Mass movements on the inner slope of a wedge at the transition from frontal accretion to frontal erosion: Evidence from a fossil analogue in the northern apennines of Italy

Submarine mass movements are sedimentary phenomena abundant in both present-day seafloor setting and in the sedimentary record of ancient basins. For this reason they represent one of the most important processes in the destruction of submarine margins and sediment redistribution. The investigation of ancient mass movements in on-land examples recorded in former sedimentary basins is particularly important because it may provide insights into the features of their modern submarine correspondent. The study of fossil analogues offers, in fact, a more detailed source of information on internal structures, depositional processes and post-depositional deformation history than the present-day submarine examples.In the Northern Apennines of Italy, both stratigraphic and structural data suggest a vast removal of the frontal part of the Ligurian accretionary prism through a large-scale process of tectonic erosion, in the early Miocene. This caused the destabilization of the forearc and the development of a distinct, large-scale mass-wasting event - the Canossa Event - which resulted in the collapse of the whole inner lower-slope of the foredeep. The slope failure is well recorded in the sedimentary infill of the external basins of the epi-Ligurian sequence, representing the lower portion of the original slope, where the sediments older than the Aquitanian are totally missing or represented by few scattered blocks ranging in size from several metres to several hundred metres (rarely few kilometres) and few hundred metres thick. As a consequence of this mass-wasting event, large submarine debris flow deposits, reworking material from the underlying accretionary Ligurian wedge, have been generated. They are now represented by the Aquitanian epi-Ligurian Val Tiepido-Canossa argillaceous breccias which in vast areas of the Northern Apennines are unconformably sealing the Ligurian substratum and few remnants of pre-Aquitanian epi-Ligurian sediments.The Val Tiepido-Canossa argillaceous breccias, extending for about 300 km along strike, ∼10 km wide and ∼300 m thick, provide a prominent fossil example of the sedimentary response to the destabilization of a slope apron in a convergent setting, as a consequence of the removal of a wedge toe through frontal tectonic erosion processes. This catastrophic mass-wasting event, implying a large tectonic reorganization of the outermost part of an accretionary prism, is one of the largest on-land examples of fossil submarine landslides and is comparable with present-day mass-wasting deposits in similar plate tectonic setting.