Late-orogenic extension ceases with waning plate convergence:The case of the Simplon normal fault (Swiss Alps)

The Simplon normal fault in the Western Alps caused tens of kilometers of orogen-parallel extension during convergence of the European and Adriatic plates, but the slip rate of the fault and the time when normal faulting ended are still debated. Here, we constrain the slip history of the Simplon fault with low-T thermochronology and thermo-kinematic modeling (Wolff et al. 2024). Closely spaced samples from an elevation profile in the center of the fault yield zircon (U-Th)/He ages (ZHe) that are nearly invariant over an altitude of 1.4 km and cluster around ~6 Ma. In contrast, apatite (U-Th)/He ages (AHe) increase with altitude from 3.4±0.3 to 4.6±0.7 Ma, while the AFT ages range from 4.4±0.7 to 5.8±1.5 Ma. In addition, recently published ⁴⁰Ar/³⁹Ar ages constrain that our samples moved through the brittle-ductile transition (i.e., ~300°C) at 8–10 Ma. Our thermo-kinematic inverse modeling shows that these age data can be explained by a single phase of normal faulting, which lasted from 19.8±1.8 to 5.3±0.3 Ma and caused 45±10 km of extension. The slip rate of the 30°-dipping model fault is 3.5±0.3 km/Myr and equivalent to an exhumation rate of ~1.8 km/Myr. Our modeling reveals that the altitude-dependent difference between ZHe and AHe ages reflects the thermal relaxation after faulting stopped at ~5.3 Ma. Since then, exhumation by erosion continued at a rate of ~0.5 km/Myr. Remarkably, the end of slip on the Simplon fault coincides with the cessation of reverse faulting at 6±2 Ma in the external crystalline massifs of the Alps (Aar, Mont Blanc, Aiguilles Rouges) and with a decrease in strain rate by one order of magnitude at 5-4 Ma in the Swiss molasse basin and the Jura mountains. This temporal coincidence suggests that normal faulting in the internal part of the Alps ceased when plate convergence waned and the under-thrusting of European continental lithosphere beneath the Adriatic plate came to an end.

References

Wolff, R., Hölzer, K., Hetzel, R., Dunkl, I., Anczkiewicz, A.A., 2024. Late-orogenic extension ceases with waning plate convergence: The case of the Simplon normal fault (Swiss Alps). Journal of Structural Geology 179, 105049. doi:10.1016/j.jsg.2024.105049.