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The Adula Nappe: Stratigraphy, Structure and Kinematics of an Exhumed High-Pressure Nappe

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The Adula Nappe: Stratigraphy, Structure and Kinematics of an Exhumed High-Pressure Nappe Unicentre CH-1015 Lausanne http://serval.unil.ch Year : 2013 THE ADULA NAPPE: STRATIGRAPHY, STRUCTURE AND KINEMATICS OF AN EXHUMED HIGH-PRESSURE NAPPE Cavargna-Sani Mattia Cavargna-Sani Mattia, 2013, THE ADULA NAPPE: STRATIGRAPHY, STRUCTURE AND KINEMATICS OF AN EXHUMED HIGH-PRESSURE NAPPE Originally published at : Thesis, University of Lausanne Posted at the University of Lausanne Open Archive. http://serval.unil.ch Droits d’auteur L'Université de Lausanne attire expressément l'attention des utilisateurs sur le fait que tous les documents publiés dans l'Archive SERVAL sont protégés par le droit d'auteur, conformément à la loi fédérale sur le droit d'auteur et les droits voisins (LDA). A ce titre, il est indispensable d'obtenir le consentement préalable de l'auteur et/ou de l’éditeur avant toute utilisation d'une oeuvre ou d'une partie d'une oeuvre ne relevant pas d'une utilisation à des fins personnelles au sens de la LDA (art. 19, al. 1 lettre a). A défaut, tout contrevenant s'expose aux sanctions prévues par cette loi. Nous déclinons toute responsabilité en la matière. Copyright The University of Lausanne expressly draws the attention of users to the fact that all documents published in the SERVAL Archive are protected by copyright in accordance with federal law on copyright and similar rights (LDA). Accordingly it is indispensable to obtain prior consent from the author and/or publisher before any use of a work or part of a work for purposes other than personal use within the meaning of LDA (art. 19, para. 1 letter a). Failure to do so will expose offenders to the sanctions laid down by this law. We accept no liability in this respect. FACULTÉ DES GÉOSCIENCES ET DE L’ENVIRONNEMENT INSTITUT DES SCIENCES DE LA TERRE THE ADULA NAPPE: STRATIGRAPHY, STRUCTURE AND KINEMATICS OF AN EXHUMED HIGH-PRESSURE NAPPE THÈSE DE DOCTORAT présentée à la Faculté des géosciences et de l’environnement de l’Université de Lausanne pour l’obtention du grade de Docteur en Sciences de l’environnement par Mattia Cavargna-Sani Master of science (MSc) in Geology, Université de Lausanne Jury Prof. Eric Verrecchia (Président du jury) Prof. Jean-Luc Epard (Directeur de thèse) Prof. Lukas Baumgartner (Expert interne) Prof. François Bussy (Expert interne) Prof. Neil Mancktelow (Expert externe) Dr Thorsten Nagel (Expert externe) LAUSANNE 2013 FACULTÉ DES GÉOSCIENCES ET DE L’ENVIRONNEMENT INSTITUT DES SCIENCES DE LA TERRE THE ADULA NAPPE: STRATIGRAPHY, STRUCTURE AND KINEMATICS OF AN EXHUMED HIGH-PRESSURE NAPPE THÈSE DE DOCTORAT présentée à la Faculté des géosciences et de l’environnement de l’Université de Lausanne pour l’obtention du grade de Docteur en Sciences de l’environnement par Mattia Cavargna-Sani Master of science (MSc) in Geology, Université de Lausanne Jury Prof. Eric Verrecchia (Président du jury) Prof. Jean-Luc Epard (Directeur de thèse) Prof. Lukas Baumgartner (Expert interne) Prof. François Bussy (Expert interne) Prof. Neil Mancktelow (Expert externe) Dr Thorsten Nagel (Expert externe) LAUSANNE 2013 MATTIA CAVARGNA-SANI THE ADULA NAPPE: STRATIGRAPHY, STRUCTURE AND KINEMATICS OF AN EXHUMED HIGH-PRESSURE NAPPE Abstract This study analyses the stratigraphy, structure and kinematics of the northern part of the Adula nappe of the Central Alps. The Adula nappe is one of the highest basement nappes in the Lower Penninic nappe stack of the Lepontine Dome. This structural position makes possible the investigation of the transition between the Helvetic and North Penninic paleogeographic domains. The Adula nappe is principally composed of crystalline basement rocks. The investigation of the pre-Triassic basement shows that it contains several Palaeozoic detrital metasedimentary formations dated from the Cambrian to the Ordovician. These formations contain also some volcanic or intrusive magmatic rocks. Ordovician metagranites dated at ~450 Ma are also a common rock-type of the Adula basement. These formations underwent Alpine and Variscan deformation and metamorphism. Permian granites (Zervreila orthogneiss, dated at ~290 Ma) have intruded this pre-structured basement in a post-orogenic geodynamic context. Due to their age, the Zervreila orthogneiss are good markers for lpine deformation. The stratigraphy of the Mesozoic and Paleogene sedimentary cover of the Adula nappe is essential to unraveling its pre- orogenic history. The autochthonous cover is assigned to a North Penninic Triassic series that testifies for a transition between the Helvetic and Briançonnais Triassic domains. The Adula domain goes through an emersion during the Middle Jurassic, and is part of a topographic high during the first phase of the Alpine rift. The