Lateral termination of the north-directed Alpine orogeny and onset of westward escape in the Western Alpine arc: Structural and sedimentary evidence from the external zone

31p.International audienceThe initial propagation of the Western Alpine orogen was directed northwestward, as shown by basement-involved and Mesozoic sedimentary cover compressional structures and by the early foreland basins evolution. The crystalline basement of the Dauphine zone recorded three shortening episodes: pre-Priabonian deformation D1 (coeval with the Pyrenean-Provence orogeny), and Alpine shortening events D2 (N-NW directed) and D3 (W-directed). The early Oligocene D2 structures are trending sub-perpendicular to the more recent, arcuate orogen and are interfering with (or truncated by) D3, which marks the onset of westward lateral extrusion. The NW-ward propagating Alpine flexural basin shows earliest Oligocene thin-skinned compressional deformation, with syn-depositional basin-floor tilting and submarine removal of the basin infill above active structures. Gravity enhanced submarine erosion gave birth locally to steep submarine slopes overlain by kilometric-scale blocks slid from the orogenic wedge. The deformations of the basin floor and the associated sedimentary and erosional features indicate a N-NW-ward directed propagation, consistent with D2 in the Dauphine foreland. The Internal zones represent the paleo-accretionary prism developed during this early Alpine continental subduction stage. The early buildup has been curved in the arc and rapidly exhumed during the Oligocene collision stage. Westward extrusion and indenting by the Apulian lithosphere allowed the modern arc to crosscut the western, lateral termination of the ancient orogen from similar to 32 Ma onward. This contrasted evolution leads to propose a palinspastic restoration taking in account important northward transport of the distal passive margin fragments (Brianconnais) involved in the accretionary prism before the formation of the Western Alps arc

Preliminary results of a paleoseismological analysis along the Sahel fault (Algeria): New evidence for historical seismic events

International audienceThe ∼60 km-long Sahel ridge west of Algiers (Tell Atlas, north Algeria) is considered as an ENE-WSW fault-propagation fold running along the Mediterranean coast and associated with a north-west dipping thrust. Its proximity with Algiers makes this structure a potential source of destructive earthquakes that could hit the capital city, as occurred in 1365 AD and 1716 AD. The first paleoseismologic investigation on the Sahel ridge was conducted in order to detect paleo-ruptures related to active faulting and to date them. From the first investigations in the area, a first trench was excavated across bending-moment normal faults induced by flexural slip folding in the hanging wall of the Sahel anticline thrust ramp. Paleoseismological analyses recognize eight rupture events affecting colluvial deposits. 14C dating indicates that these events are very young, six of them being younger than 778 AD. The first sedimentary record indicates two ruptures before 1211 AD, i.e. older than the first historical earthquake documented in the region. Three events have age ranges compatible with the 1365, 1673 and 1716 Algiers earthquakes, whereas three other ones depict very recent ages, i.e. younger than 1700 AD. Potential of these secondary extrados faults for determining paleoseismic events and thrust behaviour is discussed

Neogene and Pleistocene geodynamics: the paleoseismic evolution of Armorica (Western France)

The evolution of the passive Armorican margin (Western France) during the Neogene and Quaternary was analyzed using field data. The morphology of the margin attests to a late Hercynian shaping, further deformation during the Mesozoic mid-Atlantic opening, during the Alpine Orogeny, and ultimately, a Late Cenozoic uplift, mostly related to an onshore isostatic accommodation in response to erosion and limited tectonic activity. A very limited strike–slip dynamic, with very low seismicity, accommodated the Neogene–Pleistocene N170 strains around the rigid Armorican terrane. The South Armorican domain and English Channel floor include shear zones that adjusted the Alpine convergence, facilitating its transpressive slip to the west. The Permo-Triassic N150 faults were reactivated during the inversion phases that began after the Bartonian under the distal control of the Alpine convergence and the decrease in the Atlantic spreading rate after 34 Ma. The Armorican marine platforms were stable after the late Eocene and slightly subsident, experiencing pulsed episodes of transient lithospheric doming during the Neogene and Quaternary. Co-seismic activity onshore without surface rupture was recorded around ∼5.3 Ma, ∼3.7 Ma, ∼2.4–1.2 Ma, and ∼400–250 ka, in tandem with an inland exhumation driven by isostatic adjustment due to an intensification of periglacial erosion at the onset of the early interstadials or by agriculture. Low-magnitude and ubiquitous shallow seismic activities seem to be related today to an isostatic uplifted old brittle–ductile transition due to the accumulation of shearing strain

Partitionnement de la convergence oblique en zone de collision : Exemple de la chaîne du Zagros (Iran)

