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The Mesozoic ophiolites of the Alps: a review

S. Martin, P. Tartarotti and G. V. Dal Piaz

Abstract: 

The Alpine ophiolites are mostly concentrated in the inner and axial sectors of the Perminic nappe pile, as slices of the Jurassic Tethyan ocean (Piedmont-Ligurian and Penninic basins) which opened between the Europe and Adria (Africa) passive margins. The ophiolitic units consist of Mesozoic metasediments (caleschists-schistes lustrés-Búndnerschiefern) and mafie-ultramafic rocks, also including sfices of subcontinental mande. Metasediments with flysch affinity and huge ophiolites are predominant in the external and internal sectors of the Perminic zone, respectively. These sequences display polyphase metamorphie signatures and related deformations of Cretaceous-Tertiary age, i.c., a subduction-related eclogite (locally coesite-bearing) Io blueschist facies imprint and/or a greenschist Io amphibolite facies re-equilibration, recording the collisional thermal relaxation. Mineral and/or textural relics of pre-Alpine protoliths are locally preserved in low-strain domains, hence a comparison with equivalent sections of the oceanic fithosphere from the present-day oceans may be attempted. Despite the pervasive transposition and the absence of a convincing sheeted dyke complex, the ophiolites record place Io place a complete lithological sequenee that is typical of the world's best preserved ophiolitic complexes. Nevertheless, the ophiolitic associations frequently display anomalous stratigraphie features. Most mafie rocks are derived from tholeites with normal-MORB type affinity, which likely originated in mature ridges, even if intra plate geochemical signatures were also locally reported. A first group of supposedly syn-rift gabbros and/or subcontinental mande lherzolites may be locally associated with the occanic sequences. Contrasting models of paleogeographie restoration and kinematic evolution proposed so far for the ophiolitic units in the Alpine nappe pile are summarized in this paper. In contrast with traditional models that interpretate the Alpine ophiohtes as sutures of two or even more oceanic gaps alternating with continental blocks, other models also suggest that the Alpine ophiolites are dismembered records of a single oceanic basin, which were tectonically scattered at different structural levels of the nappe pile during the Alpine convergence.