Mélanges and chaotic units represent significant components of most subduction complexes and orogenic belts worldwide, regardless of their age (from the Precambrian to the present day), tectonic evolution, or location (e.g., from the circum-Pacific region to the Alpine-Himalayan belt). However, the mode and nature of the processes responsible for their formation, as well as the geological environments in which different mélange types develop, remain a topic of debate.

Most chaotic units preserved in exhumed subduction complexes, particularly metamorphic ones, are commonly interpreted as products of tectonic processes (i.e., underplating and return flow) occurring at intermediate to great depths during convergent stages. However, our observations from both modern and ancient subduction complexes indicate that different types of mélanges and chaotic units already form at shallow structural levels (T < 250 °C) through different processes (tectonic, sedimentary, and diapiric) and their interplay. This suggests that (i) mixing mechanisms are not exclusive of deep processes, (ii) the internal architecture of exhumed subduction complexes may be highly heterogeneous even at shallow structural levels, and (iii) tectonics is not the sole process responsible for mélange formation.

Using field-based stratigraphic and structural criteria, we show that mélanges formed through different processes and in distinct tectonic environments can be distinguished by diagnostic block-in-matrix arrangements. Therefore, the study of mélanges provides valuable constraints for improving our understanding of the tectonic evolution of Precambrian to Phanerozoic subduction complexes and metamorphic orogenic belts worldwide.

It is now recognized that there must be a wealth of interaction between processes of vastly different strain rates. One “type” example of a deformation phenomenon encompassing deformation of different rates are Slow Earthquakes (SEs). In SEs, slip occurs more slowly than in regular earthquakes, but significantly faster than can be attributed to normal plate motion. SEs have been shown to be closely associated with a range of deformation processes active at different deformation rates. Although SEs are abundant, their geophysically observed characteristics cannot be reconciled with current understanding of how rocks deform: New evidence of slip processes need to be discovered in the geological record.

Rock outcrops from an example of exhumed subducted crust in New Caledonia are interpreted to contain zones of former SEs. Microstructural characterization combining EBSD and EDS analyses deciphers controlling deformation processes, while phase petrology is used to evaluate stages of fluid ingress, production or egress. Based on our observations, we interpret that several deformation processes directly associated with the presence and movement of fluids governed rock behaviour. Relatively “slow” dissolution-precipitation creep is the main “background” deformation process responsible for the shape- and crystallographic-preferred orientations, in-grain compositional variations and grain boundary alignment. Fast brittle, hydrofracturing and local granular flow is enabled by episodic high fluid pressures induced by mineral dehydration reactions. These transient deformation processes act at much faster strain rates but occur for only short time periods.

The distinction between sedimentary and tectonic processes in the formation of chaotic block-in-matrix fabrics is especially difficult in ancient mountain belts, where sedimentary structures are usually overprinted by orogenic deformation (e.g. Festa et al. 2019, GR, 74). This also applies to the chaotic rock units occurring widespread in the allochthonous domain of the Harz Mountains. For these units, it has previously been assumed that their rock fabric was sedimentary in origin and subsequently only tectonically overprinted by late Carboniferous Variscan deformation. In contrast, it could be shown, that tectonic deformation is crucial for the formation of the "chaotic" fabric (Friedel et al. 2019, Int.J.Earth.Sc., 108). This is particularly evident in the structural characteristics of blocks of (hemi)pelagic limestone of different Devonian ages, which are incorporated in a slaty clayey matrix. So far, the blocks were mostly regarded as olistoliths and thus considered as clear proof for a sedimentary origin of the chaotic units being olistostromes. However, our investigations show that these blocks are fault-bounded, tectonically folded and internally imbricated stacks of limestone strata, whose final fragmentation and isolation occurred after folding. Similar to rootless folds, these blocks of fault-bounded imbricate stacks are an outcrop-scale diagnostic feature to identify a strong fault-tectonic overprint or even a tectonic origin of chaotic rock fabrics, provided that the tectonic character of folding and faulting is sufficiently proven. The regionally widespread occurrence of such blocks, together with meso- to micro-scale faulting criteria, suggests a predominantly tectonic origin for the chaotic rock units in the Harz.