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Within the Eocene-Oligocene syn-orogenic deposits of the Epiligurian succession (Northern Apennines of Italy), a field-based study of the Specchio Unit (lower Rupelian) reveals that this complex is made up by the amalgamation of 3 distinct mass-transport deposits (MTDs), the largest of which reaches a maximum volume of ca. 75 km3. These bodies deposited inside a complex, intraslope basin system, developed atop the submerged Ligurian accretionary prism at the collision with the Adria continental plate. The MTDs originated from catastrophic retrogressive collapses starting from the upper slope and involving progressively shallow-water environments, from distal shelfal pro-delta and delta-front sediments up to proxi-mal coastal fan-delta deposits. These recurrent and close in time, catastrophic slope failures were probably caused by tectonic and climatic triggers, such as the enhanced tectonic activity due to incipient Apenninic continental collision and the onset of harsh climatic conditions, as suggest by oxygen isotopic maxima (e.g. Oi-1a event). Although the wedge toe/foredeep system are considered the principal loci of mass transport development, usually located in deep-water settings, here we stress the importance of catastrophic mass transport events also atop the wedge, in shallow-water depositional domains. Mass transport processes also have a fundamental role in reshaping the upper physiographic profile of an evolving ac-cretionary wedge. The correct interpretation of such mass transport processes has also important implications for geohazard forecasting in modern active continental margins, for example in terms of tsunamigenic potential.

In questa comunicazione presentiamo osservazioni sulle deformazioni legate a processi da trasporto in massa ed i loro prodotti all'interno dei mélange sedimentari. Gli esempi analizzati includono i depositi da trasporto in massa oligocenici affiornati nei bacini Epiliguri di wedge-top dell'Appennino Settentrionale, e le megabrecce carbonatiche eoceniche di avanfossa del Gruppo di Hecho dei Pirenei centro-meridionali. Queste unità sono comunemente caratterizzate da un fabric tipo "block-in-matrix", (proprio della definizione di mélange) espresso da blocchi da frana deformati di varia taglia, età e forma, inclusi all'interno di una matrice fine. La caratteristica predominante di questi depositi è la presenza di una matrice clastica relativamente fine, mal cernita e caratterizzata da una forte mescolanza di litologie, che va a riempire gli spazi tra i vari clasti e blocchi. Questa matrice mostra sia strutture fluidali legate a taglio semplice in regime duttile che un fabric omogeneo e massivo, entrambi probabilmente legati a fenomeni di liquefazione/fluidificazione, e conseguentemente, alla resistenza interna delle litologie mescolate. Questa fase è dovuta mescolamento di acqua e sedimento, ed è caratterizzata da un'elevata mobilità dovuta a condizioni di sovrapressione, come testimoniato dalle evidenze di iniezioni verticali e laterali all'interno delle litologie più coerenti. A grande scala questo tipo di matrice potrebbe rappresentare le cosiddette facies acustiche trasparenti osservabili in molti profili sismici di tali depositi. Il meccanismo genetico di base è quello di una deformazione progressiva a spese di litologie non ancora del tutto litificate che avviene durante le differenti fasi evolutive del corpo di frana (i.e. collasso, traslazione, accumulo), fino alle fasi post-deposizionali. Questo meccanismo porta ad una quasi completa distruzione della stratificazione ed al mescolamento delle litologie coinvolte, sia nel corpo di frana che nel substrato sottostante. In questo contesto, il movimento del corpo di frana è favorito dallo sviluppo di zone di taglio duttile in sovrapressione, sia internamente che alla base, lungo la principale superficie di scivolamento. Riteniamo che questo tipo di matrice, e le strutture associate, siano un possibile fattore discriminante per separare il contributo sedimentario da quello tettonico nei mélange contenuti all'interno dei sistemi accrezionali, fornendo inoltre importanti informazioni sulla cinetica dei processi generatori.

We report here mass transport-related disruption processes and their artifacts within sedimentary m??langes. 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 of modern 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 the mixed lithologies. We interpreted this phase as a liquefied mixture of water and sediment, characterized by high mobility 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 m??langes within accretionary systems and, moreover, to distinguish fast- from slow-rated generating processes. ?? 2011 Elsevier B.V.

