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

Abstract

tThe Puig-reig anticline, located in the South Pyrenean fold and thrust belt, developed during the Alpinecompression, which affected the upper Eocene-lower Oligocene sediments of the Solsona and BergaFormations. In this study, we highlight the controls on formation of joints and reverse, strike-slip andnormal faults developed in the crest domain of the Puig-reig anticline as well as the relationships betweenfluids and these fractures. We integrated structural, petrographic and geochemical studies, using for thefirst time in the SE Pyrenees the clumped isotopes thermometry to obtain reliable temperatures of calciteprecipitation.Structural and microstructural analysis demonstrate that at outcrop scale fracturing was controlledby rigidity contrasts between layers, diagenesis and structural position within the anticline, whereasgrain size, cementation and porosity controlled deformation at the microscopic scale. Petrographic andgeochemical studies of calcite precipitated in host rock porosity and fault planes reveal the presence oftwo migrating fluids, which represents two different stages of evolution of the Puig-reig anticline. Duringthe layer-parallel shortening, hydrothermal fluids with temperatures between 92 and 130◦C circulatedthrough the main thrusts to the permeable host rocks, reverse and most of strike-slip faults precipitatingas cement Cc1. During the fold growth, meteoric waters circulated downwards through normal and somestrike-slip faults and mixed at depth with the previous hydrothermal fluid, precipitating as cement Cc2at temperatures between 77 and 93◦C.Integration of the results from the Puig-reig anticline in this work and the El Guix anticline indicatesthat hydrothermal fluids did not reach the El Guix anticline, in which only meteoric and evolved meteoricwaters circulated along the fold.