Detachments in high-pressure mountain belts, Tethyan examples

Using the case examples of the phyllite–quartzite nappe of Crete, the Bündnerschiefer nappe of the Engadine window in the Alps, and the infraophiolitic units of the Saih Hatat window in the Oman Mountains, we describe the tectono-metamorphic signature of large-scale extensional detachments in high-pressure belts. In these three cases retrograde P–T paths were accompanied by the formation of large-scale extensional shear zones that contributed to the exhumation and extreme thinning of the metamorphic pile. The direction of transport along later detachments was parallel to the direction of convergence, detachments being first localized within or near the brittle–ductile transition zone. Below the major shear zone, or between the shear zones in Oman, the deformation regime was more coaxial and involved a significant component of vertical shortening. Metamorphic data, mostly derived from the petrology of metapelites, show that domains characterized by coaxial strain have nearly isothermal decompression paths (type II), while those immediately below the detachments were subjected to more efficient cooling during exhumation (type I). The detachments thus affect both the deformation history and the P–T evolution of the lower plate. The existence of the two types of cooling paths is a good test of the extensional nature of a tectonic contact.