Evidence of hydrothermal fluid flow in distal rifted margins: the case study of Err nappe

In the last decades the increasing availability of high resolution seismic data and deep drill hole data have allowed to understand with greater detail the architecture of present-day passive rifted margins and to propose new models about their tectono-stratigraphic evolution. These models show how a multiphase evolution of rift systems led to a complex architecture: a proximal 30 km-thick crust separated, by a necking zone, from a thinned continental crust (<10km), followed by a wide transition zone between the continental and oceanic crusts where mantle exhumation occurs. The thermal evolution of distal margins is, however, very poorly constrained, although the presence of hydrothermal systems seems to play a key role in determining the heat fluxes. A deeper and more detailed study of such systems is thus fundamental to reconstruct the thermal and diagenetic evolution of the sedimentary successions lying above these margins. Therefore, the main aim of this study is the characterization of the hydrothermal systems in the Adriatic paleomargin and their evolution relatively to continental extension in order to figure out the relations among the hydrothermal products due to fluid flow, the stratigraphy and the main tectonic structures. The studied area is located in the southeastern part of Switzerland, where basement rocks and overlying sedimentary successions are spectacularly exposed. Since this domain escaped strong Alpine metamorphic overprint, sedimentary and structural features related to the Jurassic evolution of the margin are very well preserved. A detailed sampling was carried out on carbonate rocks cropping out along the entire margin, from the most distal to the proximal part, in order to get a complete dataset in different tectono-stratigraphic settings. Two sites in the Err nappe have been studied more in detail: Piz Val Lunga and Fuorcla Cotschna. We are focusing, in particular, on the interaction between fluids and pre- and syn-rift sediments that are, respectively, Triassic dolostones preserved as extensional allochthons, and basin-filling sedimentary breccias that reworked both the footwall and hanging wall of the detachment fault. Understanding the degree of rock-fluid interplay is crucial to develop models about the thermal history of the area. Within this context, the main goal is then to discern among different types of hydrothermal products such as cements, veins and replacement minerals that could testify any rock-fluid interaction. Field evidence, petrography and cathodoluminescence investigations reveal a strong heterogeneity in the composition of the fluids that passed along the detachment and through the sediments. Samples from the Err nappe contain saddle dolomite, sparry calcite, quartz, chalcedony and albite, suggesting a complex frame in which dolomitization, calcitization and silicification characterized the long evolution of the margin. Crosscutting relationships provide evidence of how these products refer to different steps of the evolution of the margin, from very early stages during the onset of extension until the Alpine orogeny. O and C isotopic analysis were performed on pre-rift Triassic carbonate and pre- to syn-rift calcite and dolomite veins and cements. The resulting slightly positive C and strong negative O values point to a hydrothermal origin of the fluids flowing through the sediments. These data can be easily compared with those coming from present-day Iberia continental margin (ODP Leg 103). Furthermore, we will illustrate new geochemical (main and trace elements, REE), isotopic (Sr) and fluid inclusions data that allow to better constrain the hydrothermal fluid flow pattern in the complex evolution of the Adriatic margin.