H. P. Laubscher, 1983. "Detachment, shear, and compression in the central Alps", Contributions to the Tectonics and Geophysics of Mountain Chains, Robert D. Hatcher, Jr., Harold Williams, Isidore Zietz
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Alpine cover nappes are usually characterized by both folding and stretching, suggesting simple shear between two comparatively rigid plates: subducted basement below and an “orogenic lid” above with its frontal wedges. The lid consists of thrust masses piled up in earlier stages which, being cooler and stronger due to previous deformation, constitute a comparatively rigid body. Basement nappes are formed in a variety of modes. They may emerge as steeply dipping lobes into the shear zone and there flatten out; in this case, they develop from highly “viscous” masses at elevated temperature in a sort of distributed ramp under oblique compression. In other cases, they are sheared off from their crustal underpinnings and move as rigid bodies: a famous example is the Austroalpine Silvretta nappe. At its base, widespread mylonites are in some places interrupted by pseudotachylites. These are interpreted as due to stress concentration followed by explosive failure in patches that resisted creep. In the Early Tertiary, nappes generally moved north with respect to Europe, but at a late stage there was considerable backflow toward Africa. This may be due to arrival of the North Penninic subplate boundary in the subduction zone.