New data on the initiation and development of the Pennine Basal Thrust (Swiss Alps)

Megathrusts are very large faults that allow one plate to overthrust another, accumulating much of the inter-plate displacement. They can generate very large earthquakes (Mw 8+) but if megathrusts are slipping too slowly or are sufficiently warm (>~300°C), they may creep without breaking. If the originally warmer parts of the megathrust, where new minerals form within strongly deformed rocks, are later exhumed to the surface, they can be sampled and dated. This provides important information on past plate movements and processes occurring within active megathrusts at inaccessible depths.

This study focuses on the Pennine Basal Thrust (PBT), an exhumed megathrust from the Swiss Alps. We investigated through a multi-disciplinary approach (combining field structural relationships with meso- and micro-structural analysis and Ar–Ar and K–Ar dating) the mineral white mica that grew at the same time as cleavage formed. Three cleavage generations were recognized both in the field and in thin sections as being associated with the PBT development. In our interpretation, the first event records an early accretion of trench deposits at the front of the Alps during the late Eocene. The second the intense shearing imposed by the >70 km relative displacement during the Oligocene, and the third the latest, colder early Miocene reactivation during exhumation. We also dated clay minerals formed during a younger brittle event that record the last movements prior to deactivation of the megathrust, allowing us to trace the end, beside its initiation and development.  

Reference: Cardello G.L., Di Vincenzo G., Giorgetti G., Zwingmann H., Mancktelow N., 2019. Initiation and development of the Pennine Basal Thrust (Swiss Alps): a structural and geochronological study of an exhumed megathrust. Journal of Structural Geology Volume 126, Pages 338–356.

Paper is available here: https://www.sciencedirect.com/science/article/pii/S0191814119300987?via%3Dihub

For further information please contact Gianfranco Di Vincenzo, CNR-IGG (gianfranco.divincenzo(at)igg.cnr(dot)it).