Geometry, localization and timing of deformation during orogenic front propagation : a case study from the Neuquén basin (Argentina)

Cette thèse traite de l’enregistrement de la déformation, à différentes échelles de temps et d’espace dans la chaîne plissée de Malargüe et le bloc de San Rafael, situés dans la partie septentrionale du bassin de Neuquén en Argentine. La première partie du travail comprend une analyse de la déformation finie (chaîne de Malargüe) et de la déformation active (bloc de San Rafael) à l’échelle macroscopique. De manière générale, les coupes structurales régionales proposées mettent clairement en évidence le rôle l’héritage structural lié aux structures générées lors de l’ouverture du bassin au Jurassique. Nous montrons également que les mécanismes de soulèvement actuels du bloc de San Rafael à l’est reprennent les modalités de la déformation d’âge Miocène dans la chaîne. Contrairement au mode de structuration des prismes critiques classiques caractérisés par la localisation de la déformation le long d’un niveau de décollement, la structuration de la chaîne est ici issue d’une déformation distribuée plus largement dans la lithosphère. A l’échelle de l’endommagement mesoscopique, la fracturation régionale nous a permis d’enregistrer les différents champs de contraintes régionaux liés aux phases de raccourcissement depuis le domaine pré-plissement jusqu’à la phase syn-plissement. Ces différentes phases sont en accord avec la cinématique de la convergence de la plaque Nazca depuis le Crétacé supérieur. A l’échelle microscopique, l’analyse des propriétés de susceptibilité magnétique des roches met en évidence, d’une part, une compartimentalisation de la déformation à l’échelle de la matrice liée à l’héritage structural extensif. D’autre part, la distribution spatiale de cet endommagement ne démontre pas l’existence de gradient clair des fabriques magnétiques depuis l’avant-pays vers les zones internes ce qui confirme le caractère atypique du système Andin à la latitude de 35°S. Ce dernier étant principalement contrôlé par une déformation distribuée, liée à l’héritage structural et singulièrement différente de celle attendu dans le cadre de la théorie du prisme critique.This PhD project deals with multiscale record of the tectonic signal in the Malargüe fold-and-thrust-belt (MFTB) located in the northern part of the Neuquén basin (Argentina). The first results presented rely on the study of the macroscopic finite strain in the fold-belt and the characterization of the active deformation of the San Rafael Block uplift. The cross-sections we produce show that structural inheritance related to the Jurassic extension is the main parameter controlling the belt structure. In addition we proposed that the building mechanisms that controlled the MFTB evolution by Miocene times are the same than those triggering the present day San Rafael block uplift. The second part of this work is dedicated to mesoscopic strain pattern analysis recorded by fracture networks. Throughout the MFTB, we are able to describe the occurrence of four main fractures sets emplaced in several stress regime that are linked (1) to the inheritance and (2) to the well-known compression phases from pre-folding to syn-folding settings. Finally the third part of this work describes the microscopic damage recorded by the anisotropy of magnetic susceptibility method. We mainly evidence that there is no clear gradient of magnetic fabrics from foreland to hinterland and that deformation is compartmentalized by structural inheritance. This atypical pattern of magnetic fabrics succession reveals that the matrix damage is governed by the same strain distribution as those observed at macroscopic scale thus providing a supplementary argument to consider the Andean system at these latitudes as singularly different from a classical Coulomb wedge

Matrix deformation in a basement-involved fold-and-thrust-belt: A case study in the central Andes, Malargüe (Argentina)

International audienceAn analysis of the anisotropy of magnetic susceptibility (AMS) is used to investigate the matrix strain record of weakly deformed clastic rocks, including sandstones and shales, along a cross-section of the Malargüe fold-and-thrust belt (MFTB, Argentina). We collected 339 oriented cores from the Tres Esquinas Sandstones, the red beds of the Tordillo and Neuquén formations, as well as from Shales/Silts of the Mendoza Group. Overall, ~. 90% of the magnetic fabrics are well defined and are related to the strain imprint of the layer parallel shortening. Among the tectonic-related fabrics, 40% show the development of a magnetic lineation at right angle of the shortening (type II), 38% display an apparent loss of magnetic foliation (type III) and 21% develops a magnetic foliation at right angle of the shortening (type IV). There is no lithological control on magnetic fabric typology despite contrasting ferromagnetic mineralogy between the red sandstones and dark shales. Thin-section observations highlight the good match between the orientation of both the susceptibility ellipsoid and microstructure orientation, which enables us to infer the preferred orientations of the petrofabric (i.e. finite strain directions). In this way, two trends can be recognized in the study area: shortening directions trending N110 in the north and trending N80 in the south. In addition, the spatial distribution of magnetic fabric typology and burial data suggests an atypical pattern of deformation from the foreland to hinterland. Such a pattern contrasts with previous studies in fold thrust belt settings, suggesting that, at the matrix-scale, the deformation (even in a pre-folding setting) is compartmentalized and controlled by the occurrence of deep-seated thrusts localized along inherited structures. From the analysis of cross-sections, we can distinguish several domains, each of them reflecting distinct states of matrix deformation

Evidence of active shortening along the eastern border of the San Rafael basement block: Characterization of the seismic source of the Villa Atuel earthquake (1929), Mendoza province, Argentina

ACLInternational audienceOn the 30 May 1929, a massive earthquake occurred in the San Rafael area (southern Mendoza province) leading to the destruction of the Villa Atuel and Las Malvinas towns. The region affected by the ground shaking covers a large part of southern South America. Although no surface breaks have been detected on the surface, several authors have pointed out active faults that could be related to the event of 1929. Using satellite imagery and field observations, we investigated two active faults situated on the eastern border of the San Rafael Block (SRB) close to or within the epicentral area. The most prominent faults are the c. 40 km long Las Malvinas and c. 30 km long Cerro Negro reverse faults which are located near the epicentral area. Geological and morphological observations allow us to describe late Pleistocene activity and estimate the long-term slip rates of these faults. Possible ruptures that match our observations and which are compatible with the cartographic length of these faults would account for a seismic moment magnitude of M0 = 2.8×1019 N m and a moment magnitude of MW = 6.9. The morphological signatures of these fault segments and the occurrence of the San Rafael earthquake suggests that the southern Mendoza Province is still currently submitted to shortening. © 2016 Cambridge University Press