Continental breakup by detachment faulting: field evidence and geochronological constraints (Tasna nappe, Switzerland)

The Lower Tasna Detachment (LTD) is a low-angle fault contact between serpentinized peridotite below and continental basement above. It was formed during Jurassic to Early Cretaceous rifting of a Tethyan continental margin and later captured in a thrust nappe during Tertiary plate convergence. Foliated gabbro, gabbro mylonite, and granitoid mylonite occurring along the LTD record shearing under decreasing temperatures. U–Pb dating of zircon from the gabbro mylonite yielded a Permian age, interpreted as the age of gabbro intrusion, whereas the breakup of the passive margin occurred as late as Early Cretaceous. This suggests that the gabbro belongs to a prerift, lower to middle crustal intrusion 'smeared out' along the detachment by extensional faulting. The juxtaposition of mantle and upper crust along the Lower Tasna detachment may serve as a model for several seismic reflectors observed in distal passive continental margins (e.g. S reflector of the Galicia margin).