Horizontal subduction zones, convergence velocity and the building of the Andes

We discuss the relationships between Andean shortening, plate velocities at the trench, and slab geometry beneath South America. Although some correlation exists between the convergence velocity and the westward motion of South America on the one hand, and the shortening of the continental plate on the other hand, plate kinematics neither gives a satisfactory explanation to the Andean segmentation in general, nor explains the development of the Bolivian orocline in Paleogene times. We discuss the Cenozoic history of horizontal slab segments below South America, arguing that they result from the subduction of oceanic plateaus whose effect is to switch the buoyancy of the young subducting plate to positive. We argue that the existence of horizontal slab segments, below the Central Andes during Eocene-Oligocene times, and below Peru and North-Central Chile since Pliocene, resulted (1) in the shortening of the continental plate interiors at a large distance from the trench, (2) in stronger interplate coupling and ultimately, (3) in a decrease of the trenchward velocity of the oceanic plate. Present-day horizontal slab segments may thus explain the diminution of the convergence velocity between the Nazca and South American plates since Late Miocene

Influence de la subduction d'une ride asismique sur la dynamique de la plaque continentale chevauchante: exemple de la ride de Nazca et du bassin amazonien

The Nazca ridge subduction is recorded in the eastern side of the Peruvian Andes. Its topographic response is the Fitzcarrald Arch, which induces the foredeep-foreslope-foredeep N-S partitioning of the Amazonian foreland system. Using lithospheric analogue models we evidence that horizontal subduction: i) requires the subduction of hundreds of kilometers of buoyant oceanic plateau; ii) increases the interplate friction and iii) induces uplift in the retro-lithosphere as a consequence of shortening and ridge underplating. In the Fitzcarrald Arch, the structural framework consists of two superimposed thrust-wedges. The lower wedge consists in the reactivation of a carboniferous thrust-wedge. The upper wedge is deformed by thin-skinned thrust tectonics. The structural evolution is controlled by the thickness of the paleozoic sedimentary pile involved in the thrust system. It is marked by a transfer-zone located at the northern border of a paleozoic basin. Apatite fission track analysis (AFT) allows us to determine an exhumation stage around 110 Ma linked to the opening of the South Atlantic Ocean. Since ~6Ma, AFT recorded the thrust propagation in the Camisea South of the transfer zone, where apatite thermochronometers have been reset. However, the underplating of the Nazca ridge had no influence on the short wavelength deformation of the Subandes.Les signatures de la subduction de la ride de Nazca s'expriment sur le flanc est des Andes péruviennes: l'Arche de Fitzcarrald en est la réponse topographique dans le bassin d'avant-pays amazonien. La subduction horizontale de la ride de Nazca perturbe la flexion de la lithosphère sud-américaine depuis ~4 Ma. Ce soulèvement est responsable de la configuration N-S actuelle, foredeep-foreslope-foredeep, du bassin amazonien. L'élaboration de modèles analogiques lithosphériques adaptés au contexte régional de l'étude a permis de comprendre les effets de la subduction d'un plateau océanique sur la plaque continentale chevauchante: i) le processus de la subduction horizontale requiert la subduction de plusieurs centaines de kilomètres de plateau; ii) la subduction horizontale augmente la friction interplaque et iii) s'accompagne de mouvements verticaux dans la plaque chevauchante, liés à son raccourcissement et à la flottabilité du plateau. L'architecture structurale du prisme orogénique andin au toit de l'Arche de Fitzcarrald correspond à deux prismes tectoniques superposés: un inférieur hérité de chevauchements carbonifères et un autre supérieur, la zone subandine, structuré par une tectonique de chevauchements de couverture. L'évolution structurale du système est liée aux variations d'épaisseur de la pile sédimentaire paléozoïque. Ce contrôle paléogéographique est marqué par le développement d'une zone de transfert héritée de la bordure nord du bassin paléozoïque. Les données de traces de fission sur apatites suggèrent une exhumation de cette région à ~110 Ma, liée à l'ouverture de l'Atlantique sud. La propagation des chevauchements au sud de la zone de transfert, est enregistrée à partir de ~6 Ma par les thermochronomètres remis à zéro par enfouissement lors du stade flexural du bassin de Camisea. Cependant, le sous-placage de la ride de Nazca ne semble pas avoir d'influence sur la déformation subandine à courte longueur d'onde

