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Abstract

The European North Alpine Foreland Basin is a Tertiary wedge-shaped foreland basin at the northern front of the European Alps. The Molasse Sediments are underlain by Mesozoic sedimentary successions, which include the Upper Jurassic aquifer (Malm), a major target for geothermal energy production. The Molasse Basin has been used for geothermal energy production since decades due to its specific thermal configuration. The thermal field of the basin shows increasing temperatures from north to south and pronounced positive and negative thermal anomalies at depths of exploration interest. Between these anomalies, temperature differences of more than 40 K may occur over a small horizontal distance of just a few kilometres, a phenomenon which could so far not be explained based on the present-day knowledge. Though, a high amount of data about the structure as well as the distribution of temperatures and thermal properties in the European Molasse Basin exists, knowing the temperature distribution is not enough to reduce the exploration risk in the European Molasse Basin. Rather, an understanding of the heat driving mechanisms and the origin of the pronounced temperature anomalies is of high importance to reduce the uncertainty in predicting the extraction temperature and discharge of geothermal power plants. To explain the origin of the pronounced thermal anomalies in the German Molasse Basin, first a lithospheric-scale 3D structural model was constructed based on freely available depth and thickness information which includes the Molasse Basin as well as the South German Scarpland and some parts of the Alps. Areas not covered with measured data were constrained with isostatic calculations and 3D gravity modelling. In a second step, the present-day 3D steady-state conductive thermal field of the German Molasse Basin was calculated based on the gravity constrained lithospheric-scale 3D structural model. The predicted temperature distribution indicates that the thermal field is controlled by conductive heat transport in the lithospheric mantle and the crystalline crust. Shallower parts of the thermal field are strongly controlled by a thermal interdependence between the Alpine area and the basin itself and by the underlying crystalline crust related to their contrasting thermal properties. Furthermore, the results indicate that the distinct thermal anomalies in the German Molasse Basin are partly triggered by the structural configuration of the crust and the presence of the Tauern Body. To assess the influence of fluid flow on the shallow thermal field of the German Molasse Basin, coupled fluid flow and heat transport simulation were conducted which succeeded to reproduce the observed thermal anomalies in the German Molasse Basin. In contrast to assumptions of previous studies no permeable faults were needed to reproduce these thermal anomalies. The resulting coupled thermal field indicates that the temperature distribution is primarily controlled by conductive heat transport, but also strongly affected by basin-wide as well as local fluid flow especially at shallower depths. In particular, the results show that the positive and negative thermal anomalies are caused by a combination of conductive and advective heat transport and may be correlated to the permeability of the Molasse Sediments, to the facies controlled permeability distribution in the Upper Jurassic aquifer (Malm) and to the spatial distribution of the Cretaceous Purbeck formation.