Hydrothermal fluid flow associated to the extensional evolution of the Adriatic rifted margin: Insights from the pre‐ to post‐rift sedimentary sequence (SE Switzerland, N ITALY)

Abstract This paper investigates the hydrothermal fluid circulation that was linked to the extensional evolution of the Adriatic rifted margin during the Jurassic opening of the Alpine Tethys. Remnants of this rifted margin are spectacularly preserved in SE Switzerland and N Italy. Five study areas were chosen ranging from the former proximal to the most distal part of the margin. We demonstrate an intimate link between Jurassic extensional tectonics and fluid activity affecting the pre‐ to early post‐rift sedimentary sequences. Nature, composition and origin of fluids are constrained by a multidisciplinary approach based on field observations and including geochemical (O‐C, Sr, He isotopes, U‐Pb datings, fluid inclusion microthermometry) and petrological methods. Several fluid‐related diagenetic products and processes such as dolomitization, veining, hydraulic brecciation, and silicification can be recognized. It appears that different paleogeographic settings and different stratigraphic levels document distinct phases of fluid activity. The fluids reached temperatures of up to 150°C near paleo‐seafloor. They were enriched in 18O, had high 87Sr/86Sr and low 3He/4He ratios, documenting a strong interaction between seawater and a granitic basement. Many lines of evidence point to the occurrence of over‐pressured fluids and long‐lasting fluid circulation due to fault‐valve mechanisms and high thermal gradients. Two main stages with different fluid chemistry can be recognized: (1) a carbonate‐rich stage that developed during the stretching phases and was linked to high‐angle normal faulting, and (2) a silica‐rich stage occurring during late rift exhumation of crustal and mantle rocks in the distal domains in the presence of detachment faults and high thermal gradients. This paper provides, for the first time, a large and robust characterization of fluid–rock interactions occurring during rifting along an almost complete section across a magma‐poor rifted margin.