Day 6: July 25 Tasna nappe Ocean-Continent Transition, Graubunden

Our objective was to examine both the Lower and Upper Tasna detachment faults, describe the nature of the fault rocks, and to evaluate the importance of detachment structures and their relationships to lower crustal rocks, mantle rocks, magmatic rocks and post-rift sediments.

The Tasna nappe originated from the ocean-continent transition zone between the Brianconnais terrane (SE) and the Valais ocean (NW). Two detachment faults, the Lower Tasna Detachment (LTD) and the Upper Tasna Detachment (UTD) are preserved within the Tasna nappe (Figures 1 and 2 below). The LTD juxtaposes serpentinized mantle peridotite with continental basement of the Brianconnais across a mylonitic shear zone resulting from extensional faulting in Jurassic to Early Cretaceous time. The mylonite deforms pre-rift gabbro in the footwall and basement granite in the hanging wall. The younger UTD juxtaposes serpentinized peridotite with onlapping post-rift sediments and is marked by the presence of fault breccia and cataclasite. There was virtually no syn-rift magmatism.

The following sequence of events can be interpreted from the Tasna nappe: 1) pre-rift intrusion of gabbro into future footwall of LTD, 2) LTD faulting at high temperatures, 3) UTD faulting at lower temperatures, 4) deposition of post-rift sediments
The Tasna nappe allows for the following conclusions about margin formation:

• Localized extensional faults emplace upper crust on mantle in passive margins
• Continental breakup (creation of Valais ocean) accommodated mainly by extensional shearing