Mélanges and chaotic rock units: Implications for exhumed subduction complexes and orogenic belts

Most of mélanges occurring in exhumed subduction complexes and orogenic belts are commonly inter- preted as the product of tectonic processes (e.g., underplating and return flow) acting at intermediate to great depths ( T > 250 °C, depth > 10–15 km). Conversely, observations on modern and ancient non- to poorly metamorphosed subduction complexes around the world, clearly show that the largest part (c. 64.7%) of mélanges and chaotic rock units are already formed at shallower structural levels ( T < 250 °C, depth < 10–15 km). They mainly consist of broken formations ( > 21.5%), sedimentary (c. 20%), polyge- netic ( > 13.7%), and diapiric (c. 6.7%) mélanges. Tectonic mélanges are limited to about 2.7%, suggesting that tectonics is not an efficient mixing process at shallow structural levels. We document that the sub- duction of structural inheritances (e.g., ocean-continent transition zones, and ocean plate stratigraphy) plays a significant role in forming and differentiate the different types of chaotic units at shallow depths, also controlling the location of the plate interface and the dynamics of the wedge front (i.e., tectonic accretion vs. erosion). However, not all chaotic units that formed at shallow structural levels can be sub- ducted 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. Our findings demonstrate that the evidence that the larger part of mélanges and chaotic units form at shallow depths has significant implications for a better understanding of the tectonic evolution of subduction complexes and orogenic belts, ranging from the mode and time of Precambrian Earth evolution and the onset of plate tectonics to the role of mélanges in controlling the seismic behavior.