SERPENTINITE MELANGES IN THE WESTERN ALPS METAOPHIOLITES AND THEIR SIGNIFICANCE IN DETECTING THE PRIMARY PHYSIOGRAPHY OF OCEANIC MANTLE-SEDIMENT INTERFACE

Several orogenic belts around the world as the Alpine-Hymalayan, the circum-Mediterranean, and the Appalachians contain melanges that are key records of the tectono-stratigraphic evolution of orogens.

The Alpine metaophiolites constitute the sutured Jurassic Tethys Ocean interposed between the European and African plates. Here, serpentinites are often directly covered by metasediments. In the Mount Avic serpentinite massif (Italian Western Alps), interpreted as a fossil oceanic core complex, the serpentinite-sediment contact is marked by a kilometer-scale and few meters-thick serpentinite melange. Upon massive to veined and brecciated serpentinite, the melange starts with white marble and carbonatic calcschist alternating with decimeters-thick horizons of serpentinite metabreccia and metagraywacke, including isolated metric blocks of serpentinite. The sequence continues with rounded to irregularly-shaped blocks of serpentinite, cm-to several dm in size, randomly distributed within a matrix of foliated impure marbles and calcschist.

The serpentinite melange has been severely overprinted by Alpine tectonic deformation and metamorphic recrystallization through subduction-related stretching and boudinage and collision-related folding. Nevertheless, its internal fabric still retains records of a block-in-matrix structure well consistent with mass-transport processes related to an active tectonic setting in which mantle rocks were progressively and continuously exhumed by faulting. The products of mass-transport processes and faulting are unconformably sealed by flysch-type calcschists embedding cm-sized clasts of actinolite/tremolite-schists.

This scenario has profound implications in the physiography of the Tethys Ocean. A correct reconstruction of products and processes occurred at the mantle-sediment interface is thus crucial for better interpreting the complete orogenic cycle, from intra-oceanic extensional processes to subduction-accretion and exhumation ones, affecting the Alpine ophiolites.