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A significant part of melanges 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. melanges, 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) melange fabric, forming polygenetic melanges. This ultimately complicates their distinction from true tectonic melanges, 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 melange, at various scales, around the world. These studies conclude that the understanding of this type of melange 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 melanges: a glimpse on tectonic sedimentary and diapiric recycling in convergent margins’ collection available at https://www.lyellcollection.org/cc/polygenetic-melanges

Upper Lutetian-Bartonian sedimentary mélanges, corresponding to ancient mud-rich submarine mass transport deposits, are widely distributed over an area c. 300 km long and tens of kilometres wide along the exhumed outer part of the External Ligurian accretionary wedge in the Northern Apennines. The occurrence of methane-derived carbonate concretions (septarians) in a specific tectonostratigraphic position below these sedimentary mélanges allows us to document the relationships among a significant period of regional-scale slope failure, climate change (the Early and Mid-Eocene Optimum stages), the dissociation of gas hydrates and accretionary tectonics during the Ligurian Tectonic Phase (early-mid-Lutetian). The distribution of septarians at the core of thrust-related anticlines suggests that the dissociation of gas hydrates was triggered by accretionary tectonics rather than climate change. The different ages of slope failure emplacement and the formation of the septarians support the view that the dissociation of gas hydrates was not the most important trigger for slope failure. The latter occurred during a tectonic quiescence stage associated with a regressive depositional trend, and probably minor residual tectonic pulses, which followed the Ligurian Tectonic Phase, favouring the dynamic re-equilibrium of the External Ligurian accretionary wedge. Our findings provide useful information for a better understanding of the factors controlling giant slope failure events in modern accretionary settings, where they may cause tsunamis.

Olistostrome and sedimentary mélange are two synonymous genetic terms referring to the “fossil” products of ancient submarine mass‐transport processes exhumed in orogenic belts. Lithology, stratigraphy, lithification degree, and structural anatomy of these units reflect the synergic and combined action of different mass‐transport processes leading to composite deposits developed through multistage deformation phases. The general depositional physiography, tectonic setting, and the type, scale, and rate of slide mass transformation mechanisms during the downslope motion and emplacement and postdepositional processes are the main factors controlling the final internal anatomy of olistostromes and sedimentary mélanges. These features are commonly progressively reworked by subsequent burial, diapiric, and tectonic processes and may be eventually almost completely obliterated by metamorphic processes during orogenic belt and/or subduction complex evolution. The correct recognition of olistostromal units and their intrinsic features in different orogenic belts needs extensive and careful fieldwork and ultimately provides excellent proxies for the timing of various tectonic‐sedimentary events interacting during the Wilson cycle. The basic concepts of structural geology, sedimentology, stratigraphy, and basin analysis should be jointly applied in studying the internal structure, lithological arrangement, and formation-deformation mechanisms of olistostromes and sedimentary mélanges.

The Paleogene &ldquo, megabeds&rdquo, of the Julian-Slovenian Basin are regional, basin-wide deposits, produced by catastrophic carbonate platform collapses. They record the emplacement of a bipartite slide mass behaving as a cohesive blocky/debris flow in the lower part, and as a grain to turbulent flow in the upper part. Several types of primary (sedimentary) soft sediment deformation structures testify fluid overpressure conditions during emplacement. Such structures are identified within a brecciated, fine grained matrix that encloses and intrudes slide blocks and clasts, characterized by NE-, NW- and SW-directed paleo-transport directions, indicating a depositional setting close to the basin margins. Here we present an updated review of some representative megabeds, exposed in the open-pit quarry outcrops of Anhovo (SW Slovenia). In particular, we here discuss new interpretations based on X-ray fluorescence spectrometry (XRF), thermo-gravimetry (TG) and electric resistivity tomography (ERT). Our results indicate that basal marly clasts of the megabeds are markedly different from the uppermost draping marls, suggesting two different coeval sources. The relationships with the underlying successions are strongly erosive, with deep localized scouring of the substrate and amalgamations between different megabeds, and the depositional units inside individual megabeds, supporting the geochemical differences.

