast to previously published conclusions. However, evidence for rejuvenation of the SmNd system in fluorites is also found in published data sets.

Early deformation of ultrahelvetic mélanges in the helvetic nappes (Western Swiss Alps)

The Ultrahelvetic mélanges represent an important clue for understanding the Tertiary Alpine orogeny, particularly in the external Alps of western Switzerland. The remarkable architecture of the Upper and Lower Ultrahelvetic units is classically explained by an important phase of gravity sliding and orogenic sliding accompanied by the formation of chaotic complexes and olistostromes (wildflysch), a phenomenon called ‘diverticulation’. A structural analysis reveals evidence for early phases of deformation, despite strong overprinting by late Alpine deformation. These early fabrics are observed in foliated pebbly-mudstone (a tectonic breccia, part of the mélange matrix), in disrupted flysch sequences and in dark shales. They attest to brecciation related to a complex pattern of veining, with several generations of calcite cement and several stages of failure during lithification, and they show the important role of fluids during the deformation. They present numerous analogies with the fabrics described in association with thrusts in accretionary prisms. Considering the tectonic fabrics and the relations among the infra-Prealpine mélanges, a tectonic model to explain the emplacement of the Ultrahelvetic units is proposed in which the décollements developed in three weak levels. The early fabrics evolved within a basal thrust formed in a final stage of the Valaisan subduction.

The cover-basement contact beneath the Rawil axial depression (western Alps): True amplitude seismic processing, petrophysical properties, and modelling

Since 1986, several near-vertical seismic reflection profiles have been recorded in Switzerland in order to map the deep geologic structure of the Alps. One objective of this endeavour has been to determine the geometries of the autochthonous basement and of the external crystalline massifs, important elements for understanding the geodynamics of the Alpine orogeny. The PNR-20 seismic line W1, located in the Rawil depression of the western Swiss Alps, provides important information on this subject. It extends northward from the “Penninic front” across the Helvetic nappes to the Prealps. The crystalline massifs do not outcrop along this profile. Thus, the interpretation of “near-basement” reflections has to be constrained by down-dip projections of surface geology, “true amplitude” processing, rock physical property studies and modelling. 3-D seismic modelling has been used to evaluate the seismic response of two alternative down-dip projection models. To constrain the interpretation in the southern part of the profile, “true amplitude” processing has provided information on the strength of the reflections. Density and velocity measurements on core samples collected up-dip from the region of the seismic line have been used to evaluate reflection coefficients of typical lithologic boundaries in the region. The cover-basement contact itself is not a source of strong reflections, but strong reflections arise from within the overlaying metasedimentary cover sequence, allowing the geometry of the top of the basement to be determined on the basis of “near-basement” reflections. The front of the external crystalline massifs is shown to extend beneath the Prealps, about 6 km north of the expected position. A 2-D model whose seismic response shows reflection patterns very similar to the observed is proposed.

Nappe geometry in the Western Swiss Alps

Detailed geological mapping during the last 20 years in the Western Swiss Alps has shown clearly that most of the lower basement nappes are fold nappes possessing normal and inverted limbs. Moreover their cores are made of strongly deformed gneisses indicating that important ductile strain took place during the formation of the fold nappes. It is therefore probably wrong to imagine deep basement nappes as rigid slices as often actually claimed, especially when interpreting seismic profiles. True ‘brittle type’ thrust nappes involving basement rocks only occur in the internal and upper parts of the belt. Cover nappes, on the contrary, are in most parts of the Alpine belt thrust sheets following more or less the rules of thin-skinned tectonics. Many basement fold nappes lost part of their sedimentary cover during or just before their formation, by décollement along ductile horizons. The result is that many cover thrust nappes in the external part of the Alps are directly related to their original basement fold nappes.

Processing and preliminary results of NFP/PNR20 seismic reflection profiles from the Western Swiss Alps

Results of near-vertical seismic reflection profiling conducted in the western Swiss Alps are presented. All profiles were recorded using both high-fold Vibroseis and high-energy but low-fold dynamite sources, yielding two independent data sets. The Vibroseis seismic sections provide markedly superior resolution of upper crustal units down to the top of the crystalline basement. On these sections, high-impedance contrasts within the nappes are the source of strong reflections, and the basal thrust of the Penninic nappes (Penninic front) delineates a lateral change in reflection character. By comparison, the dynamite sections provide generally higher-quality images of the deep crust. In particular, they show prominent bands of strong reflections in the lower crust and a well-defined reflection Moho deepening from ~ 12 s (about 36 km) in the north to ~ 16 s (about 48 km) in the south. It is concluded that both types of data are required to resolve accurately shallow and deep structures in the Swiss Alps. Preliminary results of the dynamite survey from the most recently recorded line support proposals that shortening of the basement in the external domain has occurred, in agreement with interpretations of the ECORS-CROP seismic reflection line further south.

