Abstract

Fluid sources and migration pathways can dramatically change during the multiphase tectonic evolution of thrust wedges. We present a multidisciplinary study of syntectonic calcite cements in the Lower Cretaceous carbonates of the external Bornes (SE France), which underwent subaerial exposure during flexural bulging, layer-parallel shortening, folding and post-folding oblique contraction. Based on stable and radiogenic isotopes, trace elements, fluid inclusions and clumped isotope thermometry, we identified two main palaeofluid types. The older iron-rich calcite cements, associated with the pre-folding and early syn-folding stages, precipitated from an ¹⁸O-enriched fluid (δ¹⁸O = +8.6 – +11.6‰ VSMOW) at temperatures between 90 and 115°C. The younger calcite cements, associated with the late syn-folding and post-folding oblique contraction stages, have low iron content and precipitated from an ¹⁸O-depleted (δ¹⁸O = −7.5 – −4.3‰ VSMOW) meteoric fluid at temperatures between 40 and 80°C. The older fluid reached thermal equilibrium with the host rock at the maximum burial conditions, whereas the second one was probably heated in the inner sector of the belt before migrating towards the foreland. ⁸⁷Sr/⁸⁶Sr ratios exclude any contribution from basement-derived ascending fluids, pointing to the migration of meteoric fluids within the laterally continuous fractured Cretaceous units.

Supplementary material: Details of the sampling sites and a complete dataset are available at https://doi.org/10.6084/m9.figshare.c.5882212