RĂ©sultat de la recherche
Quantification of strain rate in the Western Alps using geodesy: comparisons with seismotectonics
The contrasted seismotectonic regime of the Western Alps is characterized by radial extension in the high chain, combined with local compressive areas at the foothill of the belt, and everywhere occurrence of transcurrent tectonics. Here, we compare this seismotectonic regime to a large-scale compilation of GPS measurements in the Western Alpine realm. Our analysis is based on the raw GPS database, which give the measured velocity field with respect to the so called “stable Europe”, and an interpolated velocity field, in order to smooth the database on a more regular mesh. Both strain rate and rotational components of the deformation are investigated. The strain rate field shows patch-like structure, with extensional areas located in the core and to the North of the belt and compressional areas located in its periphery. Although the GPS deformation fields (both raw and interpolated) are more spatially variable than the seismotectonic field, a good qualitative correlation is established with the seismotectonic regionalization of the deformation. The rotation rate fields (both raw and interpolated) present counterclockwise rotations in the innermost part of the belt and a surprising continuous zone of clockwise rotations following the arc-shape geometry of the Western Alps along their external border. We interpret this new result in term of a counterclockwise rotation of the Apulia plate with respect to the stable Europe. This tectonic scheme may induce clockwise rotations of crustal block along the large strike-slip fault system, which runs in the outer part of the belt, from the Rhône-Simplon fault to the Belledonne fault and Southeastward, to the High-Durance and Argentera fault.
Active strike-slip faulting in the Chablais area (NW Alps) from earthquake focal mechanisms and relative locations
La région du Chablais est caractérisée par une configuration géologique et structurale complexe, résultant de la mise en place de nappes de charriage de provenances variées (nappes Préalpines) chevauchantes sur les unités externes Helvétiques durant l’Oligocène. Alors que l’historique de la mise en place de ces nappes est bien compris, le régime tectonique actuel reste à définir. L’analyse détaillée des séismes de Bonnevaux et de Samoëns présentées dans cette étude donne une bonne image du régime tectonique actuel de cette région. Le régime sismotectonique associé à ces séismes apparaît stable en fonction de la profondeur, de type décrochant, l’un au niveau du socle cristallin (17 km de profondeur), l’autre intersectant probablement l’interface socle/couverture (5 km de profondeur). Les techniques de relocalisations relatives, appliquées dans cette étude dans le domaine temporel sur la séquence sismique associée au choc de Samoëns, représentent le meilleur, si ce n’est l’unique, moyen d’identifié des failles actives dans une région où les indices néotectoniques sont rares et controversés. L’alignement sismique ainsi défini correspond au plan nodal E-W du mécanisme au foyer du choc principal, permettant de définir une faille dextre, subverticale, orientée E-W. Ce régime décrochant, replacé dans son contexte tectonique régional, correspond à celui observé dans la région Jura/Plateau Molassique, caractérisé par un système de décrochements conjugués (dextre orienté E-W, sénestre orienté NW-SE) et contraste avec celui exclusivement dextre, orienté NE-SW, observé au niveau de l’alignement Wildhorn/Martigny., Das Chablaisgebiet zeichnet sich durch einen komplexen geologischen Aufbau aus, welcher Resultat des Transportes eines aus Einheiten unterschiedlicher interner Herkunft aufgebauten Deckenstapels (die voralpinen Decken) ist, der im Oligozän über die helvetische Bedeckung der externen Zone des Alpenbogens geschoben wurde. Während strukturgeologischer Aufbau und zeitlicher Ablauf der tektonischen Bewegungen relativ gut bekannt sind, bleiben die tatsächlichen Verschiebungen und die damit verbundene Bruchtektonik weitgehend unklar. Die in der vorliegenden Studie im Detail untersuchten Erdbeben von Bonnevaux und Samoëns ergeben ein ziemlich genaues Bild der aktiven Tektonik im Gebiet des Chablais. Das damit verbundene seismotektonische Regime erscheint in Abhängigkeit der Tiefe konstant. Die Herdmechanismen beider Erdbeben deuten auf ein Blattverschiebungsregime hin, wobei eines im kristallinen Sockel in ca. 17 km Tiefe lokalisiert wurde und das andere höchstwahrscheinlich die Übergangsszone zwischen Bedeckung und Sockel in ca. 5 km Tiefe überschneidet. Die in dieser Studie angewendeten relativen Lokalisierungstechniken (in der Zeitdomäne) für die seismische Sequenz des Samoëns-Erdbebens repräsentieren den besten (wenn nicht den einzigen) Ansatz zur