A magnetic fabric study of the Aigoual–Saint Guiral–Liron granite pluton (French Massif Central) and relationships with its associated dikes.

International audienceIn the southeastern French Massif Central, the Aigoual–Saint Guiral–Liron pluton consists of porphyritic and microgranitic types. The latter is encountered within dikes forming the northern end of the pluton. Both types show prefull crystallization microstructures indicating weak subsolidus deformations. An anisotropy of magnetic susceptibility (AMS) study has been carried out to determine the granite fabric. Biotite, local hornblende, and small grains of magnetite are the main carriers of AMS in both types. Porphyritic granite and dikes display different AMS patterns related to magma dynamics and regional deformation recorded during crystallization. In the porphyritic type, the AMS lineation is mainly consistent with the regional, NW-SE to E-W trending, extensional event coeval with emplacement and crystallization of the pluton indicating an influence of regional tectonics on the linear fabric development. The dome shaped foliation pattern of the Saint Guiral massif likely corresponds to internal deformation-related processes within the pluton. In the microgranite dike swarm, the NNE-SSW trending lineations with shallow plunges record magmatic flow processes within dikes, i.e., magma injection and filling of dikes from a probable source located southwestward. Regional tectonics played a significant role in the pluton geometry and fabric development. For example, the NE-SW trend of the dikes suggests that extensional fractures took place in the same extensional strain field as elsewhere in the pluton. Different fabric development modes were therefore responsible for the contrasted fabric patterns between the microgranite and the porphyritic granite

Les traits géologiques essentiels des Andes centrales (Pérou-Bolivie)

The peruano•bolivian segment of the Andes is about 2500 km long and its direction is NW-SE north of 18° S, submeridian further south. Its frame made of precambrian and hercynian folded material, constitutes a sialic basement for the Andean orogenic belt. From a stratigraphical point of view, the peruano-bolivian Andes appear as an intracratonic chain, where continental or neritic series prevail. The paleogeographic evolution is controlled by a system of two or three basins separated by rises and generally oriented parallel to the future chain; its most internal element is a high zone, where an intense volcanic activity has been remarkably constant in time and. space during the Mesozoic. The chain built up through three brief and homo•axial tectonic phases occurring respectively at the end of the Cretaceous, the end of the Eocene and the mid•Pliocene, that develop a moderate folding accompanied by large faults and by rare and local overthrusts. From one phase to the other, the concerned zone migrates progressively to the east. The magmatic evolution appears to be clearly related to the "liminal" position of the chain, that is to the existence of a subduction zone. During the upper Cretaceous and the Tertiary the intrusion of the granodioritic batholiths takes place. Their age and volume gradually decrease to the east. Then a powerful calc•alkaline volcanism sets up, the emissive centers of which migrate too in an easterly direction through the Tertiary

Crustal thickness and velocity structure across the Moroccan Atlas from long offset wide-angle reflection seismic data: The SIMA experiment

The crustal structure and topography of the Moho boundary beneath the Atlas Mountains of Morocco has been constrained by a controlled source, wide-angle seismic reflection transect: the SIMA experiment. This paper presents the first results of this project, consisting of an almost 700 km long, high-resolution seismic profile acquired from the Sahara craton across the High and the Middle Atlas and the Rif Mountains. The interpretation of this seismic data set is based on forward modeling by raytracing, and has resulted in a detailed crustal structure and velocity model for the Atlas Mountains. Results indicate that the High Atlas features a moderate crustal thickness, with the Moho located at a minimum depth of 35 km to the S and at around 31 km to the N, in the Middle Atlas. Upper crustal shortening is resolved at depth through a crustal root where the Saharan crust underthrusts the northern Moroccan crust. This feature defines a lower crust imbrication that, locally, places the Moho boundary at 40-41 km depth in the northern part of the High Atlas. The P-wave velocity model is characterized by relatively low velocities, mostly in the lower crust and upper mantle, when compared to other active orogens and continental regions. These low deep crustal velocities together with other geophysical observables such as conductivity estimates derived from MT measurements, moderate Bouguer gravity anomaly, high heat flow, and surface exposures of recent alkaline volcanism lead to a model where partial melts are currently emplaced at deep crustal levels and in the upper mantle. The resulting model supports the existence of a mantle upwelling as mechanism that would contribute significantly to sustain the High Atlas topography. However, the detailed Moho geometry deduced in this work should lead to a revision of the exact geometry and position of this mantle feature and will require new modeling effortsThis work has been primarily funded by the Spanish MEC project CGL2007–63889. Additional funding was provided by projects CGL2010–15416, CSD2006-00041, and GL2009–09727 (Spain), CGL2008–03474-E, 07-TOPO_EUROPE_FP-006 (ESF Eurocores) and EAR-0808939 (US, NSF).Peer reviewe

Emplacement and deformation of mesozoic Gabbros of the High Atlas (Morocco): paleomagnetism and magnetic fabrics

A paleomagnetic and magnetic fabric study is performed in Upper Jurassic gabbros of the central High Atlas (Morocco). These gabbros were emplaced in the core of preexisting structures developed during the extensional stage and linked to basement faults. These structures were reactivated as anticlines during the Cenozoic compressional inversion. Gabbros from 19 out of the 33 sampled sites show a stable characteristic magnetization, carried by magnetite, which has been interpreted as a primary component. This component shows an important dispersion due to postemplacement tectonic movements. The absence of paleoposition markers in these igneous rocks precludes direct restorations. A novel approach analyzing the orientation of the primary magnetization is used here to restore the magmatic bodies and to understand the deformational history recorded by these rocks. Paleomagnetic vectors are distributed along small circles with horizontal axes, indicating horizontal axis rotations of the gabbro bodies. These rotations are higher when the ratio between shales and gabbros in the core of the anticlines increases. Due to the uncertainties inherent to this work (the igneous bodies recording strong rotations), interpretations must be qualitative. The magnetic fabric is carried by ferromagnetic (s.s.) minerals mimicking the magmatic fabric. Anisotropy of magnetic susceptibility (AMS) axes, using the rotation routine inferred from paleomagnetic results, result in more tightly clustered magnetic lineations, which also become horizontal and are considered in terms of magma flow trend during its emplacement: NW-SE (parallel to the general extensional direction) in the western sector and NE-SW (parallel to the main faults) in the easternmost structures

Précisions sur la tectonique tangentielle des terrains secondaires du massif de Pirin (Nord-Ouest du lac Titicaca, Pérou)

Le massif de Pirin s’allonge sur une trentaine de kilomètres en bordure du lac Titicaca (3 814 m) qu’il domine de quelques centaines de mètres. I1 fut rendu célèbre, au début du siècle, par un petit champ pétrolier, qui se trouve être le plus haut du monde (3 900 m environ). Par suite de son intérêt économique, plusieurs études géologiques furent consacrées à ce massif ; ce furent d’abord, il y a une vingtaine d’années, celles de A. Heim (I) et de N. D. Newell (’). Plus récemment, entre 1962 et 1966, l ’E~m presa Petrolera Fiscal 1) a réalisé l’étude détaillée (inédite) de toute la région du lac Titicaca et en particulier celle du massif de Pirin ; c’est grâce à ce travail qu’il nous a été possible, à la suite de tournées communes, de proposer une nouvelle interprétation tectonique de ce massif