Variations of slab dip and overriding plate tectonics during subduction: Insights from analogue modelling

International audienceWe present small-scale laboratory models of oceanic subduction in which plates motion is imposed by lateral boundary conditions. The oceanic plate moves trenchward at constant speed and subducts below a fixed overriding plate. In this configuration, the long-term process of subduction is not steady-state. Slab interaction with the upper mantle-lower mantle boundary results in periods of slab flattening during which the dip of the slab diminishes, followed by periods of slab steepening. The overriding plate tectonic regime is influenced by the dynamics of subduction, slab anchoring favouring trench perpendicular shortening. When the slab is anchored, slab flattening further favours shortening, while slab steepening favours extension or smaller shortening rates. Non-steady-state long-term subduction may explain part of the variability of slab geometries evidenced by statistical analyses of present-day subduction zones. Experiments suggest that, despite boundary conditions applied on the converging plates do not change, tectonics pulses within the overriding plate may be caused by this non-steady-state behaviour

Influence de la subduction d'une ride asismique sur la dynamique de la plaque continentale chevauchante (exemple de la ride de Nazca et du bassin amazonien)

TOULOUSE3-BU Sciences (315552104) / SudocSudocFranceF

Déformation quaternaire et perturbations de contraintes au front de la nappe de Digne, Alpes du sud

International audienc

Recent spatial and temporal changes in the stress regime along the southern Tunisian Atlas front and the Gulf of Gabes: New insights from fault kinematics analysis and seismic profiles

International audienceTectonic deformations in the southern Atlassic front of Tunisia (SAFT) and the Gulf of Gabes result from the Nubia-Eurasia convergence. This study, based on the inversion of geologically determined fault slip vectors, presents evidences for spatial and temporal changes in the stress state. Fault kinematics analyses reveal a temporal change in states of stress during the Late Cenozoic. A paleostress (Miocene–Pliocene) state is characterized by a regional compressional tectonic regime with a mean N134 ± 09°E trending horizontal maximum stress axis (σ1). A modern (Quaternary to present-day) state of stress also corresponds to compressional tectonic regime with a regionally mean N05 ± 10°E trending horizontal σ1. The SAFT corresponds to two distinct domains: a far-foreland Atlassic front related to contractional deformation associated with southward propagate thrusting, and eastward, the Gulf of Gabes characterized by normal component NW to WNW trending faults at the crustal margin scale probably related with a transtensional tectonic regime. We propose that the spatial and temporal changes in the stress during the Miocene–Pliocene and Quaternary may result from the geodynamic evolution of the Tunisian Atlas in relation to the rotation of Corsica–Sardinia blocks and the separation of the Sicilian continental lithosphere from the Ionian oceanic lithosphere. These temporal and spatial stress changes along the SAFT and the Gulf of Gabes are probably the engine behind the moderate seismicity, leading at the same time to a reactivation of the inherited major structures created during the evolution of the southern Tethyan margin

Pliocene-Quaternary tectono-sedimentary evolution of the Guayaquil-Tumbes forearc basin (Ecuador-Peru)

International audienceForearc systems develop largely offshore above subduction zones between the trenchand the volcanic arc. The associated forearc basins are generally parallel to the trenchand involve vertical movements (subsidence vs. uplift), influencing the dynamics of thewedge. The objectof the study is the Guayaquil-Tumbes forearc basin belonging to theEcuadorian-Peruvian subduction zone. This forearc basin has peculiar characteristics,developing on the southernmost segment of a great active continental strike-slip fault,the Dolores-Guayaquil Fault, delimiting two crustal domains, the North Andean andInca slivers. The divergent motions of the two slivers generate extension in the upperplate whose interaction with the strike-slip tectonics and subduction remains poorlyconstrained. Two major rivers, Chira and Tumbes, draining the Andean reliefs towardsthe Pacific, generate huge Pliocene-Quaternary silico-clastic sedimentary filling trappedin the basin, having an important impact on the growth of the wedge and fault activity.The objective of this study is to constrain the structural architecture of the basin bydetermining the nature, geometry and age of the major structures that controlled itsPliocene-Quaternary development. This study is carried out through the interpretationof offshore seismic profiles correlated with well data. The results show that thestructure of the basin is dominated by gravitational instabilities (classical raft tectonic)detached northwestward and westward, and active through the Pliocene- Quaternarytimes. The syn-tectonic sediments and erosional surfaces indicate that theseinstabilities are progressively younger toward the south. We propose that thisgravitational sliding has been sustained by high sedimentation rates and by the modernpropagation of the Dolores-Guayaquil Fault at depth. This major fault is interpreted asan immature strike-slip system that progressively develops towards the trench

