An accreted continental terrane in Northwestern Peru

Cet article présente une étude paléomagnétique des échantillons de plusieurs formations du bassin de Lancones (province de Piura). Les auteurs formulent l'hypothèse de l'accrétion d'une terrane sur la marge péruvienne au cours du Néocomien. Un régime de cisaillement aurait également produit des rotations in situ. L'évolution géodynamique du nord du Pérou est donc comparable aux processus observés sur les Andes septentrionales, en Equateu

Tectonic rotations and transcurrent deformation south of the Abancay deflection in the Andes of southern Peru

International audienceWe report new paleomagnetic results from 55 out of 76 sites sampled at different localities along a transect from Nazca to Cuzco where the general structures of the Peruvian Andes are strongly offset across the Abancay deflection. Nine new 39Ar/40Ar ages better constrain the timing of volcanism along the western edge of the Western Cordillera at the latitude of Nazca. A mean paleomagnetic result from 22 sites in the lower Miocene volcanics does not show significant rotation (R = −2.3° ± 7.7°) of the western margin of the Central Andean Plateau since the early Miocene. Within the Western Cordillera we sampled three structural blocks bounded to the north by the Abancay fault system. In the westernmost block, a large counterclockwise rotation (R = −65.0° ± 11.1°) is found in Mesozoic limestones and Paleocene-Eocene red beds. Magnitude of rotation decreases toward the east from (R = −35.6° ± 12.8°) in the central block to (R = −4.5° ± 8.4°) south of the town of Cuzco. The anisotropy of magnetic susceptibility (AMS) recorded by the red beds sediments is the consequence of compaction and tectonic strain during the early stages of deformation. We show that the magnetic lineations were also rotated counterclockwise as the remanent magnetizations. The present study confirms results from the Peruvian fore arc, showing that rotations are not older than circa 40 Ma and likely not younger than circa 20 Ma. The spatial variation in the amount of counterclockwise rotation suggests a large component of shear along the Abancay deflection concomitant with a broad late Eocene-Oligocene oroclinal deformation in southern Peru

La marge active nord-peruvienne. Etude paleomagnetique et modelisation analogique

SIGLEINIST T 77651 / INIST-CNRS - Institut de l'Information Scientifique et TechniqueFRFranc

Paleomagnetic study of an arcuate fold belt developed on a marginal orogen: The Cajamarca deflection, northern Peru

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Spatial structure of shell polychromatism in Cepaea hortensis in relation to a gradient of landscape fragmentation in western France.

International audienceBecause of their highly polymorphic shell patterns, Cepaea land snails have been the subject of numerous studies in ecological genetics. Here, we investigated the spatial structure of polychromatism in Cepaea hortensis in agricultural landscapes with zones from low to high hedgerow densities. Our main purpose was to search for a relationship between landscape composition and spatial structuring of chromatism. Despite significant spatial heterogeneity in the three landscapes sampled, only the high hedgerow density landscape showed a significant spatial structuring of shell polymorphism. In order to understand this result, an investigation of daily movement patterns in relation to habitat form was carried out on a mark-release experiment under semiartificial conditions. This experiment revealed a strong influence of a linear corridor on snail dispersal. In the field, spatial heterogeneity of shell polymorphism, related to the effects of genetic drift, was shaped by restricted dispersal in narrow corridors. In the more enclosed one, i.e. where hedgerow density was the highest, the significant spatial structure we detected involved a balance between local genetic drift and environmentally mediated gene flow. This isolation by distance pattern resulted from direct gene exchange through fields between neighbouring populations. When applying landscape distances based on hedgerow length, no significant spatial correlation with polychromatism was found. In the more fragmented sites, random genetic drift seemed to be the prevailing force and, at the scale of the whole sampled area, selective pressures potentially interfere with these genetic drift-dispersal events

Post-Oligocene rotations in southern Ecuador and northern Peru and the formation of the Huancabamba deflection in the Andean Cordillera

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Late Cainozoic rotation of the Peruvian Western Cordillera and the uplift of the Central Andes

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Rotación cenozoica de la Cordillera Occidental peruana y levantamiento de los Andes Centrales

Cenozoic rotation of Western Peruvian Cordillera and the Central Andes uplift Abstract A paleo-magnetic study was performed in the Lima (Peru) region from 39 reliable sampling positions of a total amount of 55 middle Cretaceous to late Miocene age (a 564 sampling), distributed through three NE-WS transverse lines cutting the Western Cordillera of Central Andes. For the entire area studied, all the paleo-magnetic declinations measured are invariable among them and systematically deviaded to the West. Such fact, confirms the existence of a counterclokwise rotation in the entire location of the Occidental Cordillera, regarding a stable South American Continent. Such rotation displays a broadness of 16° ± 6° subsequent to Cretaceous and 13.7° ± 8.1°, subsequent to Eocene. Such unpublished results are concurring to those previously obtained on the Central Peruvian Andes and strengthen the pattern recently proposed to explain the structure and elevation of the Andean Cordillera.Rotation cénozoïque de la Cordillère Occidentale péruvienne et surrection des Andes Centrales Résumé Une étude paléomagnétique a été réalisée dans la région de Lima (Pérou) sur 39 sites fiables parmi 55 (564 carottes) d'âge crétacé moyen à miocène supérieur distribués le long de trois transects NE-SW qui recoupent l'ensemble de la Cordillère Occidentale des Andes Centrales. Sur l'ensemble de ce secteur d'étude, toutes les déclinaisons paléomagnétiques mesurées sont cohérentes entre elles et systématiquement déviées vers l'Ouest. Elles démontrent ainsi l'existence d'une rotation antihoraire de l'ensemble de la Cordillère Occidentale par rapport au continent sud-américain stable. L'amplitude de cette rotation est de 16° ± 6° après le Crétacé et de 13,7° ± 8,1° après l'Eocène. Ces résultats inédits sont en bon accord avec ceux précédemment obtenus sur les Andes Centrales péruviennes et confortent le modèle récemment proposé pour expliquer l'édification et la surrection des Andes Centrales.Resumen Se ha realizado un estudio paleomagnético en la región de Lima (Perú) a partir de 39 estaciones de muestreo fiables sobre un total de 55 (564 muestras) de edad cretácea media a miocena superior distribuidas a lo largo de tres transectas NE-SW que cortan la Cordillera Occidental de los Andes Centrales. Para el sector estudiado, todas las declinaciones paleomagnéticas medidas son coherentes entre ellas y sistemáticamente desviadas hacia el oeste. Ello demuestra la existencia de una rotación antihoraria del conjunto de la Cordillera Occidental, respecto al continente sudamericano estable. Dicha rotación presenta una amplitud de 16° ± 6° posterior al Cretáceo, y de 13.7° ± 8.1° posterior al Eoceno. Estos resultados inéditos están de acuerdo con aquellos anteriormente obtenidos sobre los Andes Centrales peruanos y refuerzan el modelo recientemente propuesto para explicar la edificación y el levantamiento de los Andes Centrales.Macedo Sanchez Orlando, Surmont Jérôme, Kissel Catherine, Mitouard Pierre, Laj Carlo. Rotación cenozoica de la Cordillera Occidental peruana y levantamiento de los Andes Centrales. In: Bulletin de l'Institut Français d'Études Andines, tome 21, N°1, 1992. pp. 1-24