Éditeur(s) : HAL CCSD
Résumé : Based on fieldwork and modern analytical technics, this work presents an overview of the Mid-Cretaceous hyper-extended rift system exposed in the northern part of the Pyrenean mountain range. Its inversion during the Pyrenean orogeny allows for precious observations of the deep-seated processes occurring at the foot of distal margins. Some peculiar aspects characterizing the pre-Alpine hyper-extended domain are examined throughout the 400km long suture that is fossilized in the North Pyrenean Zone (NPZ). The High Temperature/Low Pressure (HT/LP) thermal imprint of the extensional event is studied thanks to a dataset of more than hundred peak temperature measurement by Raman spectroscopy of the carbonaceous material (RSCM). The pre- and syn-rift metasediments are characterized by an intense ductile deformation. Field and EBSD investigations of the mechanisms of deformation reveal ductile and syn-metamorphic processes. The emplacement conditions of the peridotite into crustal levels and localized exhumation is examined in the key-localities of Urdach, Saraillé-Tos de la Coustette, Moncaup, Lherz, Caussou-Bestiac and Salvezines, The results indicate an early boudinage of the crust during the rifting process, resulting in the direct superposition of pre-rift sediments on top of mantle peridotites. Field, textural and isotopic characterization of the various ophicalcite and breccias found in the vicinity of the peridotite outcrops allow to decipher their exhumation history. New Ar-Ar geochronological results allow to precise the temporal evolution of the NPZ.
Cette thèse présente une analyse structurale et thermique des marges passives crétacées inversées dans la Zone Nord Pyrénéenne (ZNP). L'application du paléothermomètre RSCM (spectrométrie Raman de la matière carbonée) sur plus de cent échantillons répartis à travers la ZNP permet de préciser l'évolution thermique de la déchirure continentale. Un métamorphisme de haute température (450 à >600°C) se développe particulièrement au droit de zones ou la croûte continentale est extrêmement amincie voire éliminée. Les sédiments pré- et syn-rift sont marqués par une intense déformation ductile. L'analyse à l'EBSD des fabriques minérales indique une déformation de Haute-température contemporaine du métamorphisme HT-BP. Les conditions de mise en place crustale des péridotites et leur exhumation localisée sont examinées dans les localités d'Urdach, Saraillé-Tos de la Coustette, Moncaup, Lherz, Caussou-Bestiac et Salvezines. Les résultats indiquent un boudinage de la croûte continentale au cours du processus de rifting. Des décollements de couverture, principalement localisés dans les évaporites triasiques conduisent à la superposition directe des sédiments pré-rift sur le manteau exhumé. La notion nouvelle de soutirage crustal est introduite pour désigner ce processus dans lequel l'écoulement gravitaire ne joue pas un rôle principal. La caractérisation sédimentologique, pétrographique et isotopique (O et C) des différentes formations de brèches et ophicalcites observées à proximité des corps de péridotites permet de reconstituer les étapes de l'exhumation du manteau en pied de marge. Enfin de nouvelles datations permettent de mieux contraindre le calendrier des évènements successifs.
https://tel.archives-ouvertes.fr/tel-00787952

tel-00787952
https://tel.archives-ouvertes.fr/tel-00787952
https://tel.archives-ouvertes.fr/tel-00787952/document
https://tel.archives-ouvertes.fr/tel-00787952/file/TheI_se-Clerc-2012.pdf

Éditeur(s) : HAL CCSD
Résumé : International audience
Sub-continental lithospheric mantle rocks are exhumed at the foot of magma-poor distal passive margins as aresponse to extreme stretching of the continental crust during plate separation. Remnants of the Northern Iberianpaleo-passive margin are now exposed in the North-Pyrenean Zone (NPZ) and represent field analogues to studythe processes of continental crust thinning and subcontinental mantle exhumation. The NPZ results from theinversion of basins opened between the Iberia and Europa plates during Albo-Cenomanian times. In the westernNPZ, the ‘Chaînons Béarnais’ ranges display a fold-and-thrust structure involving the Mesozoic sedimentarycover, decoupled from its continental basement and associated with peridotite bodies in tectonic contact withPalaeozoic basement lenses of small size. Continental extension developed under hot thermal conditions, asdemonstrated by the syn-metamorphic Cretaceous ductile deformation affecting both the crustal basement and theallochthonous Mesozoic cover.In this study, we present structural and geochemical data providing constraints to reconstruct the evolution ofthe northern Iberia paleo-margin. Field work confirms that the pre-rift Mesozoic cover is intimately associatedto mantle rocks and to thin tectonic lenses of crustal basement. It also shows that the pre-rift cover was detachedfrom its bedrock at the Keuper evaporites level and was welded to mantle rocks during their exhumation atthe foot of the hyper-extended margin. The crust/mantle detachment fault is a major shear zone characterizedby anastomosed shear bands defining a plurimetric phacoidal fabric at the top of the serpentinized mantle. Thedetachment is marked by a layer of metasomatic rocks, locally 20 meters thick, made of talc-chlorite-pyrite-richrocks that developped under greenschist facies conditions. Raman Spectroscopy on Carbonaceous Materials(RSCM), performed on the Mesozoic cover reveal that the entire sedimentary pile underwent temperatures rangingbetween 200C and 480C.We show that: (i) at the site of mantle rocks exhumation, the boudinaged pre-rift sediments have undergone drasticsyn-metamorphic thinning with the genesis of a S0/S1 foliation and, (ii) the Paleozoic basement has been ductilelydeformed, into thin tectonic lenses that remained welded to the exhumed mantle rocks. Therefore the overallcrustal rheology appears dominated by shallow levels having a ductile behavior. This rheology is related to thepresence of a thick pre- and syn-rift decoupled cover acting as an efficient thermal blanket. This new geologicaldata set highlights important characteristics of ductile-type hyper-extended passive margin that cannot be obtainedfrom the study of seismic lines. Finally, we stress that studying field analogues represents a major tool to betterunderstand the mechanisms of extreme crustal thinning associated with mantle exhumation and their structuralinheritance during tectonic inversion.
Geophysical Research Abstracts
Vienne, Austria