sediments of the late Middle Jurassic show a drowning phase associated with a tectonic activity and a breccia formation. In the neighbouring domains, coeval with the drowning phase in the Adula domain, a strong extensional crustal delamination and a scattered magmatic activity is associated with the main opening of the North Penninic domain. The Upper Jurassic of the Adula nappe is characterized by a carbonate formation comparable with those in the Helvetic or Subbriaçonnais domains. Flysch s.l. deposition starts probably at the end of the Cretaceous. These sediments are deposited on a large unconformity testifying for a Cretaceous sedimentary gap. The Adula nappe exhibits a very complex structure. This structure is formed by several deformation phases. Two ductile deformations are responsible for the nappe emplacement. The first deformation phase is associated with a folding compatible with a top-to-south movement at the top of the nappe. The second phase is dominant and pervasive throughout the whole nappe. It goes with a strong north vergent folding and the main nappe emplacement. These two phases cause the exhumation and emplacement of a coherent, although pre-structured, piece of continental crust. Two further deformation phases postdate the nappe emplacement. 7 8 MATTIA CAVARGNA-SANI LA NAPPE DE L’ADULA: STRATIGRAPHIE, STRUCTURE ET CINÉMATIQUE D’UNE NAPPE DE HAUTE-PRESSION Résumé Ce travail concerne l’étude géologique de la partie nord de la nappe de l’Adula dans les Alpes centrales. La nappe de l’Adula est l’une des nappes cristallines la plus élevée dans la pile des nappes du Pennique inférieur des Alpes lepontines. Cette position particulière permet d’étudier la transition entre les nappes des domaines helvétique et pennique inférieur. La nappe de l’Adula est principalement composée de socle cristallin : l’étude de l’histoire géologique du socle est donc un des thèmes de cette recherche. Ce socle contient plusieurs formations métasédimentaires paléozoïques du Cambrien à l’Ordovicien. Ces métasédiments sont issus de formations clastiques comprenant souvent des roches magmatiques volcaniques et intrusives. Ces métasédiments ont subi les cycles orogéniques varisque et alpin. La nappe de l’Adula contient plusieurs corps magmatiques granitiques métamorphisés. Les premiers métagranites sont Ordovicien et témoignent d’un environnement de marge active. Ces granites sont aussi polymétamorphiques. Les deuxièmes métagranites sont représentés par les orthogneiss de type Zervreila. Ce métagranite est d’âge permien (~290 Ma). Il est mis en place dans un contexte tectonique post-orogénique. Ce granite est un maqueur de la déformation alpine car il n’est pas affecté par les orogenèses précédentes. Le contenu stratigraphique des roches mésozoïques et cénozoiques de la couverture sédimentaire de la nappe de l’Adula est important pour en étudier son histoire pré-alpine. La couverture autochtone est composée d’une série d’âge triasique d’affinité nord-pennique, un faciès qui marque la transition entre les domaines helvétiques et briançonnais au Trias. Le domaine paléogéographique représenté dans la nappe de l’Adula connaît une émersion pendant le Jurassique moyen. Cette émersion marque le commencement du rift dans le domaine alpin. La sédimentation de la fin du Jurassique moyen est marquée par une transgression marine accompagnée par des mouvements tectoniques et la formation d’une brèche. Cette transgression est contemporaine des importants mouvements tectoniques et des manifestations magmatiques dans les unités voisines qui marquent la phase principale d’ouverture du bassin nord-pennique. Le Jurassique supérieur est caractérisé par l’instauration d’une sédimentation carbonatée comparable à celle du domaine helvétique ou subbriançonnais. Une sédimentation flyschoïde, probablement du Crétacé à Tertiaire, est déposée sur une importante discordance qui témoigne d’une lacune au Crétacé. La structure complexe de la nappe de l’Adula témoigne de nombreuses phases de déformation. Ces phases de déformation sont en partie issues de la mise en place de la nappe et de déformations plus tardives. La mise en place de la nappe produit deux phases de déformation ductile : la première produit un plissement compatible avec un cisaillement top-vers-le sud dans la partie supérieure de la nappe; la deuxième produit un intense plissement qui accompagne la mise en place de la nappe vers le nord. Ces deux phases de déformation témoignent d’un mécanisme d’exhumation par déformation ductile d’un bloc cohérent. 9 10 REMERCIEMENTS Cette thèse a éte financée par le Fonds national suisse de la recherche scientifique (FNS), fond no 200021_132460 et par la Société Académique Vaudoise. Comme ces quatre années, heureusement, je ne les ai pas passées tout seul sur cette thèse, je tiens à remercier les personnes qui m’ont permis d’aboutir ce travail. Je tiens à remercier
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