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The Jurassic ophiolites in the Northern Apennines and the Western Alps represent fossil mid-ocean ridge (MOR) oceanic lithosphere that formed in the Mesozoic Ligurian-Piedmont Ocean Basin (LPOB). Their sedimentary covers include chaotic rock units containing ophiolite-derived material. The processes of formation and the lithostratigraphic position of these chaotic units in the Western Alps remain a matter of debate, unlike their counterparts in the Northern Apennines. This is because of pervasive tectonic deformation and high-pressure metamorphism that affected their internal structure during collisional tectonics. A comparative analysis of these chaotic units in both mountain belts reveals the nature of processes involved in their formation. Chaotic deposits of gravitational origin occur both below and above the extrusive sequences in the ophiolites. They represent synextensional, hyperconcentrated deposits associated with the seafloor-spreading evolution of the LPOB lithosphere during Middle and Late Jurassic times. Mass transport deposits (MTDs) occur as intercalations within turbiditic sequences above the ophiolites. They represent syncontractional submarine slides that occurred on frontal accretionary prism slopes during the Late Cretaceous–Paleocene closure of the LPOB. The results of our comparative analysis imply that (1) the structure-stratigraphy of the chaotic deposits and MTDs of the Northern Apennines can be used as a proxy to better identify their metamorphosed and highly deformed counterparts in the Western Alps, (2) sedimentological processes associated with slow-spreading MOR tectonics and accretionary prism development in convergent-margin tectonics contributed to the sediment budgets of the cover sequences, and (3) magmatic, tectonic, and sedimentological processes that occurred during the formation of the Jurassic oceanic lithosphere and its sedimentary cover in the LPOB were remarkably uniform and synchronous. [ABSTRACT FROM AUTHOR]

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 strong morphological similitude of the block-in-matrix fabric of chaotic rock units (mélanges and broken formations) makes problematic the recognition of their primary forming-processes. We present results of the comparison between magnetic fabric and mesoscale structural investigations of non-metamorphic tectonic, sedimentary, and polygenetic mélanges in the exhumed Late Cretaceous to early Eocene Ligurian accretionary complex and overlying wedge-top basin succession in the Northern Apennines (northwest Italy). Our findings show that the magnetic fabric reveals diagnostic configurations of principal anisotropy of magnetic susceptibility (AMS) axes orientation that are well comparable with the mesoscale block-in-matrix fabric of mélanges formed by different processes. Broken formations and tectonic mélanges show prolate and neutral-to-oblate ellipsoids, respectively, with magnetic fabric elements being consistent with those of the mesoscale anisotropic "structurally ordered" block-in-matrix fabric. Sedimentary mélanges show an oblate ellipsoid with a clear sedimentary magnetic fabric related to downslope gravitational emplacement. Polygenetic mélanges show the occurrence of a cumulative depositional and tectonic magnetic fabric. The comparison of field and laboratory investigations validate the analysis of magnetic features as a diagnostic tool suitable to analytically distinguish the contribution of different mélange forming-processes and their mutual superposition, and to better understand the geodynamic evolution of subduction-accretion complexes. [ABSTRACT FROM AUTHOR]

One of the most compelling evidences that megathrust subduction interfaces are weak (e.g. Duarte et al., 2015), is the devastating 2011 Tohoku-Oki megaquake, whereby the megathrust accomplished a nearly complete stress drop (Hasegawa et al., 2011; Brodsky et al., 2017) and a co-seismic switching of the maximum and minimum stress orientations (Hasegawa et al., 2012).We studied the calcite veins of an exhumed thrust fault coming from the base of the Sestola Vidiciatico Tectonic Unit shear zone (SVU, Northern Apennines of Italy), interpreted as a field analogue of the shallow portion of subduction megathrusts (Tmax ̴ 100°-150° C) (Vannucchi et al., 2008). Observed crosscutting relationships suggest a cyclical formation of incompatible deformation features: extensional veins exploiting pre-existent cleavage planes at low-angle to the thrust, the main low-angle thrust faults associated with mixed hybrid-shear and implosion-breccia type veins and high-angle extension veins coupled with thrust-parallel pressure solution cleavage (Mittempergher et al., 2018).To characterize the type of fluids involved, we measured trace elements and Rare Earth Elements (REEs) concentrations by laser ablation-inductively coupled plasma-mass spectrometry (LA-ICPMS) in calcite veins and their wall rock, finding marked difference between REE patterns of extension and shear veins, suggesting different sources of fluid for each type of vein. In particular, a strongly positive Eu2+ anomaly recorded exclusively in the shear veins can derive from an exotic fluid, different in redox conditions and likely higher in temperature. To the contrary, extension veins REE patterns point out for a very local fluid circuit and source.All these microstructural and geochemical features suggest a cyclical shifting of the principal σ1 and σ3 stresses through time. This was controlled by interrelated changes in permeability, fluid pressure and composition and easily achieved thanks to the low differential stress, which represented a necessary condition for extension and hybrid veins formation.References:Duarte J. C., Schellart W. P. and Cruden A. R., 2015. How weak is the subduction zone interface? Geophysical Research Letters, 42, 2664-2673.Hasegawa A., Yoshida K. and Okada T., 2011. Nearly complete stress drop in the 2011 Mw 9.0 off the Pacific coast of Tohoku Earthquake. Earth Planets Space, 63, 703-70.Hasegawa A., Yoshida K., Asano Y., Okada T., Iinuma T. and Ito Y., 2012. Change in stress field after the 2011 great Tohoku-Oki earthquake. Earth and Planetary Science Letters 355-356, 231-243.Brodsky E., Saffer D., Fulton P., Chester F., Conin M., Huffman K., Moore J. C. and Wu H.-Y., 2017. The postearthquake stress state on the Tohoku megathrust as constrained by reanalysis of the JFAST breakout data. Geophysical Research Letters, 44, 8294-8302.Vannucchi, P., Remitti, F. and Bettelli, G., 2008. Geological record of fluid flow and seismogenesis along an erosive subducting plate boundary. Nature, 451, 699-03.Mittempergher S., Cerchiari A., Remitti F. and Festa A., 2018. From soft sediment to fluid assisted faulting at the base of the Sestola-Vidiciatico tectonic Unit in the Northern Apennines (Italy). Geological Magazine, 155(2), 438-450. [ABSTRACT FROM AUTHOR]