Identifizierung aktiver Brüche in einer Region in welcher neotektonische Indizien spärlich vorhanden und kontrovers sind. Die Ausrichtung der daraus resultierenden seismischen Linie ist in guter Übereinstimmung zu der dextralen E-W orientierten Verwerfung, welche dem Herdmechanismus entnommen werden kann. Dieses Regime ist gut mit dem regionalen Stressfeld und der aktiven Tektonik im Gebiet Jura/Molassebecken vergleichbar, unterscheidet sich aber deutlich vom rein dextralen NE-SW gerichteten Blattverschiebungsregime im Gebiet Wildhorn/Martigny., The Chablais area is characterized by a complex geological setting, resulting from the transport of nappes of various internal origins (the Prealpine nappes), thrusted in Oligocene times onto the Helvetic cover of the external zones of the Alps. While the structural setting and timing of nappe emplacement are well understood, current tectonics and associated faulting remain unclear. The detailed analysis of the Bonnevaux and Samoëns earthquakes, presented in this study, constitutes a significant contribution to the active tectonics of the Chablais area. The associated seismotectonic regime appears to be constant with depth, both focal mechanisms yielding a strike-slip regime, one in the crystalline basement at around 17 km depth and the other probably cross-cutting the cover/basement interface at around 5km depth. Relative location techniques, applied in this study to the seismic sequence associated to the Samoëns earthquake, represents the best way to identify active faults in a region where neotectonic evidence is scarce and controversial. The resulting seismic alignment corresponds to the E-W oriented nodal plane inferred from the Samoëns main shock focal mechanism, thus defining an active near vertical E-W dextral fault. This strike-slip regime, compared to the current regional stress field, corresponds to the one observed in the Jura/Molasse basin area but contrasts with the exclusively dextral and NE-SW-oriented transcurrent regime of the Wildhorn/Martigny region.
Brittle orogen-parallel extension in the internal zones of the Swiss Alps (South Valais)
De nouvelles données de fracturation tardi-alpine dans le Sud Valais (Alpes Suisses) sont présentées ici. Nous avons utilisé différentes images satellites ainsi que le Modèle Numérique de Terrain (MNT) à 25m de résolution pour identifier et cartographier différentes structures morphologiques, linéaments ou escarpements de failles. Un contrôle sur le terrain nous a permis de reconnaître que ces structures sont pour la plupart des failles. Nous avons identifié trois familles de failles, orientées E-W, NE-SW et NW-SE, qui sont postérieures à la mise en place des nappes et à toutes les structures ductiles. Ces failles sont principalement normales, avec localement une composante décrochante. La faille du Rhône, normale / dextre, contrôle la déformation cassante à proximité de la vallée du Rhône, où elle présente une forte signature morphologique. Les mesures de couples failles / stries sur 56 sites nous ont permis de calculer les paléocontraintes par la méthode d’inversion directe : l’axe d’extension δ3 est globalement orienté NE-SW (N65°). Le paramètre de forme de l’ellipsoïde des paléocontraintes Φ = (δ2- δ3)/(δ1-δ3), varie entre l’extension radiale (Φ=0) et la transtension (Φ=1). Cette direction d’extension, parallèle à la direction de structures alpines est compatible avec l’extension accommodée par la faille du Simplon. Elle se trouve dans une grande partie des Alpes nord-occidentales, du Simplon au Val d’Aoste, en passant par le Sud Valais., The latest Alpine faulting is examined in the Valais region of the Swiss Alps. We used satellite imagery and a 25m Digital Elevation Model to identify and map geomorphic features such as fault scarps and lineaments. Verification, fault characterization and fault planes / striae measurements for paleostress analysis were conducted in the field. We identified three fault families that post-date nappe emplacement, E-W, NE-SW and NW-SE oriented. A large majority of faults are normal, with some strike slip component. The normal / dextral Rhône fault system controls the brittle deformation close to the Rhône valley, where it is morphologically well expressed. This regional brittle deformation was analysed using paleostress inversion methods at 56 measurement sites. The extensional axis δ3 is is regionally consistent with a NE-SW (N65°) orientation. The deformation regime, based on the ellipsoid form parameter Φ = (δ2-δ3)/(δ1-δ3), varies between radial extension (Φ=0) and transtension (Φ=1). This orogen-parallel extension, similar to the major « Simplon fault » kinematics, is documented over a large area of the northwestern Alps : in the hangingwall of the Simplon fault, in the Southern Valais area, and in the Aosta valley region.
