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

Late Oligocene–early Miocene olistostromes (sedimentary mélanges) as tectono-stratigraphic constraints to the geodynamic evolution of the exhumed Ligurian accretionary complex (Northern Apennines, NW Italy)

“…2), including olistostromes, occur at different stratigraphic levels, within the episutural/wedge-top basin succession (e.g., Bettelli et al 1989aBettelli et al , 1989bCastellarin and Pini, 1999;Mutti et al 1995;Martelli et al 1998;Pini, 1999;Panini et al 2002bPanini et al , 2013Papani et al 2002;Pini et al 2004;Festa et al 2010aFesta et al , 2010bFesta 2011;Remitti et al 2011;Ogata et al 2012a;. Particularly, the occurrence of olistostromes in the late Oligocene -early Miocene time interval (i.e., Val Tiepido -Canossa argillaceous breccias of Panini et al 2002b; Canossa Olistostrome of Fazzini and Tacoli 1963, Papani 1963, Bettelli et al 1987; Val Tiepido-Canossa sedimentary mélange of , Bettelli et al 1989a, 1989b; see also Castellarin and Pini, 1987;Pini, 1999;Balatri et al 2002;Benini et al 2002;Bettelli et al 2002;Papani et al 2002;Gasperi et al 2005aGasperi et al , 2005bMartelli et al 2009;Remitti et al 2011) show a significant regional scale distribution (about more than 300 km long by 10-15 km wide) in almost all the outer part of the present-day Northern Apennines (Fig.…”

Carnian reefal blocks in the Slovenian Basin, eastern Southern Alps

“…The presently observed wide size range and angularity of blocks, their random distribution in a fine-grained matrix, and the intrabasinal origin of blocks are consistent with en masse gravitational movements ("type 1" mélanges in Festa et al 2010). Their undisrupted internal structure and mostly concordant position suggest that the largest blocks slid into the basin along discrete planes.…”

Cave morphology provides evidence of polyphase tectonics - an example from the Isverna karst system, South Carpathians

“…rocks), involving tectonic, sedimentary and gravitational forming processes [5]. The Urgonian limestones are karstified, all the conduits joining to a single common resurgence: Isverna Cave, at 450 m a.s.l.…”