MÉLANGES AND GEODYNAMIC SETTINGS OF THEIR FORMATION

Mélange formation commonly occurs through the interaction and/or overlapping of different processes acting during a continuum of stratal disruption and mixing. At shallow structural levels in tectonically active environments, sediments are subject to small-scale deformation immediately after deposition, at rates and in ways dependent on the interplay between gravitational deformation and tectonic burial. Here, gravitational deformation favors mixing processes forming different types of sedimentary mélanges. With the downward increase in the degree of consolidation/lithification of buried sediments, tectonic forces gradually become predominant forming broken formations. Tectonic mélanges (with exotic blocks included), however, are restricted to fault zones, plate boundaries and subduction channels. Formation of diapiric mélanges is strongly controlled by the presence of fluid overpressure, consolidation rate of sediments, and the state of stress.

A comparison of the Circum-Pacific and Tethyan mélanges shows that the internal structure of different mélange-types is controlled by tectonic, sedimentary and diapiric processes operating in their environment of origin. Discrete evolutionary stages of mélange formation, from stratal disruption to mixing, are constrained by the tectonic setting of their formation, state of consolidation of the original coherent succession, metamorphic degree, rheological contrast between component layers, strain rate, and shallow vs. deep crustal deformation. The most common mechanisms-processes of strata disruption-mixing and the tectonic setting of mélange formation are linked by a causal relationship, which we use to subdivide mélanges into six types, associated with (i) extensional tectonics, (ii) passive margin evolution, (iii) strike-slip tectonics, (iv) convergent margin–subduction tectonics, (v) continental collision, and (vi) intra-continental deformation.