ARCHITECTURE AND TECTONIC STYLE OF MÉLANGES THROUGH TIME

Mélanges represent the product of active geodynamic processes. Their occurrence and the artifacts of their block-in-matrix features (i.e., nature, composition, age, metamorphic degree of blocks and the matrix) may provide significant information on the characteristics of plate tectonics and its changes through time, during the Earth evolution from Neoarchean to Phanerozoic. However, the interpretation of mélanges and, consequently, of the characteristics of plate tectonics and convergent margins evolution, is strongly complicated by post-subduction multistage tectonic deformation and metamorphism that commonly overprint, modify, and mask their primary diagnostic block-in-matrix fabric. As a result, most of mélange occurrences in Neoarchean to Phanerozoic convergent margins are commonly interpreted as only the product of tectonic processes (e.g., underplating and return flow) acting at intermediate to great depths. This contrasts with observations on modern and Mesozoic-Cenozoic non- to poorly metamorphosed subduction complexes around the world that clearly show that a consistent part of mélanges already formed at shallower structural levels by sedimentary (i.e., gravitational) and diapiric processes, and are later subducted and/or tectonically reworked, forming polygenetic mélanges. Evidence from Mesozoic and Cenozoic metamorphic orogenic belts, also outline two main clusters of depth at which tectonic mélange and tectonic slices/blocks are exhumed (i.e., at c.30-40 km and at c.80 km), suggesting that the mélange rocks recovery is episodic, and the subducted material seems return over relatively short-lived episodes. In addition, the size of blocks/tectonic slices associated with mélange units shows apparent close relationships with the characteristics of the subducting plate (i.e., occurrence of structural heterogeneities, discontinuous vs continuous of oceanic serpentinization, etc.). In conclusion, the discovering of Precambrian mélanges, the understanding of the nature and composition of their block-in-matrix fabric, processes, and mechanisms of formation (tectonic, sedimentary or diapiric), in comparison with the diagnostic features of Mesozoic-Cenozoic and modern mélange occurrences, may have significant implications for a better understanding of early Earth’s tectonic style, and the operation of plate tectonics.