DID THE WESTERN ALPS DEVELOP THROUGH AN OMAN-TYPE STAGE - THE GEOTECTONIC SETTING OF HIGH-PRESSURE METAMORPHISM IN 2 CONTRASTING TETHYAN TRANSECTS

In the Western Alps, the Penninic metamorphism was considered as the r esult of a Cretaceous obduction similar to the Oman Mountains one. The analysis of the geodynamic evolution of both these belts leads us to abandon this hypothesis.At the onset of convergence between Africa and Eurasia, the Oman-Makran transect involved a broad ocean, partly old, partly very young (<5 Ma) and with a thick crustal section. Two subdu ction zones sensu lato accommodated convergence, one being of the ande an type, the other one being intra-oceanic and shallow-dipping (detach ement type) and resulting in the obduction of the young oceanic lithos phere onto the Arabic margin. High-pressure, low-temperature metamorph ism was effective in both zones during the Late Cretaceous. In the Wes tern Alps, sedimentation on the internal crystalline massifs (ICM, i.e . European distal margin) and the adjacent oceanic lithosphere lasted at least until the Late Cretaceous, most likely the Paleocene and loca lly the Eocene (age of the allochthonous flyschs of Ligurian origin). Eclogitic metamorphism in the Sesia zone (Austro-Alpine, Apulian margi n) is Mid-Cretaceous in age. The age of eclogitic metamorphism in the ophiolites and ICM was long considered as Eoalpine (Cretaceous) on the basis of K/Ar and Ar/Ar isotopic data; it must be Paleocene to Late E ocene respectively according to recent isotopic data (Sm/Nd and ''SHRI MP'') and in agreement with the stratigraphic record. In contrast to t he Tethys ocean between Oman and Makran, the Ligurian ocean at the ons et of convergence was narrow (400 km?), old (50 Ma), and had a thin cr ust or no crust at all through tectonic denudation of the mantle. The Alpine ophiolites, devoid of high-temperature metamorphic sole, are mi nor syn-collisional slivers. The metamorphism in the continental Penni nic crust is not obduction-related bur collision-related, The P-T-t pa ths in Oman and the Penninic Alps show similarities, they are both con cave toward the pressure axis and both show rapid initial cooling. How ever, the final cooling is slower in Oman (<10 degrees C/Ma) than in t he ICM (15-20 degrees C/Ma). In conclusion, the idea of considering th e Oman obduction as a transient but necessary stage of the evolution o f collision belts must be abandoned. Such an obduction stage never occ urred during the evolution of the Western Alps.