Abstract
The Austroalpine nappes of Switzerland represent an exhumed and tectonically imbricated segment of the passive continental margin of the Jurassic Tethys. Within one of these nappes (Err nappe), part of an upper-crustal, extensional detachment is preserved, indicating that the thinning of the crust was achieved by non-uniform extension. The Mesozoic age of the detachment is shown by comparison between its associated cataclasites and identical cataclasites that are found as redeposited components in Middle(?) Jurassic sedimentary breccias. This low-angle detachment within the basement was kinematically linked to synsedimentary high-angle normal faults at the surface.
The area of the Err nappe belongs, in terms of Jurassic paleogeography, to the most distal part of the continental margin, where both low- and high-angle normal faults dipped oceanward (that is, west to northwest in present-day coordinates). In the more proximal part of the margin, however, the high-angle normal faults dipped eastward toward the continent. Ammonite stratigraphy within the sediment prisms adjacent to the faults gives evidence for an Early Jurassic age of the faulting in the proximal part of the margin, whereas in the distal part, faulting occurred during latest Early to Middle Jurassic time. We therefore propose that the Jurassic extension of the crust, which finally led to the opening of the Piemont-Ligurian ocean, was achieved by two fault systems, which differ in geometry, fault orientation, and age. These fault systems were composed of a basal low-angle detachment with high-angle normal faults above.
The orientation of the older, eastward-dipping detachment was prone for reactivation during early Alpine crustal shortening. The present-day thrust contact between the two major tectonic units of the area, the Lower Austroalpine and the Central Austroalpine nappe complex, therefore might correspond to an eastward-dipping, Jurassic low-angle normal fault.
