Paleotectonic and paleogeographic signification of melanges: examples from the Tethyan realm
Résumé
There are two different but complementary ways to study accretion-related melanges, originating in the contexts of convergent margin (subduction/obduction): i) a local and mechanical approach, where the mélange is seen as a structural object; the aim is then to get information on the large-scale melange-forming processes, by unravelling its internal structure and complex organisation, and by explaining the timing of the different deformation phases; ii) a regional and historical approach, where the melange is considered as a witness of the history of the regional geology; the aim is to get as much information as possible on the various lithologies found in the melange, in order to reconstruct the evolution through space and time of the ocean(s) and adjacent margins contemporaneous of its formation. We want to put the emphasis on this second approach, and on the role of fossil accretion-related melanges from a paleotectonic and paleogeographic perspective. It is then necessary to give them a geodynamic signification, and to go further than the classic descriptive definition. In summary, one can distinguish between three ideal types of mélanges, regarding the origin of the material and their genesis: i) melanges related to the lower plate, reworking lithologies from the subducting oceanic lithosphere into the accretionary prism; ii) mélanges related to the upper plate, reworking lithologies both from the arc and the prism, called here fore-arc type melanges; iii) debris-flow mélanges deposited on the lower plate, and reworking ophiolitic fragments in front of an advancing obducting ophiolite. These three types of melange may be seen as ideal end-members of a simple ternary classification of melanges, based on their geodynamic setting. Examples of melanges taken from the Tethyan realm (Greece, Turkey, etc...) support this view, and their field recognition appears