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Tectonic evolution of the Ligurian accretionary wedge in Monferrato (NW Italy): new data from the analysis of tectonic, sedimentary and diapiric mélanges
No full textL’evoluzione tettonica del Monferrato è ben conosciuta a partire dagli episodi deformativi oligocenici. Al contrario, la sua evoluzione pre-oligocenica legata alle fasi di accrezione del Cretacico superiore-Eocene medio del cuneo di accrezione Ligure è ancora poco conosciuta.
Le Unità Liguri Esterne che costituiscono il substrato della successione del Bacino Terziario Piemontese in Monferrato sono conosciute come un complesso caotico indifferenziato di età Cretacico superiore-Eocene medio. Lo studio dettagliato di queste Unità ha permesso di distinguere al loro interno tre unità litostratigrafiche: le Argille varicolori (Santoniano-Campaniano), il Flysch di Monte Cassio (Campaniano superiore(?)-Maastrichtiano), e le Brecce argillose poligeniche (Oligocene superiore). Le Argille varicolori e le Brecce argillose poligeniche rappresentano il prodotto dell’interazione e sovrapposizione di processi tettonici, sedimentari e diapirici che hanno operato in momenti diversi dell’evoluzione del cuneo di accrezione Ligure formando mélange poligenici. L’analisi della deformazione e la comprensione dei rapporti di sovrapposizione tra i diversi tipi di mélanges poligenici individuati hanno permesso di: (i) distinguere più fasi deformative (Cretacico superiore, Rupeliano e Oligocene superiore); (ii) comprendere i rapporti di interazione e sovrapposizione tra processi tettonici, sedimentari e diapirici che hanno portato alla formazione di diversi tipi di mélanges poligenici; (iii) ridefinire più in dettaglio, rispetto ai dati di letteratura, i tempi della deformazione dell’intervallo di tempo Chattiano–pre Burdigaliano
Polygenetic mélanges: the interplay of tectonic, sedimentary and diapiric processes in exhumed accretionary wedges.
No full textL'interazione e sovrapposizione di processi tettonici, sedimentari e diapirici a diversi livelli strutturali di cunei di accrezione, può portare alla formazione di mélange poligenici. Il riconoscimento del ruolo avuto da ognuno di questi processi è spesso problematico a causa della forte convergenza del fabric dei loro prodotti, della sucessiva deformazione e metamorfismo che obliterano le tracce del prevalente processo di formazione, e per il fatto che questi processi possono interagire e sovrapporsi in modi complessi. In questo lavoro mostriamo come diversi mélange formati in differenti posizioni strutturali di cunei di accrezione esumati e associati bacini di wedge-top, non rappresentano il prodotto di un unico processo, come invece sono stati comunementi interpretati in letteratura. Al contrario, essi rappresentano dei mélange poligenici formati dall'interazione e sovrapposizione di processi tettonici, sedimentari e diapirici. Gli esempi discussi riguardano il Taconic Mélange della Hudson River Valey (Appalachi settentrionali), di età tardo ordoviciana, le Unità Liguri esterne di età del Cretacico superiore affioranti in Monferrato e la successione caotica messiniana del Bacino Terziaro Piemontese
Different types of Taconic mélanges s.l. and broken formations in the Hamburg Klippe (Central Appalachians, Pennsylvania).
No full textNegli Appalachi Centrali (Pennsylvania orientale), i diversi episodi deformativi dalla subduzione alla collisione continentale che hanno caratterizzato l’orogenesi taconica (Ordoviciano), hanno portato alla formazione di diversi tipi di mélanges s.l. e broken formations. Questi sono stati distinti in base alla natura ed età dei sedimenti, organizzazione interna, rapporto e relazioni tra blocchi e matrice, e contesto strutturale in cui si sono formati, in: (i) broken formations sedimentarie prive di blocchi esotici, formate in corrispondenza della transizione oceano-continente durante le prime fasi (Ordoviciano inferiore) di convergenza tra il margine passivo di Laurentia e i terreni alloctoni ubicati ad Est del bacino oceanico di Octoraro; (ii) mélanges sedimentari con blocchi esotici legati alle fasi medio ordoviciane di subduzione; (iii) broken formations legate a layer-parallel extension, e mélanges diapirici, entrambi privi di blocchi esotici, formati tra l’Ordoviciano inferiore e superiore; (iv) mélanges sedimentari con blocchi esotici legati alle fasi collisionali dell’Ordoviciano superiore; (v) broken formations prive di blocchi esotici legate a thrusting e shearing durante gli eventi tettonici post-collisionali Taconici e Alleganiani
Mélanges and mélange-forming processes: a historical overview and new concepts
No full textMélanges 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
Small-scale polygenetic mélanges in the Ligurian accretionary complex, Northern Apennines, Italy, and the role of shale diapirism in superposed mélange evolution in orogenic belts
No full textThe 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
Small-scale polygenic mélanges in the Argille varicolori of the Cassio Unit (Oltrepo Pavese, Northern Apennines)
No full textLe Argille varicolori dell’Unità Cassio (Appennino Settentrionale) sono comunemente interpretate come broken formations di origine tettonica prodotte, a partire dal Cretacico superiore, durante l’evoluzione del cuneo di accrezione ligure esterno. Il rilevamento geologico e osservazioni strutturali e stratigrafiche di dettaglio hanno messo in evidenza la natura composita di questa unità caotica che risulta costituita da diversi tipi di corpi caotici prodotti da processi tettonici, sedimentari e diapirici. Anche se parte di questi corpi caotici o mélanges s.l. non sono cartografabili, il loro riconoscimento e la comprensione dei loro processi di deformazione è importante per una dettagliata definizione dell’evoluzione strutturale (dalle fasi di subduzione a quelle collisionali e alla deformazione intracontinentale) del cuneo di accrezione ligure esterno affiorante nei settori studiati dell’Oltrepo Pavese
Calculation of lithology-specific p-wave velocity relations from sonic well logs for the po-plain area and the northern adriatic sea
