Reactivación de fallas en Picos de Europa (Cordillera Cantábrica) deducida a partir del análisis estructural y la caracterización de paleofluidos y mineralizaciones asociadas

Abstract

The Picos de Europa region is mainly conformed by marine Carboniferous limestones. It has been subjected to tectonic deformation in various episodes over the last 320 Ma. During the Variscan Orogeny, its deformation was characterised by the development of a thrust imbricate system. Subsequent deformation was associated with various tectonic contexts and did not influence its structure significantly. This thesis explores the activity of high-angle WNW–ESE trending faults formed in this period, through the combination of structural analysis and the geochemical characterisation of associated minerals (calcite, dolomite, and sulphide ores).

The study has focussed on the San Carlos and Liordes fault systems, for which kinematic data have been measured that record several deformational events. From the spatial relation between successive calcite generations and deformation structures, each fault activity period has been linked to fluids with particular physical and chemical properties, which have been characterised through clumped isotope thermometry in carbonates, and stable isotope ratios in minerals and fluid inclusions. In addition, model ages for the sulphide ores associated with the faults have been derived in order to constrain the age of fault activity.

Faults of the San Carlos system display a dominant WNW–ESE trend and dip steeply to the NNE. They display associated secondary dolostones of Permian age, which obliterated the structures related to their original activity. These faults register a normal displacement which, according to the model ages obtained from the sulphide ores, was related to the crustal extension during the Upper Jurassic to the Lower Cretaceous described in surrounding areas such as the Basque-Cantabrian and Asturian basins. During the subsequent development of the Cantabrian Mountains during the Cenozoic, within the framework of the Alpine Orogeny, the fault system was reactivated accommodating a dextral displacement with a minor reverse component (slip vector plunging ~16°).

Being aware of the relation that exists between deformation stages and the different mineral generations in the San Carlos fault system provides valuable insights in order to study other poorly outcropping faults. That is the case of the Liordes fault system, for which a reactivation history very similar to that of San Carlos has been deduced, characterised by a normal displacement during the Upper Jurassic–Lower Cretaceous extension, and a subsequent reactivation during the Alpine Orogeny, with a dextral movement with a reverse component which is more prominent than in the case of the San Carlos system (slip vector plunging 27°–62°).

Upper Jurassic to Lower Cretaceous deformation was associated with the circulation of hydrothermal fluids, which were likely a mixture of seawater and insterstitial fluids from the Pisuerga-Carrión Region. Most of the clumped isotope temperature estimates for the calcite precipitated during this stage are comprised in the range 150–250 °C. During the Cenozoic deformation, calcite was precipitated from meteoric fluids. The clumped isotope temperature estimates for these precipitates span the entire interval between 16 and 106 °C. The clumped isotope thermometer may have been subjected to variable degrees of resetting, being more marked in the oldest carbonates sampled in the areas that experienced a larger burial maximum depth. The original precipitation temperature of the Mesozoic calcite would be higher than the obtained temperature in most cases, whereas the initial temperature of the Cenozoic calcite would be either equal or slightly lower than the calibrated temperature. However, the obtained temperature values still reflect to a large extent the original temperature contrast between the Mesozoic and Cenozoic precipitates.