Characteristics and timing of hydrothermal fluid circulation in the fossil Pyrenean hyperextended rift system: new constraints from the Chaînons Béarnais (W Pyrenees)

The evolution of hyperextended rift systems is linked to complex tectonic processes in which fluid activity is much more important than previously thought. This study sheds light on the fluid-sediment interactions within the distalmost parts of the Mauléon-Arzacq hyperextended basin (Western Pyrenees) focusing on the post-depositional modifications of pre- to syn-hyperextension sediments due to hydrothermal fluids. Strong field- and petrography-based evidence demonstrates the presence of fluid-related products affecting the Jurassic to Cretaceous sediments exposed in the Chaînons Béarnais (easternmost Mauléon basin). These are supported by new U–Pb dating of carbonates and microthermometry of fluid inclusions showing temperatures of up to 250 °C and highly depleted δ18O and strongly enriched 87Sr/86Sr values. Two main stages of fluid activity can be defined: (i) a carbonate-rich stage leading to fabric-destructive replacement dolomitization of the pre-rift carbonates, widespread hydro-fracturation giving rise to different types of hydraulic breccias cemented by multi-phase dolomite and calcite dated at ~ 96 Ma; and (ii) a Na-SiO2-rich stage associated with authigenic albite and quartz, mainly affecting syn-rift deposits. Finally, the occurrence of dolomitic marbles and mylonites provide evidence for strong recrystallization and ductile shearing affecting the sediments during the latest stages of hyperextension at ~ 94 Ma. The Chaînons Béarnais represent a primary target to investigate fluid-rock interactions linked to extensional tectonics that can be used as an analogue to compare to other fossil rift systems (e.g. Adriatic paleo-rifted margin) or present-day magma-poor, hyperextended rifted margins (e.g. Iberia-Newfoundland).