Mass-transport deposits in confined wedge-top basins: surficial processes shaping the messinian orogenic wedge of Northern Apennine of Italy

Abstract

Among the many cases studied of mass-transport deposits in continental margins, the role of basin topography in controlling the types, distribution, architecture and emplacement of such deposits has not been properly remarked. In the western portion of Northern Apennine foothills, masstransport deposits form two composite Messinian mass-wasting bodies that reveal progressive development strictly controlled by basin topography. Extensively analyzed through stratigraphic and structural studies, they form two major elliptical-shaped bodies in map view; maximum 10 kilometres wide, tens of kilometres in length and with estimated volumes of about 250 km3 each, they are elongated parallel to NW-SE oriented thrust fronts. They are coalescing chaotic masses that consist, at the base, of debris flows formed by monogenic gypsum arenite or breccia and decametric blocks of primary gypsum, whereas at the top they are made up of kilometres-wide outliers of pre-gyspum deposits, which slid away from partially preserved headwall scarps. In the external accumulation zone, the mass wasted deposits show imbricate thrust-stacks composed of scraped-off gypsum debris flow deposits. The types, distribution, architecture and emplacement of the studied mass-transport deposits testify the strict control of the wedge-top basins morphology. The internal and steeper flank of the wedge-top basins was representing the depletion zone of sliding masses; whereas, the outer and less steep flank of the wedge-top basins stopped the moving masses and formed the accumulation zones. The relief of the wedge-top basins was progressively modifying during the intra-Messinian tectonic pulse that, affecting the entire Northern Apennine orogenic wedge, triggered the studied mass-transport deposits