Mass transport-related stratal disruption within sedimentary mélanges: Examples from the northern Apennines (Italy) and south-central Pyrenees (Spain)

We report here mass transport-related disruption processes and their artifacts within sedimentary mélanges. The case studies include the early Oligocene wedge-top mass transport deposits in the northern Apennines (Italy) and the Eocene foredeep carbonate megabreccias from the south-central Pyrenees (Spain). These “chaotic” units commonly share a block-in-matrix fabric expressed by variously deformed slide blocks of different size, lithology, age and shape, embedded in a fine-grained matrix. Geophysical studies ofmodern continental margins have characterized many of these deposits, which, however, remain still relatively poorly described in term of meso-scale characteristics. The prominent feature of the analyzed mass transport deposits is the occurrence of an unsorted, strongly mixed, relatively fine-grained clastic matrix, infilling space between large clasts and blocks. This matrix shows either fluidal structures related to simple shear, or a structureless, homogeneous fabric, both probably related to liquefaction/fluidization processes and thus, to the internal strength of themixed lithologies. Weinterpreted this phase as a liquefied mixture of water and sediment, characterized by highmobility due to overpressured conditions, as evidenced by both lateral and vertical injections. On a much larger scale this kind of matrix could represent the acoustically “transparent” facies separating slide blocks of many seismic examples. The inferred generating mechanism is that of a progressive soft sediment deformation, linked to different phases of submarine landslide evolution (i.e. failure, translation, accumulation and post-depositional stages), leading to an almost complete stratal disruption of involved bedded sequences, either within the slide mass and in the underlying substrate. In this framework the down-slope movement is favored by the development of ductile, overpressured shear zones, both internally and along the basal sliding horizon. Therefore, this matrix signature represents a possible discriminating factor to separate sedimentary and tectonic mélanges within accretionary systems and, moreover, to distinguish fast- from slow-rated generating processes.