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The External Ligurian Units in western Monferrato (NW-Italy) have been always described as an undifferentiated chaotic complex. This map, at 1:10,000 scale, describes in detail the tectono-stratigraphic setting of these Units in the sector of the Alps–Apennines junction. Here, the External Ligurian Units represent the northwestern prolongation of the Northern Apennines and consist of a Late Cretaceous chaotic succession represented by theArgille varicoloriand the overlaying Monte Cassio Flysch. The late Eocene–Miocene episutural succession of the Tertiary Piedmont Basin rests unconformably on the External Ligurian Units. The mapped crosscutting relationships between stratigraphic unconformities and faults allow us to describe a complex tectono-stratigraphic setting that is the product of four tectonic stages. Layer-parallel extension related to Late Cretaceous–early Eocene deformation occurred in the internal sector of the Alpine accretionary wedge and is preserved within the External Ligurian Units which is sealed by the late Eocene deposits of the Tertiary Piedmont Basin. The unconformity at the base of the Oligocene succession records the drowning of shelf sediments controlled by NW-striking left-lateral transtensive faulting. A WNW-striking and NE-verging thrust superposes the External Ligurian Units onto the late Eocene–Oligocene deposits and it is sealed by the gravitational emplacement of late Oligocene Polygenetic argillaceous breccias. Both the WNW-striking thrust and the Polygenetic argillaceous breccias are cut by NW-striking right-lateral transpressive faults that are, in turn, sealed by the Tortonian unconformity. [ABSTRACT FROM AUTHOR]

Abstract: The Hamburg Klippe of the Central Appalachian orogenic belt exposed in eastern Pennsylvania displays a complex record of poly-phase mélange and broken formation development in a convergent margin setting. It includes an imbricate stack of tectonic slices, which consist of upper Cambrian to Upper Ordovician deep-water and continental slope sedimentary rocks, emplaced by gravity sliding onto the Laurentian passive margin during deposition of the Upper Ordovician Martinsburg Formation. Based on their internal structure and stratigraphy, the block-matrix ratios and relations, and the inferred tectonic settings of origin, we have differentiated the following mélanges and broken formations, whose evolutionary stages coincide with specific deformational phases in a complete orogenic cycle from subduction to collision: (i) sedimentary broken formations without exotic blocks, formed by in situ and local down-slope remobilization during the early stages of closure of the Octoraro Sea basin in the Early Ordovician; (ii) two types of sedimentary mélanges with exotic blocks, formed at the front of the advancing accretionary wedge during the Middle Ordovician subduction of the Laurentian continental margin beneath a microcontinent–magmatic arc tectonic assembly; (iii) layer-parallel, extensional broken formation and a diapiric mélange, formed in the outer trench and at the toe of the accretionary wedge, respectively, during the early–Late Ordovician; (iv) precursory olistostromes, formed during the Late Ordovician collisional episodes as the Hamburg Klippe was emplaced in the Martinsburg Formation on the downgoing Laurentian continental margin; and, (v) contractional deformation-related broken formations, formed at the base of main thrust faults overprinting the previously formed mélanges and broken formations. This sequential development of different mélange types in the Central Appalachians was strongly controlled by the degree of consolidation of the layered strata and their rheological differences, the structural level of mélange formation within the accretionary wedge, and the kinematics of deformational processes. [Copyright &y& Elsevier]

Abstract: The Argille varicolori (Varicolored scaly clays) of the External Ligurian Units of the Northern Apennines have been widely described as a typical unmetamorphosed broken formation (i.e., a chaotic unit without exotic blocks), produced by offscraping and tectonic imbrication during the evolution of the Ligurian accretionary wedge. Geological mapping and integrated structural and stratigraphic observations show that the Argille varicolori consist of diverse types of small-scale mélanges (non-mappable at a 1:25,000 scale) forming a composite chaotic unit, in which the superposition of tectonic, sedimentary and diapiric processes resulted in the occurrence of polygenetic chaotic bodies at different scales. These mélange units record the evolution of the Ligurian accretionary wedge from subduction to collision and intracontinental deformation. Tectonically Disrupted Body 1 (TDB1) comprises boudinage and pinch-and-swell structures formed by layer-parallel extension/contraction at the wedge front of the Ligurian accretionary complex during the late Cretaceous–middle Eocene. It is interleaved with non-mappable Gravity-driven Chaotic Bodies (GCB) developed during alternating episodes of accretion and removal of material at the wedge front. The late Oligocene–early Miocene out-of-sequence thrusting related to the collisional episodes in the Apennines overprinted the previously formed chaotic bodies and formed a polygenetic tectonic mélange (Tectonically Disrupted Body 2, TDB2). This unit is characterized by a structurally ordered block-in-matrix fabric and by the gradual decrease of stratal disruption away from the regional thrust. Overpressurized fluids concentrated along the shear surfaces, and the scaly cleavage planes facilitated the diapiric upward movement of unconsolidated sediments in the early Miocene. This process produced non-mappable shale dike injections (DDB1) and mappable Diapirically Disrupted Bodies (DDB2), which show an internal zonation of deformation. This deformation reworked the previously formed chaotic bodies. Although some of these polygenetic mélanges cannot be mapped at a 1:25,000 scale, their careful documentation provides a better understanding of time-progressive, scale-independent mélange-forming processes. [Copyright &y& Elsevier]

Melanges 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 melange 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 melange concept and investigates the relationships between melange types and their tectonic settings of formation. We investigate the contribution of mass-transport versus contractional deformation processes at the onset of melange formation and throughout the evolution of different melange types, and the nature of the continuum and transition from broken formations to true tectonic melanges. A melange 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 melanges and modern tectonic environments, where melange-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 melange 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 melange formation in a single tectonic setting. Tectonic melanges are commonly subordinate to broken formations and are restricted to narrow, elongated-to-coalescent fault zones, large-scale fault zones, and plate boundaries. [ABSTRACT FROM AUTHOR]

In the peri-Adriatic region, melanges represent a significant component of the Apennine and Dinaride-Albanide-Hellenide orogenic belts as well as ancient and present-day accretionary wedges. Different melange 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 melange 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 melange 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 melange 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 melange formation. Given the differences in age, geographic location, and evolutionary patterns, we document the relative importance of melanges and broken formations in the tectonic evolution of the peri-Adriatic mountain belts. [ABSTRACT FROM AUTHOR]

The Sestola Vidiciatico tectonic Unit (SVU) accommodated the early Miocene convergence between the subducting Adriatic plate and the overriding Ligurian prism, and has been interpreted as a field analogue for the shallow portion of subduction megathrusts. The SVU incorporated sediments shortly after their deposition and was active down to burial depth corresponding to temperatures around 150 °C. Here, we describe the internal architecture of the basal thrust fault of the SVU through a multi-scale structural analysis and investigate the evolution of the deformation mechanisms with increasing burial depth. At shallow depth, the thrust developed in poorly lithified sediments which deformed by particulate flow. With increasing depth and lithification of sediments, deformation was accommodated in a meter scale, heterogeneous fault zone, including multiple strands of crack-and-seal shear veins, associated with minor distributed shearing in clay-rich domains and pressure solution. In the last stage, slip localized along a sharp, 20 cm thick shear vein, deactivating the fault zone towards the footwall. The widespread formation of crack-and-seal shear veins since the first stages of lithification indicates that failure along the thrust occurred at high fluid pressure and low differential stress already at shallow depth. Progressive shear localization occurs in the last phases of deformation, at temperatures typical of the transition to the seismogenic zone in active megathrusts. [ABSTRACT FROM AUTHOR]