Refining the picture of fracture development in folded carbonate reservoirs : Insights from U-Pb geochronology of syn-kinematic calcite mineralizations in the Provence fold-and-thrust belt, France

LA-ICP-MS U–Pb geochronology of syn-kinematic calcite in faults and fractures provides a direct means of dating brittle deformation. We present U–Pb calcite geochronological data from deformation features across a range of scales—stylolites, veins, minor faults, and major thrusts—within the Provence fold-and-thrust belt. Whether in thrust-related cover folds (Mirabeau and Bimont) or in the more complex Nerthe Massif, the results illustrate how calcite geochronology can enhance or challenge our understanding of fracture pattern development in reservoirs.

Calcite geochronology validates the sequence of fracture development during layer-parallel shortening, fold growth, and late-stage fold tightening, previously inferred from structural orientations and cross-cutting relationships, regardless of fold type. Dating of calcite jogs formed at the tips of sedimentary or tectonic stylolites further constrains the timing of deformation stages. Geochronology also helps differentiate local from regional deformation by defining a more precise chronological framework where other geological markers are absent.

Across all investigated structures, deformation features show remarkable age consistency and slight overlaps between stages, providing a continuous and detailed record of fracture development. The age overlaps may indicate that deformation lasted less than the analytical uncertainty or that fracturing was more continuous throughout folding and thrusting than previously assumed. The consistency of ages across structural scales suggests either coeval deformation or events too close in time to be distinguished by U–Pb dating. This observation supports the syn-kinematic nature of calcite mineralization in small tectonic veins, even where infills display blocky, non-stretched textures. While precipitation may lag slightly behind fracture opening in individual veins, at the vein-set scale, both processes remain coeval within dating resolution. This broadens the applicability of U–Pb calcite geochronology to diverse mesoscale structures.

The dataset reveals the multi-phase development of similarly oriented fractures, which possibly initiated during burial and were reopened or densified during subsequent tectonic episodes. Geochronology provides a robust way to test whether fractures grouped by orientation, deformation mode, and relative chronology (‘fracture sets’), as well as classical associations of veins, stylolites, and conjugate faults defined by kinematic and mechanical compatibility, truly reflect the same deformation event. Veins with up to 60° strike variation sometimes yield indistinguishable ages (within a few Myr), challenging conventional definitions of fracture sets and implying local stress variations. This questions the presumed stability of the stress field in tectonic reconstructions.

Regionally, clusters of U–Pb calcite ages, if not reflecting sampling bias, hint towards variations in fluid activity, redox conditions, and/or uranium mobility, or distinct pulses of brittle rock damage and fluid flow. The latter interpretation suggests two deformation phases—late Cretaceous (81–67 Ma) and late Paleocene–Eocene (59–34 Ma)—separated by a Paleocene tectonic quiescence, matching the two already recognized Pyrenean shortening phases and indicating a likely, though not systematic, link between regional tectonic activity, brittle rock damage, fluid circulation, and calcite mineralization.

These examples demonstrate how U–Pb calcite geochronology not only constrains the timing and duration of brittle deformation but also helps reassess models of fracture development and fold–fracture relationships.