Understanding mélange and mélange-forming processes: new constraints for the tectonic evolution of the Alpine orogenic system

Chaotic rock units with block-in-matrix structure, commonly known as mélanges and broken formations, are one of the hallmarks of the Alpine orogenic system and of many exhumed convergent plate margins throughout the world. They commonly form in different tectonic settings and structural levels during the evolution of convergent plate margins, reflecting a close relationship between the active processes (tectonic, sedimentary and diapiric), and the physical and mechanical conditions experienced by rocks. Hence, systematic and process-oriented, inter-disciplinary studies of these units may provide much-needed information about the evolution of the Alpine orogenic system and convergent plate margins in general. The relationships between the final block-in-matrix structure and the processes of their formation commonly are, however, obscure and controversial, because of the complex and seemingly chaotic nature. The overlap of shearing, tectonic mixing, and metamorphic recrystallization lead to the reworking and change of the initial block-in-matrix structures formed by different processes (e.g., sedimentary or diapiric), and to the formation of polygenetic mélange. In this framework, block-in-matrix units formed by sedimentary processes may result very difficult-to non-distinguishable when placed in settings such as, for example, accretionary wedges or in a subduction channels or after a complete cycle from subduction to collision, exhumation and intra-continental deformation. We present here different examples of block-in-matrix bodies from the Alpine orogenic system, showing that at shallower structural levels, mélange-forming processes commonly operate in complex and mutual interactions, hence in partial superposition and rarely in isolation. In the geological record of mélanges we commonly observe as the best preserved evidence the artifacts of the last and/or more pervasive process. In most cases, the latter is consistent with tectonics, although sedimentary and/or diapiric-derived structures still characterize notably examples. We show here a comparative analysis of on-land exhumed examples of mélanges and tectonic environments where mélange-forming processes are currently operating. Main types of deformational mode leading to mélange formation include extensional, strike-slip, convergent margin/collisional tectonics, and intra-continental deformation. We discuss: (1) possible relationships between various mélange types and their tectonic setting of formation; (2) contribution of mass-transport versus contractional tectonic deformation processes at the onset of mélange formation (3) nature of the “continuum” and transition from broken formations to true tectonic mélanges.