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Evidence of hydrothermal activity is reported for the Mesozoic pre‐ and syn‐rift successions of the western Adriatic palaeomargin of the Alpine Tethys, preserved in the Western Southalpine Domain (NW Italy). The products of hydrothermal processes are represented by vein and breccia cements, as well as dolomitization and silicification of the host rocks. In the eastern part of the study area, interpreted as part of the necking zone of the continental margin, Middle Triassic dolostones and Lower Jurassic sediments are crossed by veins and hydrofracturing breccias cemented by saddle dolomite. The precipitation of dolomite cements occurred within the stratigraphic succession close to the sediment–water interface. Despite the shallow burial depth, fluid inclusion microthermometry and clumped isotopes show that hydrothermal fluids were relatively hot (80–150°C). In the western part of the study area, interpreted as part of the hyperextended distal zone, a polyphase history of host‐rock fracturing is recorded, with at least two generations of veins cemented by calcite, dolomite and quartz. Vein opening and cementation occurred at shallow burial depth around the time of deposition of the syn‐rift clastic succession. Fluid inclusion microthermometry on both quartz and dolomite cements indicates a fluid temperature of 90–130°C, again pointing to hydrothermal fluids. Both in Fenera‐Sostegno and Montalto Dora areas, O, C and Sr isotope values, coupled with fluid inclusion and clumped isotope data, indicate that hydrothermal fluids derived from seawater interacted with crustal rocks during hydrothermal circulation. Stratigraphic and petrographic evidence, and U–Pb dating of dolomitized clasts within syn‐rift sediments, document that hydrothermal fluids circulated through sediments from the latest Triassic to the Toarcian, corresponding to the entire syn‐rift evolution of the western portion of the Adriatic palaeomargin. The documented hydrothermal processes are temporally correlated with regional‐scale thermal events that took place in the same time interval at deeper crustal levels. [ABSTRACT FROM AUTHOR]

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). [ABSTRACT FROM AUTHOR]