Polygenetic mélanges: a glimpse on tectonic, sedimentary and diapiric recycling in convergent margins

Abstract

A significant part of mélanges recognized in exhumed convergent margins around the world has been recently documented to have chiefly originated from masse transport and subsurface remobilization and disruption (i.e. mélanges, from sedimentary and mud–serpentinite diapiric processes and from in situ fluidification–disruption). Tectonic and/or sedimentary processes occurring during subsequent multiple deformational events of convergent margin evolution commonly overprint and significantly rework the primary (sedimentary or diapiric) mélange fabric, forming polygenetic mélanges. This ultimately complicates their distinction from true tectonic mélanges, masking part of the recorded tectono-sedimentary evolution of the associated convergent margin. The contributions gathered in this thematic collection explore with different approaches (from field structural and stratigraphic observations to geophysical analyses) different types of polygenetic mélange, at various scales, around the world. These studies conclude that the understanding of this type of mélange may provide crucial insights for a more detailed interpretation of the evolution of ancient and modern convergent margins, and of processes and mechanisms triggering potential natural hazards (earthquakes and tsunamis). Case studies include the Apennines in the Central Mediterranean region, the Carpathians in Central Europe and the Nankai Prism in Japan. Thematic collection: This article is part of the ‘Polygenetic mélanges: a glimpse on tectonic sedimentary and diapiric recycling in convergent margins’ collection available at https://www.lyellcollection.org/cc/polygenetic-melanges

A tectonic carpet of Variscan flysch at the base of an unrooted accretion prism in NW Iberia: U-Pb zircon age constrains from sediments and volcanic olistoliths

“…In our reinterpretation the Figueruela dacite is an olistolith contained in a basal block-in-matrix unit placed below the San Vitero coherent primary unit and above the black Silurian condensed unit. In this section, parts of the block-in-matrix unit and the Silurian black sequence are sheared, forming a polygenetic mélange in the meaning proposed by Festa et al (2019Festa et al ( , 2020. Thus, in our general model, the Figueruela dacite belongs to the syn-orogenic LPa as proposed by González Clavijo et al (2016).…”

Is complex fault zone behaviour a reflection of rheological heterogeneity?

“…(a) Fault zone heterogeneity at shallow depths Subduction-related mélanges commonly preserve evidence for early, soft sediment deformation structures overprinted by later faults and fractures [48,49] (figure 2a). A common interpretation is therefore that subduction plate interface deformation style progressively changes from neartrench distributed, ductile deformation of unconsolidated sediments to localized, frictional sliding along faults in sedimentary rocks as the strata consolidate and lithify with progressive displacement and burial [50,[54][55][56].…”

Effects of block shape and inclination on the stability of melange bimrocks

“…Different processes may lead to the formation of melanges: the adjectives "tectonic", "sedimentary", "diapiric" and "polygenetic" (i.e. formed by the interplay and superimposition of tectonic, sedimentary and/or diapiric contributions) are therefore used to indicate their origin (Festa et al 2020).…”