Terranes affinity and variscan transpressional evolution of rheic-related units in SW Iberia

Resumen

The Rheic Ocean closure led to the Variscan collision between Laurussia and Gondwana and concluded with the formation of the Pangea supercontinent in late Paleozoic time. Other minor terranes were also involved in the Variscan Cycle, e.g. Avalonia and Armorica. This Thesis deals with the Variscan tectonometamorphic evolution of the SW Iberian Variscides, constituted by the southern Central Iberian Zone (CIZ), the Ossa-Morena Zone (OMZ) and the South Portuguese Zone (SPZ). The main focus is put on the OMZ/SPZ boundary, which is interpreted as the Rheic Ocean suture and attests the collision of Avalonia (SPZ) with a north-Gondwanan terrane (OMZ). Three tectonics units have been classically related to the OMZ/SPZ suture: (i) the Beja-Acebuches amphibolites (BAA), a metamafic unit interpreted as a Rheic ophiolite; (ii) the Pulo do Lobo unit, a low-grade metasedimentary unit with minor MORB-like metabasalts, considered as the remnant of a Rheic subduction-related accretionary prism; and (iii) the allochthonous Cubito-Moura unit, which appears emplaced onto the southern OMZ and contains Ordovician? MORB-featured rocks and Devonian? high-pressure metamorphic assemblages. Concerning the palaeogeographic significance of the SPZ, a good number of SHRIMP U/Pb detrital zircon age populations from Devonian-Carboniferous rocks of the northern SPZ have been obtained in order to estimate provenance sources and maximum depositional ages. The results are compatible with an Avalonian affinity of the SPZ, particularly regarding the latest Devonian Horta da Torre formation of the Pulo do Lobo unit. The lower formations of the Pulo do Lobo unit (Pulo do Lobo and Ribeira de Limas formations) and that of the Iberian Pyrite belt (Ronquillo formation) can be considered as equivalent according to their similar inherited zircon content, depositional age, lithology and polyphasic deformation, thus challenging the classical interpretation of the Pulo do Lobo unit as exotic with respect to the SPZ. New structural and geochronological data have been obtained in order to improve the knowledge of the geometry and timing of the Variscan tectonometamorphic evolution of the OMZ/SPZ suture-related units. The interplay between folding and high- to low-temperature ductile shearing of the BAA has been studied, with particular emphasis on structural mapping, which, in turn, has allowed the recognition of the large-scale Quintos fold. We have also characterized the polyphasic Devonian to Carboniferous deformation of the Pulo do Lobo unit at different scales, from thin-sections to regional kilometric cross-sections. The MORB-like metamafic rocks of the Pulo do Lobo belt have yielded (SHRIMP U/Pb on zircons) Mississipian ages, thus having been reinterpreted (as previously was done for the BAA) as formed during an Early Carboniferous intraorogenic transtensional and magmatic event that obscured the former Rheic suture zone. The collision was resumed after this transtensional stage in an oblique left-lateral scenario that gave way to the majority of structures described in this Thesis. The very low- to low-grade metamorphic evolution of the Pulo do Lobo unit has been studied by applying three different methodologies. Two metamorphic events have been distinguished based on phyllosilicate growth in relation to the regional foliations: the Devonian M1 corresponds to the epizone, and the Carbonifeous M2 to the anchizone/epizone boundary. The white-mica b parameter and celadonite content indicate low-pressure conditions, which question the interpretation of the Pulo do Lobo belt as an accretionary prism. Finally, the large-scale kinematics of the Variscan collision between the CIZ/OMZ/SPZ has been quantified by using all the available data (fold, shear zone and fault patterns, tectonic fabrics and finite strain) on suitable transpressional scenarios for each structural domain. The total collisional convergence surpasses 1000 km, most of them corresponding to left-lateral displacement parallel to terrane boundaries. This left-lateral kinematics contrasts with the dextral component that prevailed elsewhere in the Variscan orogen, having been explained in the context of an Avalonian plate promontory (currently represented by the SPZ) that entered the northern Gondwanan or Armorican margin.