LAUSR

Abstract Clumped isotopes thermometry (Δ47) and U-Pb dating of carbonates have been used to elucidate the temperature-time conditions of 10 tectonic calcite veins in the hanging wall of the South Pyrenean Frontal Thrust (Spain), the youngest thrust unit in the south-central Pyrenees. The Δ47 values indicate precipitation temperatures below ∼90 °C, in agreement with fluid inclusion analyses. 6 veins have been successfully dated, giving ages between 61 and 14.5 Ma with all but one value below 24 Ma. On the basis of our new results, we propose that several the studied veins record the end of Pyrenean tectonics in the area during the Burdigalian, possibly up to the Serravalian, younger than estimates based on magnetostratigraphic data from continental syn-tectonic deposits. Calculated δ 18 O values of the precipitating fluid are either very positive, suggesting a local source that has interacted with the host rock, or negative, suggesting the downward incursion of meteoric waters. For undated or pre-burial veins, the Δ47 values derived from temperature history reordering model (THRM) show that the measured Δ47 values may have been significantly altered during burial, preventing their use as a reliable thermal or paleohydrological marker. Although applied to a limited number of samples, these results highlight the great potential of the U-Pb/Δ47 approach to decipher the history of vein or cement formation and fluid flow in complex tectonic zones, provided that (i) the thermal evolution is known and (ii) the extent of clumped isotope reordering is systematically modelled.