This manuscript present the results of the tectonic analysis of two major dextral strike-slip faults in the Zagros fold-and-thrust belt of Iran : the Main Recent Fault (MRF) and the Kazerun Fault (KF). The MRF accommodates partitioning of a part of the obliquity of the convergence between Arabia/Eurasia at the rear of the northwestern Zagros. In the central Zagros, the KF, an inherited basement fault, affects the entire width of the belt. This fault, together with a series of N- to NNW-trending strike-slip faults, such the Karehbas fault, define an orogen-scale fan shaped fault pattern pointing towards the southeastern MRF tip.Structural and kinematic study of these faults allows to consider a re-organisation of the structures and deformation modes of the Zagros in the Neogene. Between 9 and 5 Ma, distributed transpressional deformation operating at the rear of the Zagros belt became partitioned along the newly formed MRF. Around 3 Ma, the southeastern tip of the MRF connected to the northern tip of the KF. Through the fan shaped strike-slip faults pattern of the central Zagros, slip along the MRF became transmitted to the thrusts and folds of the eastern Zagros especially along the bent terminations of the three fault zones of the KF. This fault system corresponds to as an orogen-scale horse-tail termination of the MRF. Geomorphic offset features analysis and dating (cosmogenic 36Cl exposure ages, U-Th dating) allow to determine the Quaternary horizontal slip rate of the two strike-slip faults. The MRF slip rate is of 6 ± 1 mm/yr. The slip rate along the strike of the KF varies from north to south, from 4 mm/yr to 0 mm/yr; and the Karehbas slip rate is of 6 ± 1.5 mm/yr. Considering the available GPS data over the study area, these results argue for a complete partitioning of the convergence along the MRF and support the hypothesis of the slip from along the MRF being transmitted to the eastern Zagros belt by the southern bent termination of the three fault zones of the KF and associated faults.Right-lateral slip along this strike-slip fault system initiated as a consequence of the re-organization of the Arabia/Eurasia collisional domain at 5 ± 2 Ma. We suggest that the activation of the MRF was favoured by the slab break-off of the Arabian lithosphere subducting beneath central Iran.Ce travail présente les résultats de l'analyse tectonique des deux décrochements dextres majeurs de la chaîne du Zagros formée lors de la collision des plaques Arabie/Eurasie au Néogène. La Main Recent Fault (MRF), marquant la limite nord-occidentale du Zagros, accommode une partie de l'obliquité de la convergence Arabie/Eurasie. Dans le Zagros central, la faille de Kazerun (KF), faille de socle de direction NS, recoupe entièrement la largeur de la chaîne. Elle est associée à des failles décrochantes, dont la faille de Karehbas, qui définissent un éventail ouvert vers le SE et pointant vers la terminaison SE de la MRF.L'étude structurale et cinématique de ces failles permet de documenter une ré-organisation des structures et de la déformation dans le Zagros au cours du Néogène. Entre 9 et 5 Ma, la déformation transpressive distribuée à l'arrière de la chaîne a cessé au moment de la mise en place de la MRF impliquant le partitionnement de la convergence. Vers 3 Ma, la terminaison SE de la MRF s'est connectée à la terminaison nord de la KF. Par l'intermédiaire des failles du Zagros central formant le système en éventail, le déplacement de la MRF se voit dès lors distribué vers l'est sur les plis et les chevauchements du Zagros oriental. A l'échelle du Zagros, ces failles peuvent donc être perçues comme formant la terminaison en queue de cheval de la MRF.L'analyse de marqueurs géomorphologiques datés (datation 36Cl, datation U/Th) et décalés par ces failles permet de déterminer le taux de déplacement horizontal le long des décrochements sur les derniers 140 ka. Il est de 6 ± 1 mm/an pour la MRF. Il varie du nord au sud de la KF de 4 mm/an à presque 0 mm/an. Il est de 6 ± 1.5 mm/an pour la faille de Karebhas. Ces vitesses, au regard des données GPS disponibles, permettent de conclure à un partitionnement total de l'obliquité de la convergence le long de la MRF, et à un transfert progressif du mouvement de la MRF du NW vers le SE sur les terminaisons courbées et chevauchantes des trois zones de failles de la KF et des failles associées. L'activation de ce système de failles dextres est liée à la ré-organisation de la collision Arabie-Eurasie, à 5 ± 2 Ma. L'hypothèse d'un détachement du panneau lithosphérique Arabique subduit sous le Zagros et l'Iran central est considéré comme une cause de la mise en place du partitionnement

Partitionnement de la convergence oblique en zone de collision (exemple de la chaîne du Zagros (Iran))

Ce travail présente l analyse tectonique de deux décrochements majeurs dextres de la chaîne du Zagros en Iran : la Main Recent Fault (MRF) et la faille de Kazerun (KF). L analyse de marqueurs géomorphologiques décalés par ces failles et datés permet de déterminer un taux de déplacement horizontale de 6 +- 1 mm/an pour la MRF, et variant du nord au sud de la KF de 4 mm/an à presque 0 mm/an. Ces deux failles contribuent à la ré-organisation régionale de la déformation au cours du Néogène et au partitionnement de la convergence oblique Arabie/Eurasie. Elles sont connectées et permettent la distribution vers l est du déplacement de la MRF sur les plis et les chevauchements du Zagros oriental.AIX-MARSEILLE3-BU Sc.St Jérô (130552102) / SudocSudocFranceF