Mélanges and mélange-forming processes: a historical overview and new concepts

TLDR

Mélanges are widespread chaotic rock bodies in collisional and accretionary orogenic belts, and the term has evolved to encompass tectonic, sedimentary, and diapiric processes, yet debate persists over the precise definitions and significance of block‑in‑matrix chaotic rocks. This study reviews the historical evolution of the mélange concept and investigates how different mélange types relate to tectonic settings, focusing on the roles of mass‑transport versus contractional deformation and the transition from broken formations to true tectonic mélanges. The authors classify mélanges by comparing exhumed ancient on‑land units with modern tectonic environments, grouping them into extensional, passive‑margin, strike‑slip, subduction, collisional, and intracontinental deformation categories. Sedimentation and contractional deformation are key drivers of mélange formation across all tectonic settings, with late‑stage processes dominating internal structure, and tectonic mélanges are typically subordinate to broken formations and confined to narrow fault zones and plate boundaries.

Abstract

Mélanges represent a significant component of collisional and accretionary orogenic belts and occur widely around the world. Since its first introduction and use, the term has evolved to cover both processes (tectonic, sedimentary, and diapiric) and tectonic settings of mélange formation. The meaning and significance of various terms referring to the origin of ‘block-in-matrix chaotic rocks’ are still subject to debate. This study presents a historical overview of the evolving mélange concept and investigates the relationships between mélange types and their tectonic settings of formation. We investigate the contribution of mass-transport versus contractional deformation processes at the onset of mélange formation and throughout the evolution of different mélange types, and the nature of the continuum and transition from broken formations to true tectonic mélanges. A mélange is a mappable chaotic body of mixed rocks with a block-in-matrix fabric whose internal structure and evolution are intimately linked to the structural, sedimentary, magmatic, and metamorphic processes attending its origin. On the basis of a comparative analysis of exhumed, ancient on-land mélanges and modern tectonic environments, where mélange-forming processes are at work, such units are classified into those related to extensional tectonics, passive margin evolution, strike-slip tectonics, subduction zones, collisional tectonics, and intracontinental deformation. Sedimentation and contractional deformation contribute significantly to mélange formation in all these tectonic environments, although the internal structure of deposits is strongly controlled and overprinted by processes that prevail during the last stages of mélange formation in a single tectonic setting. Tectonic mélanges are commonly subordinate to broken formations and are restricted to narrow, elongated-to-coalescent fault zones, large-scale fault zones, and plate boundaries.