Multiple episodes of deformation during the tectonic evolution of orogenic belts and ancient subduction-accretion complexes cause obfuscation of primary block-in-matrix fabric of melanges, and thereby making the recognition of their tectonic, sedimentary or diapiric origin difficult. Here we present a comprehensive overview and synthesis of a diverse set of field-based stratigraphic and structural criteria, which are at the base of geological mapping rules, to differentiate between various melange types, developed by disparate geological processes and mechanisms. We first define the current concepts of melange and melange nomenclature, and describe the most diagnostic features of tectonic, sedimentary and diapiric melanges at different scales. We discuss some of the main issues complicating the application of these diagnostic criteria, such as: (i) similarities between the block-in-matrix fabric of different melange types formed in partially lithified sediments at shallow structural levels, (ii) transformation of fabric elements with increased depth due to tectonic reworking and recrystallization processes, (iii) significance of “exotic” versus “native” blocks in melange matrix, and (iv) age relationships between blocks and matrix in a melange. We introduce two additional criteria in approaching these complexities and in recognizing different processes of polygenetic melanges formation in the field when primary diagnostic fabrics were reworked by multiple deformational events. These new criteria are based on (i) the coherence between lithological compositions of melange components (blocks and matrix) and characteristics and tectonic evolution of the geodynamic setting of their formation (“tectonic environment criterion”), and (ii) specificity and kinematic coherence in the distribution of deformation between blocks and the matrix (“deformation criterion”). The discussed diagnostic criteria can be applied to all field-based investigations of melanges and broken formations in orogenic belts and exhumed subduction-accretion complexes around the world, regardless of their location, age, and tectonic history.

We present a critical appraisal of the role of subducted, medium (10-1000 km(2)) to giant (>= 1000 km(2)) and heterogeneous, mud-rich mass transport deposits (MTDs) in seismic behavior and mechanisms of shallow earthquakes along subduction plate interfaces (or subduction channels) at convergent margins. Our observations from exhumed ancient subduction complexes around the world show that incorporation of mud-rich MTDs with a "chaotic" internal fabric (i.e., sedimentary m langes or olistostromes) into subduction zones strongly modifies the structural architecture of a subduction plate interface and the physical properties of subducted material. The size and distribution of subducted MTDs with respect to the thickness of a subduction plate interface are critical factors influencing seismic behavior at convergent margins. Heterogeneous fabric and compositions of subducted MTDs may diminish the effectiveness of seismic ruptures considerably through the redistribution of overpressured fluids and accumulated strain. This phenomenon possibly favors the slow end-member of the spectrum of fault slip behavior (e.g., Slow Slip Events, Very Low Frequency Earthquakes, Non-Volcanic Tremors, creeping) compared to regular earthquakes, particularly in the shallow parts (T

The Paleogene carbonate “megabreccia” units of the Friuli Basin are composite deposits produced by catastrophic shallow-water carbonate platform collapses re-deposited in relatively deep-water inner foredeep settings developed at the front of the advancing Dinaric thrust system. These thick, basin-wide mass transport deposits (MTDs) record the catastrophic emplacement of bipartite slide masses, comprising a lower coherent/cohesive blocky flow and an upper grain/turbulent flow. We here present the results of micro- to outcrop-scale structural analyses, constrained by stratigraphic and sedimentologic observations, performed to identify the internal deformation mechanisms and the emplacement processes of four of the largest MTDs exposed in two large three-dimensional outcrops: the Vernasso (NE Italy) and Anhovo (W Slovenia) open-pit quarries. Our results reveal a variety of primary (sedimentary) soft sediment deformation structures testifying fluid overpressure conditions within the brecciated, fine-grained matrix that sustain, intrude and surround slide blocks and clasts. Meso-scale structural analyses unraveled paleo-transport directions toward the N for the Vernasso quarry and toward the S for the Anhovo quarry. This suggests a forced propagation of the mass transport events controlled by the shape of basin, and reinforces the interpretation of different source areas related to multiple collapses from a carbonate platform rimming the southeastern tip of the basin. These units are thought to represent exhumed fossil examples of the MTDs extensively mapped in the present-day, carbonate-dominated continental margins, and thus, considered as reliable analogues for integrated studies.

The terms melange and broken formation have been used in different ways in the literature. The lack of agreement on their definition often leads to confusion and misinterpretations. An evaluation of the various uses of these terms allows us to consider several types of chaotic rock bodies originated by tectonic, sedimentary and diapiric processes in different tectonic settings. Our review of stratal disruption and mixing processes shows that there exists a continuum of deformation structures and processes in the generation of melanges and broken formations. This continuum is directly controlled by the increase of the degree of consolidation with burial. In tectonically active environments, at the shallow structural levels, the occurrence of poorly consolidated sediments favors gravitational deformation. At deeper structural levels, the deformation related to tectonic forces becomes gradually more significant with depth. Sedimentary (and diapiric) melanges and broken formations represent the products of punctuated stratal disruption mechanisms recording the instantaneous physical conditions in the geological environment at the time of their formation. The different kinematics, the composition and lithification degree of sediments, the geometry and morphology of the basins, and the mode of failure propagation control the transition between different types of mass-transported chaotic bodies, the style of stratal disruption, and the amount of rock mixing. Tectonically broken formations and melanges record a continuum of deformation that occurs through time and different degrees of lithification during a progressive increase of the degree of consolidation and of the diagenetic and metamorphic mineral transformation. Systematic documentation of the mechanisms and processes of the formation of different broken formations and melanges and their interplay in time and space are highly important to increase the understanding of the evolutionary history of accretionary wedges and orogenic belts. (c) 2012 Elsevier B.V. All rights reserved.

