Abstract
• Most of chaotic units (>64%) form at shallow depth ( T <250 °C) of convergent margins. • Deep ( T >250 °C) mélanges are mostly polygenetic, reworking shallow chaotic units. • Chaotic units have implications for convergent margins evolution and seismicity. • New potential constraints for Precambrian plate tectonics and modern seismicity . Most of mélanges occurring in exhumed subduction complexes and orogenic belts are commonly interpreted 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%), polygenetic (>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 subduction 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 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. 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.
Highlights
Mélanges and, chaotic units are a significant component of most of subduction complexes and orogenic belts around the world, regardless of their age, tectonic evolution, and location
Most of mélanges occurring in exhumed subduction complexes and orogenic belts are commonly interpreted as the product of tectonic processes acting at intermediate to great depths (T > 250 °C, depth >10–15 km)
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
Summary
Chaotic units are a significant component of most of subduction complexes (i.e., accretionary, and not-accretionary margins) and orogenic belts around the world, regardless of their age (from Precambrian to present day), tectonic evolution, and location (e.g., from the circum-Pacific region to the circumMediterranean area, up to the Alpine-Himalayan and Asian orogenic belts and suture zones). The above observations suggest that part of mélanges occurring in exhumed metamorphic and polydeformed subduction complexes and orogenic belts not necessary represent the exclusive product of intermediate to deep processes, but they may document the subduction-related tectonic reworking of chaotic rock units formed at shallower structural levels (Raymond, 1984; 2019; Cowan, 1985; Krohe, 2017; Festa et al, 2019; Wakabayashi, 2011, 2019). The term mélange (i.e., chaotic rock units with “exotic” blocks, Section 2) is incorrectly used to describe broken formations (sensu Hsü, 1968), which represent a different type of chaotic rock unit (i.e., without “exotic” blocks), formed by different processes, and characterized by different internal components (Section 2) The consequence of this incorrect and unclear terminology can lead to inaccurate interpretation of the tectonic evolution of the tectonic/geodynamic setting in which chaotic rock units occur. Earth evolution and the onset of plate tectonics, and iii) the role of mélanges and chaotic units in controlling the seismic behavior
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