Mass Transport Complexes in bacini confinati a controllo strutturale: l'Unità Epiligure di Specchio (Appennino Settentrionale)

The recent increasing in geophysical exploration of continental margins and the concomitant progress of increasingly more sophisticated seismic- and acoustic-imagery technologies have shown the widespread occurrence of large accumulations of remolded sediments generated by submarine landslides, and commonly referred to as Mass Transport Deposits or Complexes (MTD and MTC, respectively). These units are being intensively investigated, not only for strictly scientific reasons, such as their environmental significance, understanding of triggering processes, and their role in the transformation of density flows, but also because of economic and social implications, mainly in terms of hydrocarbon exploration and production, and geohazards. On the other hand, field-based studies on ancient examples of MTDs are relatively scarce with respect to the huge amount of data derived from marine geology. Many problems arise from the tentative comparison of the data derived from these two different approaches, mainly because of the resolution limits and scaling problems between these methods, and for the limited occurrences of comparable geodynamic contexts and relative unambiguous interpretations (i.e. collisional/convergent versus divergent margin settings). This is particularly evident for accretionary systems, where several controlling factors combine to produce “chaotic units” at different scales. This kind of situation highlights the need of a systematic comparison and an integrated approach to the study of such units for a better understanding of their geologic significance, especially for those depositional environments located on top of accretionary prisms, which represent an ideal setting for studying the role of mass transport deposits in the wedge evolution, in spite of their yet relatively poor recognition in the modern and ancient rock record. Keeping in mind the above problems and in order to partly fill this gap, this study has been carried out through a field-based work carried on an ancient example of MTC, known as the Specchio Unit among the Apennine geologists, and typically developed on top of an accretionary prism. This unit occurs within the lower Rupelian rocks of the syn-orogenic sedimentary record of the Eocene-Oligocene Epiligurian succession, cropping out in the eastern side of the Northern Apennines (Italy) as isolated sedimentary remnants (i.e. outliers), representing the fill of local intra-slope basins developed on top of the translating Ligurian Nappe (i.e. proto-Apenninic accretionary wedge). This study has been developed through cartographic- to microscopic-scale stratigraphic and structural analyses, collected directly on the slide bodies and on the over- and under-lying sedimentary succession, along with an attempt of systematic comparison with so far recognized modern analogs, focusing on both the slide-related features and the overall physiography of the original depositional setting. The first important result coming out from these detailed outcrop studies, is the subdivision of this MTC into five sub-units, representing at least four distinct MTDs: the lower ones, of local significance, derived from the southern sectors, and the upper ones, of basin-wide extent, derived from the northern sectors. The largest MTD reaches an inferred volume of involved material of about 150 km3. The vertical stacking of these MTDs and the progressively “shallower water nature” of the internal components, seems to represent the deposition from closely spaced events, originated through a retrogressive failure process, involving progressively more proximal areas. Careful observations carried on the internal elements of these units (i.e. matrix and blocks) allow considerations on slide mechanics, with the detailed characterization of sedimentary bodies generated by catastrophic processes, which include both slump- and debris flow-like facies (i.e. blocky-flow deposits of Mutti et al., 2006), and the influence exerted by structural confinement on the slide emplacement, mainly in term of forced slide direction, localized over-thickening, substrate coupling (bed erosion in a sedimentological sense) and margin-induced strain partitioning. In particular, this study highlights the likely occurrence of a generalized lateral buckling (compression + transpression), transversally to the main sliding direction, and an overall unidirectional shearing in the longitudinal sense, giving important information on the slide kinematics. This study also contributes to the understanding of the local intra-slope basin configuration, highlighting the differences in facies associations between pre- and post-slide emplacement sedimentary successions, and the possible existence of an overall shallow level tectonism (gravity-related?). This kind of “slope-tectonics”-type regime forecasts the synergic development of thrust- and strike slip-related shale diapirism and local, small-scale MTDs, affecting basin margins and intrabasinal highs, and contributing to the development of an overall above-grade slope physiography. Moreover, some regional considerations on the palaeogeograhic setting of the lower Rupelian Epiligurian succession can be made, integrating the so far accepted hypothesis from the literature with the results of this study, mainly in terms of transport directions, provenance and environmental significance of the involved material. The evidence of a far-located calcalkaline volcanism (in particular here I report the first recognition of resedimented volcaniclastic beds in the sedimentary succession on top of the Specchio unit, in the Pessola Valley outlier), along with the climatic- and tectonic-related signatures characterizing the restored sedimentation, could contribute in constraining the possible triggering mechanisms and preconditioning factors of slide development, thus giving indirect information on the general configurations of source areas. These observations demonstrate that further specific studies on previously overlooked “chaotic” units may significantly contribute to a better understanding of the Apenninic orogenic system, and, at the same time, introducing new challenges for their application into other orogenic belts worldwide. Part of these conclusions are in contrast with the nowadays-generalized belief that seismically active convergent margins are characterized by the occurrence of relatively few and smaller scale MTDs, if compared to divergent margin settings. In such contexts, submarine landslides are thought to develop close to the main deformation front, commonly located in deep-water settings. Apart from some specific convergent margins (e.g. southern Cascadia, NE New Zealand, western Aleutinian Arc), particularly those involving a continental collision (e.g. NW Borneo), where some catastrophic, large-scale MTDs are observed to occur, in typical convergent margin settings (particularly those involving oceanic crust consumption; e.g. offshore Perù, Costa Rica), MTDs sharing huge dimensions with those of divergent margins are instead characterized by overall slow-rate motion, and are thought to be commonly linked to subduction dynamics (e.g. colliding seamount). In spite of these interpretations, this work seems to confirm the possible occurrence of catastrophic, large-scale MTDs on the internal depositional domains of an accretionary prism (i.e. internally from the main tectonic front), which may originate from shallow-water regions, also possibly involving near-shore areas, with consequent important implications, for example, in terms of geohazard purposes. In summary, it is possible to suggest that an integrated field-based approach to the study of MTDs may significantly contribute to solve some scaling problems about submarine landslide deposits. In particular, this approach should be useful in facing the difficulties regarding the interpretation of “chaotic” units cropping out in orogenic belts, as well as those characterizing modern submarine collisional belts.