Chaos and geodynamics: mélanges, mélange-forming processes and their significance in the geological record. Preface to Special Issue

Mélanges occur extensively in accretionary and collisional orogenic belts around the world, and represent disrupted and chaotic units including sedimentary, magmatic and metamorphic rocks (commonly as exotic blocks) mixed by stratal disruption and tectonic, diapiric and/or sedimentary processes Although numerous papers and books have been published on mélanges, the definition and the geological significance of mélanges are still a matter of debate and controversy. The mechanisms that are responsible for the formation of mélanges may occur in a range of geological environments from the shallowest geosphere to significant crustal depths, associated with subduction zone tectonics, en-mass sedimentary processes, slope tectonics, in situ fluidization and mud diapirism, strike-slip tectonics, or due to the interplay of some of these processes. Although mélanges may characterize diverse geodynamic environments of formation, they are commonly associated with subduction of oceanic lithosphere, collisional events (arc-continent and continentcontinent), and intra-continental deformation including rifting and passive margin evolution. Hence, mélanges are cogenetic with emplacement of ophiolites, advancement of thrust and nappe sheets, evolution of foreland basins, formation of submarine landslides (olistostromes) and seismic events. Mélange formation is also involved in the mechanical stability of accretionary and orogenic wedges, and in the redistribution of earth materials through the processes of offscraping, underplating, mass-transport movements, subduction channel flow, and mud diapirism. Therefore, mélanges and mélange-forming processes are intimately linked with tectonics and tectonically induced geological processes in crustal evolution, and they constitute a significant component of the Earth history. Hence, further systematic and process-oriented, inter-disciplinary studies of mélanges should provide much-needed information about orogenic processes and crustal growth. Furthermore, the results of these mélange studies should also bemost insightful for the recognition of these chaotic rock bodies in the Precambrian greenstone belts. This Special Issue is a result of series of scientific sessions on mélanges that we convened at the GSA Global Meeting, Tectonic Crossroads, held in Ankara, Turkey in October 2010, the European Geosciences Union General Assembly in Vienna, Austria in May 2010, and the American Geophysical Union Fall Meeting in San Francisco, California, in December 2011. The papers in this Special Issue present the most up-to-date observations and interpretations on various mélange types and mélange forming processes from around the world. The geographic locations of the mélanges covered in this Special Issue are shown in Fig. 1. This Special Issue makes a significant contribution to the mélange concept, streamlines the definitions and classifications ofmélanges and broken formations, and provides a rich archive of well-documented mélange occurrences from diverse geological environments.