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

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.

Keywords:

• mélanges
• broken formations
• olistostromes
• mud diapirs
• subduction processes
• obduction

Acknowledgments

This synthesis is based on many years of field studies by the authors on chaotic rock bodies in different places around the world. We thank friends and colleagues who led insightful field excursions, helped organizing fieldwork in different mélanges, and introduced us to the geology of mélanges and surrounding landscapes. Among many others, we thank Juan Luis Alonso, Peter Ballance, John Bradshaw, Angelo Camerlenghi, Alberto Castellarin, Darrel Cowan, Jorge Gallastegui, Francesca Ghisetti, Ron Harris, Ken-ichiro Hisada, Claudio Corrado Lucente, Alberto Marcos, Michele Marroni, Emiliano Mutti, Kei Ogata, Yujiro Ogawa, Jarg Pettinga, Luis Quintana, Annie Rassios, Ender Sarifakioglu, Mustafa Sevin, Minella Shallo, Bernhard Spörly, Angela Suarez, Livio Vezzani, and John Wakabayashi for passionate discussions on mélanges and mélange-forming processes while in the field in different orogenic belts around the world. Our discussions and exchanges of observations about mélanges with all these colleagues contributed significantly to the development of our ideas in this article.

During the editorial process, we received the sad news that Peter Balance had passed away in October 2009. Peter valuably contributed to advancing geological knowledge of New Zealand, Pacific accretionary wedges and arcs, and chaotic units. We will always remember his suggestions and encouragement, as well as his friendship and hospitality.