This work discusses the stratigraphy and facies analysis of the Ranzano Sandstone, in the northern Apennines (Italy), a confined low-efficiency turbidite system deposited in a series of small piggy-back basins, which show strong analogies with intraslope minibasins commonly observed in divergent margins. The detailed physical stratigraphy of these deposits, which are the counterpart of the Annot Sandstone of the classic “Trilogie Priabonienne” cropping out in the western Alps, shows a cross-current asymmetrical distribution of the facies related to basin morphology. In an E-W oriented transect, roughly perpendicular to the general paleocurrents directed towards SSW, coarse-grained amalgamated massive sandstones that onlap against the eastern basin margin, grade towards the west, over very short distances, into a stratigraphic succession dominated by coarse-grained sandstones reworked in megaripples. This westward lateral facies change is also associated with: a) progressive deviation of the megaripple paleocurrents towards the west, b) progressive eastward increase in the sandstone/mudstone ratio, impact flow structures and beds with mudstone clasts, and c) progressive westward decrease in bed thickness characterizing massive facies. This cross-current transition is interpreted as related to bipartite turbidity currents experiencing deceleration against a relatively steeper eastern margin, producing flow decoupling and consequent bypass of upper turbulent flows that can spread out towards the less steep western bounding slope. The lateral spreading of turbulent flows causes reworking of the coarse-grained massive facies, producing tractive structures that indicate westward flow divergence. The lack of medium, fine-grained sands and mud testifies that these grain sizes must have been transported by turbulent flows into another adjacent basin, allowing the described deposits to be interpreted as recording a “flow stripping” phase as found in the ponded intraslope basins of the Gulf of Mexico and other confined settings, such as wedge-top basins and inner foredeeps.

We present the integrated outcrop-geophysical study of two mass transport complexes, the exhumed Specchio unit in the Northern Apennines of Italy and the Holocene Poverty unit in the Hikurangi margin of New Zealand. The combination of micro- to meso-scale structural, stratigraphic and sedimentologic analyses carried on continuous three-dimensional outcrops, with large-scale structural and morphologic data deriving from seismic/acoustic imaging of the present-day continental margins, allow important considerations on submarine landslide processes and mechanisms through the broader (up-scaled and down-scaled) understanding of the mass transport-related structural associations. We compare the discontinuous high-amplitude, reverse-polarity reflectors observed within the Poverty with the syn-sedimentary, ductile shear zones found within the Specchio mass transport complex. The seismic signature of such structures suggests localized fluid overpressure along detachment/thrust zones due to shearing and loading of undrained, water-saturated, fine-grained material, developed along with the slide mass movement. The outcrop expression of these structures is tentatively attributed to m- to tens of m-thick shear zones comprising large amounts of sedimentary matrix which separate and accommodate the differential movements of the internal slide components (e.g. slide blocks, olistoliths). The sedimentary matrix is an unsorted, lithologically mixed medium characterized by a scale-invariant “block-in-matrix” fabric (i.e. brecciated, mud-supported), that injects, sustain and surrounds discrete slide elements (from particles to blocks) and interpreted as a hyper-concentrated (liquefied/fluidized) suspension of water and scattered sediments developed in fluid overpressure conditions. We highlight the fundamental role of shearing-related liquefaction as one of the main factors controlling slide mobility through the “lubrication” of the internal and basal friction forces. The analysis of such features can therefore provide important information for the characterization of mass transport deposits developed from potentially catastrophic, long run-out mass transport events, and consequently, to better understand their possible socio-economic impact in terms of tsunamigenic potential.

Within the Eocene-Oligocene syn-orogenic deposits of the Epiligurian succession (Northern Apennines of Italy), a field-based study of the Specchio Unit (lower Rupelian) reveals that this complex is made up of three distinct but amalgamated mass- Transport deposits (MTDs), the largest of which reaches a maximum volume of ca. 150 km3. These bodies were deposited inside the complex system of intraslope basin systems, developed atop the submerged Ligurian accretionary prism at the collision with the Adria continental plate. The MTDs originated from catastrophic retrogressive collapses starting from the upper slope and involving progressively shallow-water environments, from distal shelfal pro-delta and deltafront sediments up to proximal coastal fan-delta deposits. These recurrent and close in time, catastrophic slope failures were probably caused by tectonic and climatic triggers, such as the enhanced tectonic activity due to incipient Apenninic continental collision and the onset of harsh climatic conditions, as suggested by oxygen isotopic maxima (e.g., Oi-1a event). Although the wedge toe/foredeep systems are generally considered the principal loci of such, usually located in deep-water settings, here we stress the importance of catastrophic mass transport events also atop the wedge, in shallow-water depositional domains. Mass transport processes also have a fundamental role in reshaping the upper physiographic profile of an evolving accretionary wedge. The correct interpretation of such mass transport processes has also important implications for geohazard forecasting in modern active continental margins, for example in terms of tsunamigenic potential. © Springer Science+Business Media B.V. 2012.