Fault-controlled groundwater recharge from Alpine units into Upper Jurassic Limestone of the North Alpine Foreland Basin (SE Germany)

Fluid temperatures in the Cenozoic basin fill of the North Alpine Foreland Basin (SE Germany) are locally significantly lower compared to adjacent areas of the basin. In the greater Rosenheim area, formation temperatures at a depth of 4000 mbs range ca. 80 K lower than expected with respect to a typical regional geothermal gradient of 28-30 K/km. Possible explanations for this so-called Wasserburg Trough anomaly include thermal blanketing by rapid deposition of cold sediments, effects of convective and advective heat transfer in Cenozoic sediments, long-term effects of glacial thermal overprint, increased gravity-driven recharge due to karstification in the underlying Upper Jurassic Limestone, and heat transfer towards the Tauern Window due to a thermal chimney effect. Recent studies on formation fluid ages in the Upper Jurassic Limestone, a prolific, hydrostatically pressured geothermal aquifer, show comparatively young fluid ages of <20 ka which points at local freshwater infiltration at greater depth. Freshwater influx may reduce heat flow, act as a conductive heat barrier and favour karstification. However, fluid overpressure in shales of the Cenozoic overburden does not allow for direct vertical fluid infiltration across the stratigraphic column.

We propose a tectonic control mechanism responsible for freshwater infiltration with the Bavarian Inntal Fault Zone, a normal fault system that was formed during indentation of the Southern Alps in Oligo-Miocene times, acting as a conduit fault. This fault zone is indicated by a steepening of W-E striking fold axes towards the Bavarian Inntal, and the existence of several, valley-parallel sets of NNW-SSE striking normal faults proving WSW-ENE directed extension. Total vertical displacement inferred from cross-sections and field data yield at ≥250 m which is probably sufficient to ensure hydraulic contact between sedimentary strata of the Alpine nappes and underlying Upper Jurassic Limestone in the deeper subsurface. Thereby, freshwater from the Alps could bypass the overpressure zone in the Bavarian Inntal and infiltrate into the Upper Jurassic Limestone aquifer of the foreland basin.