MELANGE AND MELANGE-FORMING PROCESSES: CASE STUDIES FROM THE APENNINES, APPALACHIANS AND NEW ZEALAND

Mélange-forming processes (tectonic, sedimentary, and diapiric) commonly operate in tandem and locally synchronously in various discrete tectonic settings, but rarely in isolation. They are linked through complex and mutual interactions and occur in partial superposition. In the geological record of mélanges we commonly observe the artifacts of the last and/or more pervasive process as the best-preserved evidence. In most cases, the latter is consistent with tectonics, although few examples in which sedimentary or diapiric processes have also been described in the literature.

We present here a comparative analysis of on-land examples of mélanges and tectonic environments where mélange-forming processes are currently operating. Main types of deformational mode leading to mélange formation include extensional, strike-slip, convergent margin/collisional tectonics, and intra-continental deformation. We discuss: (1) possible relationships between various mélange types and their tectonic setting of formation; (2) contribution of mass-transport versus contractional deformation processes at the onset of mélange formation, and (3) nature of the “continuum” and transition from broken formations to true tectonic mélanges.

Mélange formation commonly develops through the interaction and/or overlapping of several mechanisms and processes acting during a “continuum” of stratal disruption. This phenomenon defines a broad spectrum of products, represented by two end-members consisting of undeformed stratigraphic successions and block-in-matrix bodies. Different sedimentary, tectonic, and diapiric mélange types developed in this “continuum” of deformation conceptually represent discrete evolutionary stages of stratal disruption processes constrained by the geodynamic setting of their formation, state of consolidation of the original coherent succession/metamorphic degree, rheological contrast between component layers, strain rate, shallow vs. deeper deformational mechanisms. We show that some notable examples, mainly from the Apennines (Italy), Ordovician Taconic belt in Eastern USA, and Hikurangi margin in New Zealand, display clear structural evidence for the operation of different mélange-forming processes and mechanisms at different scales and in different tectonic settings during their formation. These processes appear to have interacted with each other and reworked in turn their products, leading to the genesis of several types of mélanges recognizable in other orogenic belts.