DETACHMENTS IN HIGH-PRESSURE MOUNTAIN BELTS, TETHYAN EXAMPLES

Using the case examples of the phyllite-quartzite nappe of Crete, the Bundnerschiefer nappe of the Engadine window in the Alps, and the infr aophiolitic units of the Saih Hatat window in the Oman Mountains, we d escribe the tectono-metamorphic signature of large-scale extensional d etachments in high-pressure belts. In these three cases retrograde P-T paths were accompanied by the formation of large-scale extensional sh ear zones that contributed to the exhumation and extreme thinning of t he metamorphic pile. The direction of transport along later detachment s was parallel to the direction of convergence, detachments being firs t localized within or near the brittle-ductile transition zone, Below the major shear zone, or between the shear zones in Oman, the deformat ion regime was more coaxial and involved a significant component of ve rtical shortening. Metamorphic data, mostly derived from the petrology of metapelites, show that domains characterized by coaxial strain hav e nearly isothermal decompression paths (type II), while those immedia tely below the detachments were subjected to more efficient cooling du ring exhumation (type I). The detachments thus affect both the deforma tion history and the P-T evolution of the lower plate. The existence o f the two types of cooling paths is a good test of the extensional nat ure of a tectonic contact. (C) 1998 Elsevier Science B.V. All rights r eserved.