CHARACTERIZING AND DATING SYNDEFORMATIONAL FLUIDS IN FAULTS OF THE SPANISH PYRENEES FOLD-THRUST BELT USING STABLE AND RADIOGENIC ISOTOPES

The fluid history of the southern Pyrenees fold-and-thrust belt has been explored for several decades. Research has focused on the stable isotopic composition of veins and fluid inclusions to provide information about syntectonic fluid migration through crustal faults during the evolution of the belt. Dating the presence or passage of fluid along a structure during deformation, however, is a new task. In this study, we present evidence of fluid involvement during two stages of the Pyrenean mountain belt evolution, constrained in time using ⁴⁰Ar/³⁹Ar dating of secondary, fluid-mediated clay mineral growth and stable isotope analysis for fluid sourcing. Samples from two major thrusts from the southern Pyrenean zone, Jaca and Cotiella thrust faults, were collected for H-isotopic measurements (δD_VSMOW) and illite dating. The Jaca thrust fault, a splay originating from the Gavarnie thrust, is 39.9 ± 4.9 Ma, with a corresponding δD value of -74 ± 2 ‰. The age for movement on the Cotiella thrust at the Castillo Mayor Klippe is 25.9 ± 6.5 Ma, with a corresponding δD_VSMOW value of -91 ± 2 ‰. Using published temperature estimates from these fault zones, we calculate the δD value of the mineralizing fluids as -38 ‰ and -69 ‰, respectively. These isotopic compositions are interpreted as two distinct stages of fluid evolution within this belt. The Jaca thrust fault fluid is similar to formation brines, and, given that the age corresponds to the early end of Gavarnie thrusting, this stage is representative of within-basin fluid flow where fluids are locally sourced. The Cotiella fault fluid, in contrast, is depleted in δD, characteristic of high altitude meteoric fluids. As this sample is surprisingly young for the region, it is interpreted as late-stage Eocene fault reactivation associated with ongoing uplift of the Hercynian axial zone. This late deformation resulted in opening of the fluid-rock system, allowing the infiltration of meteoric waters. As demonstrated in this study, paired stable and radiogenic isotopic analysis of secondary clay minerals constrains the source(s) and timing of fluids in evolving fault systems.