© 2014 © 2014 Taylor & Francis. In the Northern Apennines of Italy, mud-rich olistostromes (sedimentary mélanges) occur at different stratigraphic levels within the late Oligocene-early Miocene sedimentary record of episutural/wedge-top basins. They are widely distributed along the exhumed outer part of the Ligurian accretionary complex, atop the outer Apenninic prowedge, over an area about 300 km long and 10-15 km wide. Olistostromes represent excellent examples of ancient submarine mass-transport complexes (MTCs), consisting of stacked cohesive debris flows that can be directly compared to some of those observed in modern accretionary wedges. We describe the internal arrangement of olistostrome occurrences in the sector between Voghera and the Monferrato area, analysing their relationships with mesoscale liquefaction features, which are commonly difficult to observe in modern MTCs. Slope failures occurred in isolated sectors along the wedge front, where out-of-sequence thrusting, seismicity, and different pulses of overpressured tectonically induced fluid flows acted concomitantly. Referring to the Northern Apennines regional geology, we also point out a gradual lateral rejuvenation (from late Oligocene to early Miocene) toward the SE and an increasing size and thickness of the olistostromes along the strike of the frontal Apenninic prowedge. This suggests that morphological reshaping of the outer prowedge via mass-transport processes balanced, with different pulses over a short time span, the southeastward migration and segmentation of accretionary processes. The latter were probably favoured by the occurrence in the northwestern part of the Northern Apennines of major, inherited palaeogeographic features controlling the northward propagation of the prowedge. Detailed knowledge of olistostromes, as ancient examples of MTCs related to syn-sedimentary tectonics and shale diapirism, and of their lateral variations in term of age and size, provides useful information in regard to better understanding of both the tectono-stratigraphic evolution of the Apenninic prowedge and the submarine slope failures in modern accretionary wedges.

We present the integrated outcrop-geophysical study of two mass transport complexes, the exhumed Specchio unit in the Northern Apennines of Italy and the Holocene Poverty unit in the Hikurangi margin of New Zealand. The combination of micro- to meso-scale structural, stratigraphic and sedimentologic analyses carried on continuous three-dimensional outcrops, with large-scale structural and morphologic data deriving from seismic/acoustic imaging of the present-day continental margins, allow important considerations on submarine landslide processes and mechanisms through the broader (up-scaled and down-scaled) understanding of the mass transport-related structural associations. We compare the discontinuous high-amplitude, reverse-polarity reflectors observed within the Poverty with the syn-sedimentary, ductile shear zones found within the Specchio mass transport complex. The seismic signature of such structures suggests localized fluid overpressure along detachment/thrust zones due to shearing and loading of undrained, water-saturated, fine-grained material, developed along with the slide mass movement. The outcrop expression of these structures is tentatively attributed to m- to tens of m-thick shear zones comprising large amounts of sedimentary matrix which separate and accommodate the differential movements of the internal slide components (e.g. slide blocks, olistoliths). The sedimentary matrix is an unsorted, lithologically mixed medium characterized by a scale-invariant “block-in-matrix” fabric (i.e. brecciated, mud-supported), that injects, sustain and surrounds discrete slide elements (from particles to blocks) and interpreted as a hyper-concentrated (liquefied/fluidized) suspension of water and scattered sediments developed in fluid overpressure conditions. We highlight the fundamental role of shearing-related liquefaction as one of the main factors controlling slide mobility through the “lubrication” of the internal and basal friction forces. The analysis of such features can therefore provide important information for the characterization of mass transport deposits developed from potentially catastrophic, long run-out mass transport events, and consequently, to better understand their possible socio-economic impact in terms of tsunamigenic potential.