Results of NFP 20 seismic reflection profiling along the Alpine section of the European Geotraverse (EGT)

SUMMARY Recent deep-crustal seismic reflection profiling across the central Alps of eastern and southern Switzerland has provided a detailed image of this continental collision zone. As part of the Swiss National Science project NFP 20, a series of reflection profiles was recorded during two phases along N-S transects crossing the Alpine edifice. The eastern and southern profiles coincide with the Alpine segment of the European Geotraverse (EGT) and in part with the earlier Swiss Geotraverse. These data together with other geophysical and extensive geologic information form a unique and comprehensive volume of crustal information for this region of the Alps. On a larger scale, the new reflection data also add to a growing set of profiles crossing the Alpine fold belt which includes an additional traverse carried out by NFP 20 across the western Swiss Alps and the ECORS-CROP profile across the western Alps. The initial results of the reflection data across the central Alps outline a crustal framework involving northward indentation of the Adriatic hinterland into the subducting European foreland. This occurs beneath the collapsed oceanic basins of the Penninic allochthon which is defined by its highly reflective crystalline nappes. The present crustal thickness within the Alpine collision zone has doubled to about 60 km by both vertical and horizontal displacements along an inferred complex detachment system controlled by the Adriatic wedge. The south-plunging European foreland implies that portions of the thickened crust have been subducted into the upper mantle. This general framework of flake tectonics is laterally consistent with the ECORS-CROP results, from the western Alps. However, the western Alps show a higher degree of crustal imbrication which may be related to the presence of the rigid Ivrea body and an overall greater amount of crustal shortening compared to the central Alps.

Facies model for deglaciation in an overdeepened alpine valley (bulle area, Western Switzerland)

This study is concerned with glaciolacustrine sedimentation at the front of the Western Swiss Alps, at the former meeting point of the subsidiary Sarine glacier and the ice stream of the Rhône glacier. During the last glaciation (Late Würmian), these two glaciers eroded into pre-existing glacial sediments and excavated two overdeepened basins in the bedrock. The infill of these basins by glaciolacustrine sediments occurred in two phases. First, diamicts and turbidites were deposited in stagnant or flowing water conditions under the glacier. This episode was followed by deposition of eskers, glaciolacustrine and glaciofluvial outwash, deltaic deposits, alluvial fans and intraglacial moraines in a proglacial lake basin. The glacial sediments are overlain by clay, marl and chalk deposited in a temperate climate. The Quaternary sedimentary succession is a record of rapid climatic change: in 10,000 years, the climate evolved from polar to temperate.

Zoned high-pressure assemblages in pillow lavas of the Zermatt-Saas ophiolite zone, Switzerland

Pillow lavas of the Zermatt-Saas ophiolite zone of the western Swiss Alps in places consist of zoned high-pressure parageneses. These assemblages, which developed subsequent to the metamorphic peak, are of lawsonite eclogite in the cores of pillows and dolomite-bearing blueschist or glaucophane eclogite in the rims. The matrix between the pillows comprises dolomite-glaucophane-rich material. An analysis of reactions in the system Na 2OCaOFeOMgOAl 2O 3SiO 2H 2OCO 2 (NCFMAS-H 2OCO 2) indicates that one possible model is that these assemblages formed cofacially due to systematic intra-pillow variations in the chemical potentials of H 2O and CO 2. Pillow rims equilibrated at higher values of μ CO 2 and lower values of μ H 2O than pillow cores. Textural evidence suggests that infiltration has controlled the distribution of CO 2 generated by the breakdown of dolomite-bearing assemblages during the return of the rocks to the surface. Variation in H 2O content was caused by ocean-floor hydrothermal alteration. In these rocks, therefore, blueschist assemblages have been stabilised by higher values of μ CO 2 and eclogites by higher values of μ H 2O . If fluids were present (as suggested by various lines of evidence) then blueschist pillow rims equilibrated at higher CO 2 ( CO 2+ H 2 O) than eclogitic pillow cores.

BIOSYSTEMATIC STUDIES OF THE RUMEX ACETOSELLA-COMPLEX II. THE ALPINE REGION

SUMMARY The present paper is a sequal to similar ones dealing with populations in Belgium and France. About 80 population samples were gathered in the following areas: the southern part of the German Federal Republic, the Swiss, Austrian and (partly) Italian Alps, and northern Austria. Of these samples the occurrence of angiocarpy vs. gymnocarpy and the chromosome numbers were determined, special attention being paid to the incidence of foliar multifidy. In the Alpine region proper the large majority of the individuals is gymnocarpic. In southern Germany and Lower Austria the plants are nearly always angiocarpic, however. The distribution of the levels of ploidy is rather intricate and roughly as follows: gymnocarpic diploids occur in the southernmost French Alps, in the southern and western Swiss Alps, and in a single population in the Austrian Wachau on the NE escarpment of the Alps; gymnocarpic tetraploids in the Austrian Alps, and rather numerous hexaploids, with some angiocarpy, in the intervening border area of the central Alps. In southern Germany and Lower Austria mostly angiocarpic hexaploids were encountered, with predominantly gymnocarpic tetraploids especially in the border area between Austria and Czechoslovakia (the Bohemian Forest). In the latter region hybridisation between tetra- and hexaploids frequently takes place. There are correlations between this chorological pattern and certain consistent phytogeographical phenomena in the Alpine region associated with its Pleistocene geological history, such as the presence of several refugia and the so-called Rein-Traun gap in the central Alps. In addition it is suspected that antropogenous influences have disturbed the original situation to an appreciable extent. The degree of dissection of the leaves, as a character included in the population analyses for the first time, is present to the same extent in alpine di- and tetraploids (in which the leaves are more or less multifid), but in the Alpine and the German and Lower Austrian hexaploids, foliar multifidy is hardly developed. Some possible causes and plausible conclusions are discussed. The single population in the Wachau refugium contains a singularly high percentage of plants with multifid foliage and this is, among other things, the reason why this population is considered to belong to a different population complex than do the Alpine ones. In the discussion some space is devoted to difficulties that may be expected if a future, novel taxonomic classification of the complex is attempted.