Significance of ophiolitic mélanges and chaotic rock units in the evolution of subduction complexes and orogenic belts

Most ophiolitic mélanges and chaotic rock units in exhumed subduction zone complexes and orogenic belts are commonly interpreted as the products of tectonic processes (e.g., underplating and return flow) acting at intermediate to great depths (depth > 10–15 km, T > 250 °C) at convergent margins. Conversely, observations from modern and ancient, non- to poorly metamorphosed subduction–accretion complexes (recognized as mélanges and chaotic rock units) around the world show that these rock associations: (1) likely formed at shallow structural levels first, and (2) were later subducted and became tectonically reworked. As such, they mainly consist of broken formations (> 21.5%), and sedimentary (c. 20%), polygenetic (> 13.7%) and/or diapiric (c. 6.7%) mélanges. Tectonic mélanges are limited to <3.0% (in surface distribution), suggesting that tectonic processes do not make efficient mixing mechanisms at shallow structural levels. Subduction of structural inheritances (e.g., ocean-continent transition zones, and lithological and structural heterogeneities in ocean plate stratigraphy – OPS – assemblages) plays a more significant role in forming mélanges and chaotic rock units at shallow depths; it can also control the origin and location of plate interface and the dynamics of the wedge front (i.e., tectonic accretion vs. erosion). However, not all chaotic rock units that formed at shallow structural levels may become subducted; but, if subducted, their fate might be different depending on whether they become part of the plate interface or if they become part of the lower plate. Our global field observations, suggesting that most mélanges and chaotic rock units form at shallow depths, have significant implications for the tectonic evolution of subduction zone complexes and orogenic belts.