Numerical modelling is used to understand the regional scale flow dynamics of the fault-hosted orogenic geothermal system at the Grimsel Mountain Pass in the Swiss Alps. The model is calibrated against observations from thermal springs discharging in a tunnel some 250 m underneath Grimsel Pass to derive estimates for the bulk permeability of the fault. Simulations confirm that without the fault as a hydraulic conductor the thermal springs would not exist. Regional topography alone drives meteoric water in a single pass through the fault plane where it penetrates to depths exceeding 10 km and acquires temperatures in excess of 250 ºC. Thermal constraints from the thermal springs at Grimsel Pass suggest bulk fault permeabilities in the range of 1.75e-15 m² – 5.2e-15 m². Reported residence times of >30000 years and 7 years for the deep geothermal and shallow groundwater components in the thermal spring water, respectively, suggest fault permeabilities of around 2.5e-15 m². This range of permeabilities estimated from observational constraints is fully consistent with a subcritical single-pass flow system in the fault plane. We show that the long residence time of the deep geothermal water is likely a consequence of low recharge rates during the last glaciation event in the Swiss Alps which started some 30 000 years ago. Deep groundwater discharging at Grimsel Pass today thus infiltrated the Grimsel fault prior to the last glaciation event.