We report here mass transport-related disruption processes and their artifacts within sedimentary m??langes. The case studies include the early Oligocene wedge-top mass transport deposits in the northern Apennines (Italy) and the Eocene foredeep carbonate megabreccias from the south-central Pyrenees (Spain). These "chaotic" units commonly share a block-in-matrix fabric expressed by variously deformed slide blocks of different size, lithology, age and shape, embedded in a fine-grained matrix. Geophysical studies of modern continental margins have characterized many of these deposits, which, however, remain still relatively poorly described in term of meso-scale characteristics. The prominent feature of the analyzed mass transport deposits is the occurrence of an unsorted, strongly mixed, relatively fine-grained clastic matrix, infilling space between large clasts and blocks. This matrix shows either fluidal structures related to simple shear, or a structureless, homogeneous fabric, both probably related to liquefaction/fluidization processes and thus, to the internal strength of the mixed lithologies. We interpreted this phase as a liquefied mixture of water and sediment, characterized by high mobility due to overpressured conditions, as evidenced by both lateral and vertical injections. On a much larger scale this kind of matrix could represent the acoustically "transparent" facies separating slide blocks of many seismic examples. The inferred generating mechanism is that of a progressive soft sediment deformation, linked to different phases of submarine landslide evolution (i.e. failure, translation, accumulation and post-depositional stages), leading to an almost complete stratal disruption of involved bedded sequences, either within the slide mass and in the underlying substrate. In this framework the down-slope movement is favored by the development of ductile, overpressured shear zones, both internally and along the basal sliding horizon. Therefore, this matrix signature represents a possible discriminating factor to separate sedimentary and tectonic m??langes within accretionary systems and, moreover, to distinguish fast- from slow-rated generating processes. ?? 2011 Elsevier B.V.

Although the 1978 Penrose Conference stated that “Mélanges are mappable, internally fragmented and mixed rock bodies” formed by “tectonic movements, sedimentary sliding, or any combination of such processes”, an exclusive origin from tectonic processes is often aprioristically assumed. Due to the recent advances in the investigating methods and the increasing work of comparison between mélanges in mountain ranges world-wide, an increasingly larger number of sedimentary mélanges is recognized. Although strongly deformed by post-depositional, tectonic and/or mud-diapiric processes, these mélanges prevailingly originated from sedimentary processes. Moreover, the chaotic sedimentary bodies known as olistostromes in the literature fall inside the more general category of sedimentary mélanges, further increasing their relative abundance and diffusion. Sedimentary mélanges are prevailingly related to mass-transport processes. They often are, therefore, excellent fossil examples of mass-transport complexes (MTC), cropping out in well preserved and continuously exposures. This communication will show the results of the holistic study of both sedimentary mélanges and more classic cases of fossil MTC, with a focus on the Apennines of Italy. Fossil MTC, especially the large-scale, basin-wide ones, are complex units involving the entire spectra of mass-wasting processes and often originated by retrogressive sliding of basin margins up to the shelf. The translation of the bodies is enabled by the relative movement of discrete masses, with progressive stratal disruption of rocks/sediment involved and flow transformation. Fluid overpressure inside either diffuse or concentrated shear zones enable these movement and trigger the stratal disruption. Detailed information on internal geometry and structures and on the mechanisms of deformation can be gathered from fossil MTC, which are less readily achieved by submarine geophysical studies of modern submarine landslides. The synergic study of modern and fossil MTB should result in a better knowledge of mass-transport processes and bodies, in relation with the basin floor geometries.

In the peri-Adriatic region, mélanges represent a significant component of the Apennine and Dinaride-Albanide-Hellenide orogenic belts as well as ancient and present-day accretionary wedges. Different mélange types in this broad region provide an excellent case study to investigate the mode and nature of main processes (tectonic, sedimentary, and diapiric) involved in mélange formation in contrasting geodynamic settings. We present a preliminary subdivision and classification of the peri-Adriatic melanges based on several years of field studies on chaotic rock bodies, including detailed structural and stratigraphic analyses. Six main categories of melanges are distinguished on the basis of the processes and geodynamic settings of their formation. These mélange types are spatially and temporally associated with extensional tectonics, passive margin evolution, strike-slip tectonics, oceanic crust subduction, continental collision, and deformation. There appears to have been a strong interplay and some overlap between tectonic, sedimentary, and diapiric processes during mélange formation; however, in highly deformed regions, it is still possible to distinguish those melanges that formed in different geodynamic environments and their main processes of formation. This study shows that a strong relationship exists between melange-forming processes and the palaeogeographic settings and conditions of mélange formation. Given the differences in age, geographic location, and evolutionary patterns, we document the relative importance of mélanges and broken formations in the tectonic evolution of the peri-Adriatic mountain belts.