Mechanisms and processes of sedimentary, tectonic and diapiric stratal disruption and mixing in the development of mélanges and broken formations

Mass-transport deposits, olistostromes and mélanges represent significant components of both modern and ancient continental margins. Tectonic forces and processes constitute the most common triggering mechanisms to downslope movement and formation of these chaotic rock bodies. At shallow structural levels in tectonically active environments, sediments are subject to small-scale deformation immediately after deposition, at rates and in ways dependent on the interplay and gradual transition between gravitational deformation and tectonic forces during burial. The downward increase of both the consolidation and lithification in buried sediments and the tectonic stress, control the progressive increase of deformation and stratal disruption. The result of these conditions is a continuum of deformation structures of the originally coherent stratigraphic successions where strata disruption and mixing processes are central in the genesis of broken formations and mélanges. The continuum of deformation in chaotic rock units has been described rarely, perhaps because their chaotic structure obfuscates the structural evidence of the deformation path history. The transition between sedimentary and tectonic chaotic processes is marked by an overlapping zone, in which their block-in-matrix fabric formed by tectonic stratal disruption commonly shows a strong convergence of fabric with sedimentary chaotic products. Moreover, the superposition of different mechanisms and processes of disruption and mixing of rocks may lead to the reworking of existing mélange products and to the formation of polygenetic mélange types. We present a synthesis of data from the Apennines (Italy) and Ordovician Taconic belt in Eastern USA, that show that exists a continuum of stratal disruption and mixing across structural levels or depths of burial in which various processes operate. At shallow structural levels, the occurrence of poorly consolidated sediments favors gravitational deformation whereas at deeper structural levels, with the downward increase of consolidation, the deformation related to tectonic forces becomes gradually more significant. Sedimentary (and diapiric) mélanges represent punctuated stratal disruption mechanisms recording the instantaneous physical conditions in the geological environment at the time of their formation. The different velocities of movement, the composition and lithification degree of sediments, the geometry and morphology of the depositional setting, and the mode of failure propagation control the nature of the transition between different types of mass-transport chaotic bodies. Tectonic mélanges and broken formations record the history of a progressive deformation event, which may result in different degrees of lithification during the evolution of a tectonic setting. The sediments are affected by a brittle to more ductile deformation that follows their progressive dewatering and strengthening due to burial with depth. Systemic documentation of the mechanisms and processes of the formation of different broken formations and mélanges and their interplay in time and space are highly important to increased understanding of the evolutionary history of accretionary wedges and orogenic belts.