No full textOne of the most useful petrophysical parameters in hydrocarbon reservoir studies is the velocity of the seismic waves propagating in the Earth’s subsurface. Seismic velocities have multiple applications in geophysical exploration, well log interpretation and petrophysical and geomechanical characterization. In this study we used publicly available well data (VIDEPI database) covering the Po Plain and the northern Adriatic areas to calculate the P-wave sonic velocity from the analysis of well profiles (1:1000 scale). Data were collected from 134 wells located inside the region of interest that included sonic log registrations. From each of the wells the cuttings interpretation log, the available spontaneous potential or gamma ray logs and the sonic log were digitized from existing profiles whereas the hydrocarbon-bearing-marker (resistivity log readings) and the geological formation log were constructed. The lithological and the geological formation logs were used to analyse the regional stratigraphy while the resistivity log was used to identify and exclude the hydrocarbon bearing intervals affecting the sonic log readings. The various lithologies reported on the well profiles were combined to characterize 9 main lithological groups (6 clastic, 1 marly, 2 carbonatic). For each group a linear regression was applied to extract the relation of velocity versus depth. The results show a gradual velocity increase with depth for most of the lithologies, while limestones and dolomites present constant velocities independently of the depth. Furthermore, at approximately 3.5-4 km the velocities of all lithologies tend to stabilise at a value that remains relatively constant even for larger depths. The results of this study can prove helpful for the construction and calibration of velocity models and for the calculation of dynamic geomechanical parameters (e.g. Young’s modulus), which are crucial for the mechanical characterization of the rock during geomechanical studies
Integrated InSAR-GPS Monitoring of an Underground Gas Storage (UGS) Field: The Minerbio Pilot Test
No full textIn 2014 the Italian Oil & Gas Safety Authority published technical guidelines (Italian Guidelines for monitoring the seismicity, ground deformation and pore pressure) to regulate the monitoring of all existing and future underground activities on the Italian territory. The guidelines suggested that ground deformation monitoring of underground activities was carried out by integrating InSAR and continuous GPS techniques to detect possible surface deformation phenomena linked to the monitored activities during the considered time interval, and to provide information of their space-time variations with respect to the background conditions. The guidelines application on underground gas storage (UGS) was tested on the Minerbio gas field under the supervision of the National Institute of Geophysics and Volcanology (INGV). At the time of the pilot test, Minerbio had a high-quality record of about 15 years of InSAR data and 10 years of CGPS data (from the MINE station). The presented case study shows how the Minerbio ground deformation monitoring system was implemented during the testing phase. The time series of MINE, sited in 2008 in a peripheral area of the field, show that it measures seasonal oscillations in the North-planar component and a certain seasonality in the East-planar component: considering such results, the InSAR data were used to identify a proper site for a second CGPS station with the aim to integrate the results of MINE. The new station MIN2 was placed in 2019 in the center of the field, near the vertical and East-planar InSAR amplitude peaks. The correlation analysis between the CGPS signals and the cumulative curve of UGS volumes of the Minerbio field indicates a strong correlation in the North-planar component for the MINE station and a strong correlation in the East-planar and vertical components for the MIN2 station. According with ILG target, such results indicate that: (i) MIN2 effectively integrates the MINE results by detecting the peak of UGS-related, vertical and East-planar oscillations, (ii) the current geometric configuration of the two-station CGPS system effectively integrates the InSAR information by giving information on the spatial distribution of the North-planar component of ground deformation, and (iii) the integrated InSAR-CGPS monitoring system provides a complete picture of the distribution of UGS-related ground deformations on the Minerbio field
Mass-transport deposits, olistostromes and mélange formation in the Ligurian accretionary complex (NW-Italy)
No full textAncient accretionary complexes are a natural laboratory to investigate the mode and nature of mass-transport processes and their mutual interactions with tectonic and diapiric processes, and the formation of different types of mélange. The External Ligurian accretionary wedge and the related wedge-top basins (Epiligurian Units) in the NW- Apennines (Italy) provide different examples of ancient MTDs emplaced during the late Cretaceous Miocene evolution of this accretionary wedge. These MTDs consist of sedimentary mélanges or olistostromes and display heterogeneous deformation controlled by the degree of sediment consolidation and the velocity of gravitational processes. Crosscutting relationships between MTDs and coherent successions, tectonic mélanges - broken formation and shaly-dykes and diapirs allow us to document their time-progressive development, the correlation with tectonic and diapiric processes, and the material redistribution forming polygenetic mélanges in the frontal part of the External Ligurian accretionary wedge.
Out-of-sequence “megathrust” and strike-slip faulting, fluid overpressure and presence of low-permeable layers in the sedimentary column were the main factors that controlled the emplacement of various MTDs. In all the examples described, mass-transport was closely associated and had mutual interactions with tectonic and diapiric processes. Tectonics played the most prominent role (directly and indirectly), whereas fluid flow and overpressure strongly controlled the mechanical behavior of sediments and facilitated the emplacement of kilometer-to-hundreds of square kilometers of MTDs. All the examples described from the External Ligurian accretionary wedge here are comparable in size and geodynamic selling types to those documented from the frontal part of several modern accretionary and/or subduction complexes (e.g., Nankai accretionary wedge), providing important information on their evolution, slope instability and hazard mitigation
Types and Evolutionary Processes of Formation of the Ordovician Taconic Mélanges in the Central and Northern Appalachian Orogenic Belt, Eastern USA.
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