Detachment folding-related Miocene submarine slope instability in the Romagna Apennines (Italy).

The relationships between slope failure and growing thrust-related structures have been explored in the Miocene Marnoso-Arenacea foredeep basin (Romagna Apennines). Impressive fossil slump layers are distributed along the footwall of the regional Mount Nero thrust, which evolved from detachment fold to breakthrough thrust. During the detachment fold stage, development of growth strata on the rotating forelimb was preceded by the emplacement of slump layers that are currently part of the Mount Nero thrust footwall. In this process, sliding is assumed to nucleate along the contacts between different lithologies in the semiconsolidated rock mass. The conditions of sliding, specifically the critical angle of sliding acrit, are quantified by adding the influence of cohesion of sediments to the equilibrium analysis reported in Hubbert and Rubey's (1959) classical work. The resulting angles acrit are discussed with respect to the depth of the basal surface of sliding and pore fluid pressure states. On the basis of such an analysis and field work results, an evolutionary model is proposed in which slide sheets come from the crest and upper forelimb of growing detachment folds and attain a deeper detachment depth as the forelimb rotates and steepens. Pore fluid pressure is found to play a major role by radically decreasing the critical angle of sliding. Likewise, localization of overpressures may also control the depth of sliding during progressive forelimb rotation. To allow sliding along gently dipping bedding surfaces, slope failure is interpreted as being related to the reactivation of anisotropies represented by lithological discontinuities.