Mélanges and their significance in the evolution of subduction-accretionary complexes and orogenic belts

Mélanges and chaotic units represent significant components of most subduction complexes and orogenic belts worldwide, regardless of their age (from the Precambrian to the present day), tectonic evolution, or location (e.g., from the circum-Pacific region to the Alpine-Himalayan belt). However, the mode and nature of the processes responsible for their formation, as well as the geological environments in which different mélange types develop, remain a topic of debate.

Most chaotic units preserved in exhumed subduction complexes, particularly metamorphic ones, are commonly interpreted as products of tectonic processes (i.e., underplating and return flow) occurring at intermediate to great depths during convergent stages. However, our observations from both modern and ancient subduction complexes indicate that different types of mélanges and chaotic units already form at shallow structural levels (T < 250 °C) through different processes (tectonic, sedimentary, and diapiric) and their interplay. This suggests that (i) mixing mechanisms are not exclusive of deep processes, (ii) the internal architecture of exhumed subduction complexes may be highly heterogeneous even at shallow structural levels, and (iii) tectonics is not the sole process responsible for mélange formation.

Using field-based stratigraphic and structural criteria, we show that mélanges formed through different processes and in distinct tectonic environments can be distinguished by diagnostic block-in-matrix arrangements. Therefore, the study of mélanges provides valuable constraints for improving our understanding of the tectonic evolution of Precambrian to Phanerozoic subduction complexes and metamorphic orogenic belts worldwide.