Crestal graben fluid evolution during growth of the Puig-reig anticline (South Pyrenean fold and thrust belt)

The Puig-reig anticline, located in the South Pyrenean fold and thrust belt, developed during the Alpine compression, which affected the upper Eocene-lower Oligocene sediments of the Solsona and Berga Formations. In this study, we highlight the controls on formation of joints and reverse, strike-slip and normal faults developed in the crest domain of the Puig-reig anticline as well as the relationships between fluids and these fractures. We integrated structural, petrographic and geochemical studies, using for the first time in the SE Pyrenees the clumped isotopes thermometry to obtain reliable temperatures of calcite precipitation. Structural and microstructural analysis demonstrate that at outcrop scale fracturing was controlled by rigidity contrasts between layers, diagenesis and structural position within the anticline, whereas grain size, cementation and porosity controlled deformation at the microscopic scale. Petrographic and geochemical studies of calcite precipitated in host rock porosity and fault planes reveal the presence of two migrating fluids, which represents two different stages of evolution of the Puig-reig anticline. During the layer-parallel shortening, hydrothermal fluids with temperatures between 92 and 130 degrees C circulated through the main thrusts to the permeable host rocks, reverse and most of strike-slip faults precipitating as cement Cc1. During the fold growth, meteoric waters circulated downwards through normal and some strike-slip faults and mixed at depth with the previous hydrothermal fluid, precipitating as cement Cc2 at temperatures between 77 and 93 degrees C. Integration of the results from the Puig-reig anticline in this work and the El Guix anticline indicates that hydrothermal fluids did not reach the El Guix anticline, in which only meteoric and evolved meteoric waters circulated along the fold. (C) 2016 Elsevier Ltd. All rights reserved.