The transition between thin-to-thick-skinned styles of deformation in the Western Pyrenean Belt

This PhD Thesis describes the kinematics of the Burgalesa Platform in the Basque-Cantabrian Pyrenees by the integration of i) surface geology; ii) seismic and well data; and iii) vertical axis rotation paleomagnetic studies. The integration of all the data allow to understand the relationship between the Burgalesa Platform and the Ebro and Duero foreland basins during the Pyrenean Orogeny. The principal contribution of the Thesis is to present a new structural model, that fits all the constraints imposed by the stratigraphy, the pre-configuration of the area prior to the contractive deformation and both the deep and the surface data. This model allows to understand the transition between thin-skinned and the thick-skinned tectonics. The combined work between surface geology and well data has allowed to reinterpret the Zamanzas structure located at the northern boundary of the Burgalesa Platform. Previously, this structure had been interpreted as a set of extensional faults. This Thesis pointed out the mismatches and reinterpret the Zamanzas structure as a popup generated during the Pyrenean Orogeny. The combined interpretation of wells and seismic sections allow to partially comprehend the structure of the area during the extensional event as well as the result of the Pyrenean Orogeny occurred later. This study allowed to determine the distribution of the Lower Cretaceous depocenters during the North Atlantic and the Bay of Biscay opening. Moreover, during this stage salt mobilisation triggered by extension generated areas where the Triassic salt layer was thickened as demonstrated by the well data and the outcropping diapirs. This process accentuated the development of the depocenters. The extensional event also generated the onlap sedimentary geometries in the syn-rift sediments. The extensional model proposed is a combined model with the development of cover forced folding with salt migration at the southern boundary. The Triassic salt decoupled the extension between the basement and the cover and contractional structures were developed such as the back-thrust in the Huidobro area that produced the duplication of the Mesozoic. The early stages of the Pyrenean Orogeny resulted in the development of the San Pedro Structure, with north-directed and basement-involved thrusts, later fossilised by the Duero foreland sediments. To the north, the Burgalesa Platform, characterised by south-directed thrusts rooted into the Triassic salts, was southwards displaced until the San Pedro act as a backstop forcing the Burgalesa Platform to extrude laterally towards the southeast overriding the Ebro Foreland Basin. The paleomagnetic study focussed on determine the possible vertical axis rotations along the Pyrenees, denotes that the amount of sites that some areas have is not present in the Burgalesa Platform and vicinities that lack of sites. For that reason, another objective of this Thesis is to provide new paleomagnetic sites for the study area. The study carried out is made up of twenty-six new sites widely distributed and that allow to characterise the vertical axis rotation pattern. The results obtained reveals that i) the magnetisation of the Cenozoic sediments is prior to deformation and that ii) clockwise and counter clockwise rotation occurred in the Burgalesa Platform and adjacent areas. The evolution models previously proposed for the Burgalesa Platform were based either from surface geology of from geophysical data thus fitting some of the constraints. Because any model integrated all the observations and available data, a new model explaining the Burgalesa Platform evolution has been proposed. This model is characterised by presenting thick-skinned deformation at the western sector whereas, thin-skinned at the eastern one when the Triassic salt layer thickens point in which the thrusts are detach the cover from the basement. The results of this Thesis demonstrate that the pre-configuration of the extensional basin and its vicinities have a strong implication in the development and evolution of the Burgalesa Platform during the Pyrenean Orogeny. Finally, during this evolution, clockwise and counter clockwise rotations occurred in the different sectors as shown by the paleomagnetic studies carried out during this Thesis.