Melanges and their significance in the evolution of convergent margins

Mélanges and, chaotic units represent significant components of most of subduction complexes and orogenic belts worldwide, regardless of their age (from Precambrian to present day), tectonic evolution, and location (e.g., from the circum-Pacific region to the circum- Mediterranean area, the Inner Western Carpathians, and extending to the Alpine-Himalayan and Asian orogenic belts and suture zones). Despite nearly a century having passed since Greenley (1919) introduced the term “mélange,” a comprehensive understanding of the processes leading to their formation, as well as the geological environments in which they develop, remains incomplete and continues to be the subject of active debate within the geoscientific community. A major challenge lies in the identification of the original, diagnostic block-in-matrix fabric of mélanges, which provides critical clues about where and how these chaotic rock assemblages formed, and what they reveal about the interplay between crustal and submarine processes during their development.
Historically, most mélanges and chaotic units preserved in exhumed subduction complexes, particularly those affected by metamorphism, have been interpreted as the exclusive product of tectonic processes operating at intermediate (250°C<T<400°C, corresponding to ~10–15 km depth) to great depths (T>400°C, corresponding to >15 km depth) during convergent stages. The transfer of isolated slices and blocks detached from the downgoing slab to the overriding plate and/or within the plate boundary interface, primarily through tectonic underplating, and return flow (i.e., flow mélanges), has been widely recognized as an effective mixing mechanism responsible for their formation at such depths.
However, an increasing body of field-based evidence indicates that mélange formation is not limited to these deeper structural levels or exclusively linked to convergent margin settings. On the contrary, our observations on well-documented mélange occurrences worldwide demonstrate that various mélange and chaotic unit types can originate at shallow structural levels (T<250°C). This suggests that (i) mixing mechanisms are not exclusive to deep-seated processes and convergent settings, (ii) the internal architecture of exhumed subduction complexes may already exhibit significant heterogeneity at shallow structural levels, and (iii) tectonic processes are not the sole mechanisms responsible for mélange formation (see Raymond, 1984; Pini, 1999; Festa et al., 2016). These observations imply that some mélanges found in exhumed metamorphic and polydeformed orogenic belts may not exclusively represent the product of intermediate- to deep-level tectonic processes. Rather, they may preserve evidence of subduction-related tectonic reworking of pre-existing chaotic units formed at shallower structural levels, through different processes and within a range geodynamic environments (Festa et al., 2018).
Consequently, mélanges observed in exhumed orogenic belts often record a more complex evolutionary history than previously assumed. They may reflect multi-phase deformation, tectonic overprinting, and material recycling occurring within the dynamic and evolving environment of a convergent margin (Festa et al., 2022). In this context, distinguishing between mélanges formed predominantly by tectonic mechanisms (e.g., slicing, underplating, return flow), sedimentary processes (e.g., slumping, mass-transport deposition), diapirism, or combinations thereof is essential for geologists seeking to reconstruct the tectono-stratigraphic evolution of a convergent system (Figure 1).
Crucially, any robust classification and interpretation of mélanges must be based on detailed, field-based investigations employing consistent stratigraphic and structural criteria (Festa et al., 2019, 2022). This approach enables the distinction between different mélange types, such as tectonic, sedimentary, diapiric, and polygenetic (see Raymond, 1984; Festa et al., 2010) and provides insights into their respective