insu-01510103
https://hal-insu.archives-ouvertes.fr/insu-01510103

Éditeur(s) : HAL CCSD Elsevier Masson
Résumé : International audience
We provide new constraints on the modes of mantle exhumation and denudation in the pre-orogenic Pyrenees by investigating the poorly known peridotites located east of the Lherz body, in eastern Ariè ge (Vicdessos, Urs, Bestiac, Caussou and Prades main peridotitic bodies). We show that: (1) numerous peridotite bodies, not described so far, are constituted by decametric to sub-metric outcrops scattered between and around the main outcrops identified by Monchoux (1970); (2) most of the peridotites form blocks and lenses of various sizes, from millimetric monocrystalline grains to hectometric olistoliths, embedded in debris flows involving mainly Mesozoic metasediments; these peridotites can therefore be linked to the S-type peridotites of Lagabrielle et al. (2010), and share similarities with the peridotites of Lherz region (Clerc et al., 2012). All originated from tectono-sedimentary processes associated with mantle exhumation in relation with extreme thinning and breakoff of the continental basement; (3) the various petrographic characters of these peridotites and their heterogeneity in serpentinisation degree imply a peculiar mode of exhumation, which merges mantle fragments inherited from different structural levels; (4) the lack of tectonic structures such as faults and shear planes associated with the ultramafic-rich breccia formation and the presence of neoformation of metamorphic minerals within the matrix preclude their origin as cataclasites related to the Latest Cretaceous and Tertiary Pyrenean contractional deformation. The peridotites of eastern Ariè ge are indisputable witnesses of the presence of an inverted hyper-extended Cretaceous passive margin forming the current North Pyrenean zone. The geometry of the extensional system was complex and probably associated with strike-slip faults and transcurrent motion. ß 2016 The Authors. Published by Elsevier Masson SAS on behalf of Acadé mie des sciences. This is an open access article under the CC BY-NC-ND license (http://creativecommons. org/licenses/by-nc-nd/4.0/).
ISSN: 1631-0713

Droits : http://creativecommons.org/licenses/by-nc-nd/
insu-01280502
https://hal-insu.archives-ouvertes.fr/insu-01280502
https://hal-insu.archives-ouvertes.fr/insu-01280502/document
https://hal-insu.archives-ouvertes.fr/insu-01280502/file/1-s2.0-S1631071316000043-main.pdf
DOI : 10.1016/j.crte.2015.12.003

Éditeur(s) : HAL CCSD Elsevier
Résumé : International audience
The identification of the structures and deformation patterns in magma-poor continental rifted margins is essential to characterize the processes of continental lithosphere necking. Brittle faults, often termed mantle detachments, are believed to play an essential role in the rifting processes that lead to mantle exhumation. However, ductile shear zones in the deep crust and mantle are rarely identified and their mechanical role remains to be established. The western Betics (Southern Spain) provides an exceptional exposure of a strongly thinned continental lithosphere, formed in a supra-subduction setting during Oligocene-Lower Miocene. A full section of the entire crust and the upper part of the mantle is investigated. Variations in crustal thickness are used to quantify crustal stretching that may reach values larger than 2000% where the ductile crust almost disappear, defining a stage of hyper-stretching. Opposite senses of shear top-to-W and top-to-E are observed in two extensional shear zones located close to the crust-mantle boundary and along the brittle-ductile transition in the crust, respectively. At locations where the ductile crust almost disappears, concordant top-to-E-NE senses of shear are observed in both upper crust and serpentinized mantle. Late high-angle normal faults with ages of ca. 21 Ma or older (40Ar/39Ar on white mica) crosscut the previously hyper-stretched domain, involving both crust and mantle in tilted blocks. The western Betics exemplifies, probably better than any previous field example, the changes in deformation processes that accommodate the progressive necking of a continental lithosphere. Three successive steps can be identified: i/a mid-crustal shear zone and a crust-mantle shear zone, acting synchronously but with opposite senses of shear, accommodate ductile crust thinning and ascent of subcontinental mantle; ii/hyper-stretching localizes in the neck, leading to an almost disappearance of the ductile crust and bringing the upper crust in contact with the subcontinental mantle, each of them with their already acquired opposite senses of shear; and iii/high-angle normal faulting, cutting through the Moho, with related block tilting, ends the full exhumation of the mantle in the zone of localized stretching. The presence of a high strength sub-Moho mantle is responsible for the change in sense of shear with depth. Whereas mantle exhumation in the western Betics occurred in a backarc setting, this deformation pattern controlled by a high-strength layer at the top of the lithosphere mantle makes it directly comparable to most passive margins whose formation lead to mantle exhumation. This unique field analogue has therefore a strong potential for the seismic interpretation of the so-called “hyper-extended margins”.
ISSN: 0264-8172

insu-01310895
https://hal-insu.archives-ouvertes.fr/insu-01310895
https://hal-insu.archives-ouvertes.fr/insu-01310895/document
https://hal-insu.archives-ouvertes.fr/insu-01310895/file/Frasca-MarPetGeol-2016.pdf
DOI : 10.1016/j.marpetgeo.2016.04.020