SIGNIFICANCE OF MÉLANGES IN THE EVOLUTION OF SUBDUCTION-ACCRETION COMPLEXES AND OROGENIC BELTS

Most of mélanges occurring in exhumed subduction-accretion complexes and metamorphic orogenic belts are commonly interpreted as the product of tectonic processes (e.g., underplating and return flow) acting at intermediate to great depths. Field and analytical data outline two main clusters of depth at which mélange and tectonic slices/blocks are exhumed: at c. 30-40 km (i.e., downdip of the seismogenic zone) and at c. 80 km. The mélange rocks recovery seems to be episodic and the subducted material seems return over relatively short-lived episodes, representing only a fraction of subduction lifetime. In this scenario, the open question is whether the mélange record in subduction-accretion complexes and metamorphic orogenic belts reliably reflects depths of processes at which tectonic slices/blocks get scraped off or instead some other process, preventing the recovery of already detached material, or episodic switches between subduction accretion and erosion. Field examples in non-metamorphic subduction-accretion complexes and orogenic belts around the world clearly show that the subduction of structural inheritances (e.g., ocean-continent transition zones, and ocean plate stratigraphy) may play a significant role in forming mélanges already at very shallow depths (T<250°C). This also controls the location of the plate interface and the dynamics of the wedge front (i.e., tectonic accretion vs. erosion). However, not all mélanges that formed at shallow structural levels can be subducted and, as subducted, their fate could be very different if they become part of the plate interface or if they share the fate of the lower plate. Evidence that a consistent part of mélanges form at shallow depths has significant implications for a better understanding of the tectonic evolution of subduction complexes and metamorphic orogenic belts around the world.