The link between deep groundwater flow and serpentinization-sourced H2 production in rift inversion orogens: the example of the Engadine valley (SE Switzerland)

The Engadine valley, located in the Grischun area in SE Switzerland, presents multiple mineralized springs distributed along the Engadine fault. Hydrogen (H2) concentration measured along the Engadine fault can reach up to 1900 ppm, indicating the presence of both, a deep groundwater flow system and a deep-seated 'kitchen'. These observations suggest that the Engadine fault may control the regional hydrodynamics and likely also the hydrogen production along the Engadine valley. A key factor to identify and understand the location of the H2 kitchen, fluid pathways and related water in- and H2 out-flow is the understanding of the nappe stack in the Grischun area and its relation to the Engadine fault. The latter, represents a major SW-NE striking >100km long structure that resulted from post-collisional oblique strike-slip movements during Oligocene-Miocene time. It transects the Late Cretaceous Austroalpine nappe stack, floored by the Pennine, ultramafic rocks bearing ophiolites, inherited from the closure of the Alpine Tethys proto-oceanic domain. Thus, a key question is whether there is a hydrodynamic link between the ultramafic source rocks flooring the rift-inversion nappe stack, the Engadine fault, acting as a possible conduit for deep water circulation, and the occurrence of springs and H2 anomalies in the soil gas. To answer to this question, we constructed a numerical hydrodynamic model of the Engadine and surrounding area, including the Engadine fault. This model allows us to carry out regional-scale simulations to investigate the interplay between topography and a deep, permeable conduit (e.g. Engadine fault) and its control on hydrothermal circulation. The model couples groundwater flow, heat transport and solute transport, and will be calibrated with surface observations (location of springs and chemical anomalies in water and soil gas). First results suggest that fluid upwelling occurs SW of St.Moritz and NE of Scuol along the Engadine valley, whereas the fault-segment between St.Moritz and Scuol corresponds to a region of meteoric recharge. This SW-NE distribution of deep upwelling correlates well with first geochemical field measurements. Future work will include chemical fluid-rock interaction to fully understand the hydro-chemical conditions of H2 formation and H2 pathways to the surface along the Engadine valley. Ultimately, this well-constrained, regional scale model, will serve as an exploration tool, allowing us to quantitatively evaluate the potential for energy-related exploitation (H2 and/or geothermal).