Evolución geodinámica del Oroclinal Ibero Armoricano. Geología estructural, modlización análoga y geocronologí

Abstract

The Ibero-Armorican orocline is a central component of the Western European Variscan Belt, a complex continental-scale orogen (1000 km wide and 8000 km long) that formed through a series of protracted collisional events extending from ca.420 Ma to 320 Ma. Variscan deformation represents the closing of at least two – and possibly four – oceans between Laurentia, Baltica, Gondwana, and several micro-continents during the Paleozoic amalgamation of the Pangea supercontinent. The Ibero-Armorican orocline is characterized by the arcuate structural trend that traces an arc from Brittany across the Cantabrian Sea into western Iberia, where it is truncated by the Cenozoic Betic-Alpine front in southern Spain. New studies consider the Ibero-Armorican orocline as part of a coupled bend together with the southern Central-Iberian arc. The Ibero-Armorican orocline is a curved orogenic system characterized by a 180o bend of the Variscan structural grain. The Ibero-Armorican orocline has been the object of many studies, especially at its core. The aforementioned studies have attempted to decipher the curved mountain belt kinematics, and a wealth of different hypotheses have been proposed: a primary arc inherited from a Neoproterozoic embayment; a progressive arc resulting from indentation of a point-shaped block situated either in Gondwana or in Avalonia, an oblique collision producing a non-cylindrical orogen, a thin-skinned origin produced by a progressive change in the transport direction of the thrust units similar to a photographic iris, a large scale trans-continental shear zone, and more recently a true orocline formed by the rotation around a vertical axis of an originally linear orogen. In this PhD thesis the kinematics and dynamics of the Ibero-Armorican orocline have been studied at a lithospheric scale through structural analysis, analogue modelling and detrital zircon geochronology. With the data presented in this thesis and all the previous data published, a plausible overall interpretation is that the Variscan orogen was folded around a vertical axis during the Pennsylvanian during a period that lasted about 10 m.y during the Late Pennsylvanian. The structures developed during the formation of Iberian-Armorican orocline buckling suggest that this process occurred due to a large change in the stress field from E-W to N-S (in present day coordinates), which implies that the folding of the orogen was produced by the mechanism of buckling. The buckling process affected the whole lithosphere, which would have been deformed by a dominant mechanism of longitudinal-tangential strain. According to the experimental analogue models, the root formed in the lithospheric-mantle beneath the core of oroclinal was probably caused by lithospheric folding. This root became gravitationally unstable at around the Carboniferous-Permian boundary. At that time it could begin to develop a Rayleigh-Taylor instability ending with the detachment and sinking of the lithosphericmantle in the asthenospheric-mantle. This process of lithospheric-mantle detachment would have produced an inversion of the topography as recorded by the detrital zircons

Daniel Pastor-Galán

Científico Titular and Assistant Professor (助教) of Tectonics

My research interests include plate tectonics, paleogeogrphy and geodynamics.