Differential spreading along the NE Atlantic ridge system and post-breakup deformation of the adjacent continental margins
L'ouverture différentielle de l'océan Atlantique Nord-Est et ses effets sur les déformations postbreak-up des marges continentales
Résumé
One of the main assumptions of the theory of plate tectonics is that all lithospheric plates are rigid. However, reconstructions of the opening of the NE Atlantic Ocean, on the basis of two rigid plates (Eurasia and Greenland), lead to gaps and overlaps between the plates. The area between Iceland and the Jan Mayen Fracture Zone (JMFZ) had a complex spreading history, including progressive separation of the Jan Mayen Microcontinent (JMMC) and a ridge jump from the Aegir Ridge to the Kolbeinsey Ridge. Moreover, post-breakup compressional structures developed along the continental margin of NW Europe, but apparently not on the East Greenland Margin. We therefore investigate how compressional deformation of the NW European Margin may have resulted from variations in the direction and rate of sea-floor spreading along the various ridges. In order to reconstruct the complex spreading history of the NE Atlantic and to study the evolution of the European Margin during sea-floor spreading, we have developed a method for palinspastic reconstructions of the opening of an ocean, using magnetic anomalies and fracture zones. The best kinematic reconstructions result from subdividing the NE Atlantic into three oceanic segments: Reykjanes, Jan Mayen and Mohns. The method allows all oceanic segments to spread at a different rate and results in accurate determinations of spreading rates and relative displacements between the segments. The model ensures a good fit of the magnetic anomalies and predicts differences in direction and rate of spreading between the Reykjanes, Kolbeinsey/Aegir and Mohns ridges. This differential sea-floor spreading generated leftlateral slip between the oceanic segments: (1) from Early Eocene to Late Oligocene, along the Faeroe Fracture Zone (FFZ); and (2) from Late Eocene to Early Oligocene, as well as during the Miocene, along the JMFZ. Such left-lateral motion and the relative rotation between the oceanic segments are compatible with the development of inversion structures on the NW European Margin at these times. Field observations of Jurassic outcrops in NE Scotland have provided additional evidences for post-Jurassic right-lateral reactivation of the Great Glen Fault (GGF), under transpression. The period of reactivation may be from Late Eocene to Late Oligocene, which coincides with (1) an exhumation episode in Scotland, (2) intraplate stress from the Alpine Orogeny (3) a pulse of the Iceland Mantle Plume, and (3) left-lateral slip along the FFZ. Such left-lateral slip along the FFZ is compatible with right-lateral reactivation of the GGF. The driving forces may have come from the Iceland Mantle Plume, which was in a suitable position to generate differential sea-floor spreading along the NE Atlantic and resulting deformation of the NW European margin.
La théorie de la tectonique des plaques implique que les plaques lithosphériques soient rigides. Or les reconstructions de l'ouverture de l'océan Atlantique NE, utilisant deux plaques rigides (Eurasie et Groenland), conduisent à des zones de recouvrement. De plus, l'ouverture océanique de la zone située entre l'Islande et la zone de fracture de Jan Mayen (JMFZ) fut très complexe, incluant la formation progressive du microcontinent de Jan Mayen (JMMC) et un saut de ride. J'ai développé une méthode de reconstruction palinspastique d' ouverture de l'Atlantique NE, à partir de données d'anomalies magnétiques et de zones de fracture, méthode qui permet d'obtenir un bon ajustement des anomalies magnétiques. Le modèle prédit des différences de direction et de taux d'ouverture entre les segments océaniques. Cette ouverture différentielle a induit des mouvements décrochants sénestres le long des zones de fractures océaniques, compatibles avec le développement de structures compressives le long de la marge continentale européenne, (1) de l'Éocène inférieur à l'Oligocène supérieur, le long de la zone de fracture des îles Féroé (FFZ), et (2) de l' Éocène supérieur à Oligocène inférieur, et au Miocène, le long de la JMFZ. Aussi,j'ai mis en évidence, par une étude de terrain, une réactivation Cénozoïque en dextre de la Great Glen Fault, en Ecosse, que j'interprète comme étant due aux mouvements senestres le long de la FFZ. Je suggère également que les forces motrices proviennent du point chaud, actuellement sous l'Islande, qui était en mesure de générer une ouverture différentielle de l'Atlantique NE et les déformations compressives de la marge continentale européenne.