Structural evolution of the superimposed Provençal and Subalpine fold-thrust belts (SE France)

International audienceHighlighting how crustal shortening is accommodated in space and time in fold-thrust belts is a major issue for understanding the long-term tectonic evolution of orogenic systems. In this study, we combine surface and subsurface data to build a 150 km-long sequentially restored balanced cross-section across two superimposed foreland fold-thrust belts in southeastern France: the upper Cretaceous-Eocene Provençal belt and Oligocene-Neogene Subalpine belt. These belts are composed of inverted Paleozoic-Mesozoic basins with Mesozoic halokinetic structures, basement thrusts, and cover thrusts-nappes detached in the Triassic evaporites. The multiphase growth of the Provençal and Subalpine belts has been constrained along the studied cross-section by syn-orogenic deposits and kinematic indicators of thrusting. The pre-orogenic palinspastic reconstruction of the cross-section to Late Santonian shows a large uplift zone in the center of the section (Durance and Valensole highs), which led to the separation of the Beausset basin (South Provence basin) to the south from the Barles-Digne basin (Vocontian basin) to the north. The Provençal shortening propagated ~NNE-ward from the Beausset basin up to folds in the Barles area during the Latest Santonian to Eocene times. Shortening value reaches 38 km and has been mainly accommodated by the inversion of the South Provence basin-Durance high. The Subalpine shortening propagated ~SW-ward from the Barles-Digne basin up to the Mediterranean coast during the Oligocene-Miocene to Quaternary times. It reaches 35 km and has been mainly consumed by the inversion of the thick Digne basin. The Provençal thrust wedge is characterized by distributed basement thrusts reworking numerous structures inherited from the Variscan belt and Permian-Mesozoic rifts. This structural style might have favored the development of confined foreland basins, as the Arc and Rians basins. In contrast, the vertical stacking of the thick Digne Nappe and Barles basement triangle zone in the Subalpine thrust wedge might have controlled the large flexure of the Valensole foreland basin. The lack of Triassic evaporites in the Valensole high probably explains that shortening was not transferred into the cover of this domain. Consequently, the Provençal then the Subalpine shortenings might have been transferred more deeply to induce the reactivation of basement faults into the external zones. The Mesozoic halokinetic structures also strongly influenced the location of contractional deformation. This study highlights that the crustal structural inheritances influenced the structural styles and development of extensional basins and subsequent Provençal and Subalpine belts

Keys and pitfalls in mesoscale fault analysis and paleostress reconstructions, the use of Angelier's methods

International audienceWhereas most of the stress inversion methods using fault slip data only minimize the angle between themeasured striation and a computed shear stress to find the best fitting reduced stress tensor, Angelier(1990) proposed an alternative method named INVD that also takes into account the relative shear stressmagnitude which allows the fault to move. Using artificial datasets and particular fault geometries we comparethis method with one of the classical methods based on the minimization of the shear-slip angles (R4DT;Angelier, 1984) and we show that in most cases the new method has improved the quality of the results. Furthermore,as proposed by Angelier, we point out that the quality of the stress inversion primarily depends onthe quality of the field data. We give advice and warn about some pitfalls concerning determination of senseof slip on fault planes, recognition of successive faulting events and their chronology, drawer (or wedge)faults, stress permutations, faults in vertical bedding. We also argue that, in case of tilted sequences, faultdiagrams should not be presented without bedding planes. But we show that stress inversions, when realizedwith caution and with the correct method, can havemuch more applications than reconstructing stress fields,like for determining: the paleo-horizontal, the nature and the sense of motion of large faults, the chronologyand age of large structures