The role of olistostromes and argille scagliose in the structural evolution of the Northern Apennines. Pre-Congress Field Trip B13

The influence of gravity in accretionary wedges is implied in the Coulomb critical wedge theory. Internationally, slope failure and mass wasting phenomena are increasingly considered to have had a strong impact on the geometry and internal features of accretionary wedges. They can directly influence important processes, such as movement and balancing of rock masses, erosional activity and exhumation of deep rocks, and the sedimentary budget in satellite and trench/foredeep basins. Therefore, they may control the mechanisms of accretion, offscraping/underplating and obduction. Mass wasting phenomena are often part of the geology of a peculiar group of rocks, characterized by high internal deformation, up to the loss of the lateral continuity of beds (stratal disruption), and by different amounts of rock mixing. These rock units, frequently defined as mélanges, crop out as a consistent part of on-land fossil accretionary wedges and, thus, may represent opportunities to study the role of mass wasting processes in accretionary wedges through direct field observations. The term mélange, after the re-introduction by K. Hsu, has been used to indicate stratally disrupted and chaotic rocks elsewhere. The term should be used in a descriptive, non-genetic sense and the adjectives “tectonic” and “sedimentary” can be added to point out the different origin of the rocks. However, mélange has been prevailingly used with the sense of tectonic product in mind by several authors in the American and International literature, with some exceptions suggesting a possible origin from mass wasting processes. On the contrary, in the Apennines of Italy and in Sicily until the 1980’s, the origin of chaotic units was almost unanimously related tochaotic unit have been used for bodies disrupted by mass wasting processes at different scales . In the last 20 years the term mélange has been restricted to sedimentary stratally disrupted rocks, which mostly contain exotic blocks, whereas, other bodies, which retain their original stratigraphic coherence but contain no exotic blocks, have been defined as broken formation or tectonosomes. Notably, tectonosomes and sedimentary melanges all have variably disrupted bed packages and block-in-matrix rocks. Notwithstanding the similar aspect, there is now a general agreement on the aforesaid distinction and a general consensus on the distinctive criteria. The latter are based on the careful definition of the age and composition of rocks and on detailed study of fabric at outcrop to microscopic scale. There is now a large diffusion of geologic maps based on these criteria distinguishing units of different origin and evolution into afore considered chaotic masses. Some other issues about the internal features of the disrupted and chaotic units of the Apennines still remain open and are currently being investigated: 1) the discrimination between mass-wasting and tectonic processes that led to early stratal disruption within broken formations/tectonosomes; 2) the mechanisms that cause the stratal disruption in shallow level tectonics and their association with compaction and diagenesis; 3) the influence of different gravity-mass movement processes in olistostromes; and 4) the possible role of mud diapirism in the formation and deformation of olistostromes. Moreover, some disrupted units, such as the basal complexes of the Modino, Pievepelago and Sestola-Vidiciatico units, are associated at the regional scale with the evolution of the Oligocene-Miocene foredeep complexes of the paleo-Apennines, which are some of the best exposed and preserved foreland-basin turbiditic successions elsewhere. These disrupted units disclose the role of large-scale gravitational collapse and mass wasting during the evolution of the paleo-Apenninic accretionary wedge and associated foredeep. This field trip has been proposed as a part of the 32nd IGC Congress to display and discuss all the aforementioned issues, which are, in our opinion, of the greatest interest for the geology of mélanges and accretionary wedges elsewhere.