IMPACT OF METEORIC FLUID FLOW ON THE THERMAL EVOLUTION OF THE GOTTHARD MASSIF; INSIGHTS FROM ALPINE RAILWAY TUNNELS

The Gotthard Base Tunnel provides a unique window into the subsurface of the Swiss Alps and into the complex thermal evolution of the Gotthard Massif. The 57 km tunnel trends north-south and provides an orogen perpendicular cross section through the backbone of the Swiss Alps, the Aar and Gotthard massifs. The modeled temperature profile in the tunnel mimics topography; increasing from 11°C at the north entrance to 42°C in the middle where overburden exceeds 2300 meters. A 15°C negative temperature anomaly exists 37 km south of the Erstfeld portal at the intersection of the Piora Zone aquifer. The Piora Zone is a syncline comprised of a pervasively deformed and kartsified Triassic dolomite that overlies the gneissic basement of the Gotthard Massif. This aquifer is a conduit that funnels cold Alpine meteoric water deep into the massif. The water absorbs heat from the adjacent bedrock, creating an advective thermal regime and a local negative temperature anomaly.

Apatite (AHe) and zircon (ZHe) (U-Th)/He ages will be measured for 30 samples collected from the length of the tunnel through the Piora Zone, as well as corresponding surface transects, in order to trace the extent to which meteoric fluid has depressed the underlying isotherms throughout the exhumation history of the complex. Samples adjacent to the Piora Zone have been analyzed and show AHe ages of ~2.0 Ma, which represent cooling of the massif through 70°C, unperturbed by the Piora Zone aquifer. Ages within the Piora Zone that are significantly older will indicate that meteoric fluid has depressed isotherms substantially enough to affect the closure of apatite and zircon during exhumational cooling. The resulting data will reveal the topologic evolution of the AHe and ZHe closure temperature isotherms through times t₁ (tunnel samples) and t₂, (surface samples) shaped by the coupled effect of meteoric fluid-flow and topography.