Aseismic deformation in the Alps: GPS vs. seismic strain quantification
Neotectonics of the Western and Central Alps is characterized by ongoing widespread extension in the highest zones of the chain and transcurrent/compressive tectonics at the external limits of the belt. The overall geodetically measured deformations also indicate extension across the Western Alps. There is a good qualitative coherency between seismotectonic and geodetic approaches. Here we attempt to quantify the seismic part of the deformation. The seismic strain is compared to the deformation derived from geodesy. In sub-areas of homogeneous seismic stress/strain, we computed the total seismic moment tensor and related strain tensor. This study provides new quantitative elements about the ongoing geodynamic processes in the alpine belt. The important discrepancies obtained between seismic strains and geodetically-measured deformations raise the issue of aseismic deformation in the Alps, which could be related to elastic loading, creeping and/or a slower ductile-style deformation.
Brittle deformation in the inner NW Alps : from early orogen-parallel extrusion to late orogen-perpendicular collapse
Internal parts of the Alps have undergone widespread extensional deformation in the course of their Neogene exhumation history. Palaeostress inversion methods are used to map the prevailing stress fields and their evolution through time. Here we present new data from 100 sites with a total of about 2000 faults/striae couples, covering a large portion of the inner north-western Alps. Palaeostress tensors are mostly extensional, although one-third of them are transcurrent. The dominant direction of minimum horizontal stress axes (3) is in an orogen-parallel (N30° to N70°) orientation around the bend of the north-west alpine arc. A comparison between this older (Neogene, post-metamorphic) stress field with the current stress and strain field determined from seismotectonics and geodesy indicates a change in deformation mode from early orogen-parallel extrusion to a late and ongoing orogen-perpendicular spreading.
Origin of the current stress field in the western/central Alps : role of gravitational re-equilibration constrained by numerical modeling
We interpret the strain and stress fields of the western / central Alpine arc on the basis of 2.5D finite element modelling and a recent seismotectonic synthesis. Models have fixed boundary forces and different crustal geometries, so that they respond to buoyancy forces (variations in gravitational potential energies). The seismotectonic regime, characterized by orogen-perpendicular extension in the high topographic core of the belt and local orogen-perpendicular compressional / transpressional deformation in the external zones, appears to be very close to the modelled gravitarional regime. Rotation of Apulia has a minor effect on the current strain or stress fields of the Alpine realm. Nevertheless, it could help to explain the orogen-parallel dextral faulting that is observed all along external zones, from the northern Valais to the Argentera external crystalline massif. Our results highlight the consequences for the Alpine realm of ongoing convergence between the African and European plates. Our interpretation is that collision is no longer ongoing and that buoyancy-driven stresses dominate the present-day geodymamics of the western / central Alps.
Present-day geodynamics in the bend of the western and central Alps as constrained by earthquake analysis
The contrasted tectonics of the western/central Alps is examined using a synthesis of 389 reliable focal mechanisms. The present-day strain regime is mapped and interpolated for the entire Alpine belt based on a newly developed method of regionalization. The most striking feature is a continuous area of extension which closely follows the large-scale topographic crest line of the Alpine arc. Thrusting is observed locally, limited to areas near the border of the Alpine chain. A majority of earthquakes within the Alps and its forelands are in strike-slip mode. Stress inversion methods have been applied to homogenous subsets of focal plane mechanisms in order to map regional variations in stress orientation. The stress state is confirmed to be orogen-perpendicular both for δ3 in the inner extensional zones and δ1 in the outer transcurrent/transpressional zones. Extensional areas are well correlated with the part of the belt which presents the thickest crust, as shown by the comparison with the Bouguer anomaly and the average topography of the belt. In the northwestern Swiss Alps, extension is also correlated with currently uplifting zones. These observations and our strain/stress analyses support a geodynamic model for the western Alps in which the current activity is mostly a result of gravitational 'body' forces. Earthquakes do not provide any direct evidence for ongoing convergence in the Alpine system, but a relationship with ongoing activity of complex block rotations of the Apulian microplate cannot be ruled out.