Impact of meteoric fluid flow on the thermal evolution of Alpine exhumation : insights from the Gotthard Base Tunnel, Switzerland
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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 58 km tunnel trends roughly N-S and provides an orogen perpendicular cross section through the backbone of the Swiss Alps - the Aar and Gotthard massifs and parts of the Lepontine Dome. In the tunnel, the measured rock temperatures increase from 11°C at the north entrance to a maximum of 42°C in the tunnel where overburden exceeds 2300 meters. The observed temperature profile shows negative anomalies underneath major valleys (e.g., Upper Rhine) and directly mimics the topography, with high temperatures under peaks. Not correlated with topography, a marked 15°C negative temperature anomaly exists 35 km south of the Erstfeld portal centered around the Piora Zone syncline and related aquifer. The Piora Zone is a syncline comprised of a deformed and kartsified, porous Triassic dolomite that is infolded into the gneissic basement of the Gotthard massif. This water infiltration zone forms a conduit that funnels meteoric water deep into the massif. The water flow cooled the syncline and adjacent bedrock, creating an advective thermal regime and a pronounced local negative temperature anomaly. The interplay of uplift and erosion conductive heat loss, and radiogenic heat production, along with the coupled effect of topography and meteoric fluids can influence near surface geothermal gradients and therefore, apatite (U-Th)/He cooling histories. Numerical modeling demonstrates that the Piora thermal anomaly is in dynamic equilibrium and thus older than 50 kyr. (U-Th)/He low temperature thermochronometry was employed to examine both the regional cooling history of the Gotthard massif and the age of the fluid-induced thermal anomaly. High-density sampling across the Piora syncline from tunnel and surface transects allows for a comprehensive reconstruction of the Alpine exhumation and cooling history since 12 Ma. Apatite (U-Th)/He ages from along the Gotthard base tunnel show reproducible background cooling ages of ~ 2.5 Ma that increase to ~ 5.5 Ma in the Piora Zone, recording dramatic isotherm depression due to meteoric fluid infiltration. Zircon (U-Th)/He ages from the tunnel and surface (~8.0-10.0 Ma) show rapid cooling related to exhumation of the Alpine External Massifs. In addition, 1-D and 2-D thermal modeling was used to determine the topology of the AHe partial retention zone and its deflection due meteoric fluid flow consistent with the (U-Th)/He data. Inverse thermal modeling quantifies both the Alpine exhumation and the thermal history of the Piora Zone. The results show that the Piora anomaly is at least 5.5 Ma old, but cannot be older than ~8 Ma. These results point to late Miocene onset of fluid infiltration in the Piora Zone that is likely driven by the exhumation and uplift of the External Massifs and response to increase in relief and regional hydrologic head, but predates the onset of Alpine glaciation. This case study demonstrates the power of low-temperature thermochronometry in examining the interface between endogenic and exogenic thermo-tectonic processes and forcing factors.