Extensional metamorphic domes in mountains belts, syn-orogenic and post-orogenic extension.

Exhumation of metamorphic rocks and extension are often intimately related. The role of extension in the removal of the overburden has been proven in numerous examples, even though compression and erosion play important parts . We recall a number of observations on Tethyan examples in order to focus the debate on the respective roles of erosion and extension in the formatio n of syn- and post-orogenic extensional metamorphic domes. The occurrence o f pressure gaps across major contacts, the distribution of retrograde P-T p aths of types I and II, the thinning of the nappe pile during exhumation an d the frequent association with sedimentary basins make unescapable the cal l to a significant extension. Syn-orogenic exhumation is in general more ef ficient than post-orogenic exhumation. The finite geometry is similar in th e syn- and post-orogenic cases bur. the P-T evolution is different, colder in the synorogenic case. Distinguishing between syn- and post-orogenic exte nsion is important to unravel the tectonic history of a region because the two processes can coexist along a single cross-section and can happen seque ntially. (C) 2000 Academie des sciences / Editions scientifiques et medical es Elsevier SAS.