In obtaining an evolutionary model of the northern Sicilian continental margin during the Quaternary, this research is concerned with the analysis of morphostructural elements, fluid escape structures, and selective and mass movement transport mechanisms, both in offshore and onshore settings, in the sector spanning between the Egadi Islands and Cefalù meridians. In particular the following have been studied in detail: a) morphostructural elements, in order to gather information pertaining to the recent evolution of the area, focusing on the recent and active faults, also integrating seismicity data of the region, taken from online databases; b) structures linked to the fluid escape, subsequent to shallow and deep processes within the investigated crustal sector, integrated with data from geochemical analysis of pore water derived from core sediment samples; c) structures produced by mass movement mechanisms and in particular those triggered by fluid migration, seeking to highlight the relationships between different mechanisms and their contribution to the recent evolution of the margin. In addition to a meticulous bibliographic search which allowed for an in-depth analysis and an up-to-date account on the subjects, pre-existing data has been used (provided by the Marine Geology group of the University of Palermo), as well as those derived from established databases and bibliographic sources, and overall newly acquired. Regarding the latter, morpho-bathymetric and single-channel seismic reflection data was collected during several oceanographic cruises between 2009 and 2013 (PUMA 09, MACS_2010, MASVE_2013) carried out in the area between the Egadi islands and Cefalù (in which the writer participated), in the context of the MaGIC project (Marine Geohazards along the Italian Coasts). The data collected was edited and/or processed accordingly before being interpreted. The multi-channel seismic reflection data acquired by ENI/AGIP and provided in digital format by the Italian Ministry of Economic Development in the context of the VIDEPI project has been inserted into a georeferred database and converted into seg-y format via a dedicated software, so as to reconstruct the surface of the M-reflector and verify the actual existence of faults in the sectors where fault escarpments, incisions, pockmark alignments and mounds have been identified through the morpho-bathymetric model. The single-channel profiles have been utilized to reconstruct the depositional architecture and the structural characteristics of the Plio-Quaternary succession, also allowing for an analysis of the structures of the morpho-bathymetric model. In addition, on a sediment core sample collected in proximity to a fluid escape structure (pockmark) in the Gulf of Palermo, at 414 m b.s.l., granulometric analysis as well as pore-water analysis have been performed; the latter aimed to chemical and isotopic characterization of the fluid phase occupying pores so as to determine its origin. The specific data obtained from online databases (http://www.idromare.it; http://www.ingv.it) and from literature sources is: meteo-marine, meteorological and sea surface current data; elements necessary to evaluate the parameters relative to submarine dynamics and seismological data (location and focal mechanisms of earthquakes). In this study neotectonic elements have been identified through interpretation of morpho-bathymetric and seismic data. Active faults, in a submarine environment, are recognized as displacements of the seafloor or shallow sedimentary layers by faults that clearly displace deeper geological strata at the same localities; evidence of fault control on late Quaternary sedimentation; displacement of the seabed or near-surface sediments where evidence of large-scale land-sliding or slumping occurs; displacement of the seafloor and deeper strata in the opposite sense to the local bathymetric gradient; evidence of morphologies on the seabed as pockmark and mound. The alignment of the earthquake epicenters with recognized seabottom and buried structures, allowed to recognize seismogenic faults, whilst focal mechanisms have permitted the reconstruction of the fault kinematics and the regional stress field. The fluid escape structures have been recognized thanks to interpretation of the morpho-bathymetric model, presence of wipeout zones, backscatter, reflector geometry, change of velocity and phase inversion in seismic reflection profiles. They have been distinguished as mounds or pockmarks. The latter have been characterized according to the following parameters: diameter, height, depth, distance from coastline and shelf edge, proximity to faults and landslides. Mass wasting structures including canyons and submarine slope failures have been studied through morpho-bathymetric analysis, conducting morphometric analyses, such as slope steepness and aspect, sinuosity index and thalweg length, estimation of volume of sediments mobilized, all key factors for understanding slope dynamics. In particular, these structures have been analyzed for their interaction with the processes related both to the ascent of fluids as well as to those related to tectonics. In addition, the longitudinal profiles of submarine canyons were compared to those present in other geodynamical settings (Carlsbad, Oceanside, East Break), so as to make inferences about the margin type and evolutionary geological model from the physical features (concave or convex) and depositional architecture. The evolution of the north-Sicilian continental margin during the Quaternary, reconstructed through analysis and interpretation of different types of data, appears linked to: • the deposition of the Quaternary succession, which is organized in depositional sequences associated with the interaction between tectonic, sedimentation and eustatic oscillations. It is made up of seaward dipping clastic and terrigenous Pleistocene deposits, truncated by an erosional surface formed during the last glacio-eustatic oscillation, on which lie the late Quaternary depositional sequence. • the recent tectonic that operates mainly by two fault systems: 1) a NW-SE trending extensional and/or transtensional system, which produces the recent articulation of the seabed, and 2) an ENE-WSW trending compressive system, highlighted by high-angle reverse faults and thrusts in accordance to those found onshore. The integrated study of the morpho-structural and seismological data has highlighted the relationship between the focal mechanisms of earthquakes and major tectonic structures recognized in the study area, both being consistent with a compressional tectonic model with NW-SE trend. Moreover, the earthquake clusters of 2002 (M 5.6 06/09/02), 2010 to 2013, located in the northern Gulf of Palermo and aligned with the ENE -WSW trending fault recognized in “La Barra High”, and the clusters aligning in NW-SE direction such as that of the earthquake of 1998 and the recent one (13/04/2012) located in the offshore Sferracavallo upper continental slope (M 4.2), aligned with the tectonic structures, which are recognized in the offshore Palermo Salient, whose focal solutions reveal mechanisms compression, which allow to define these two systems as active and especially seismogenic systems. • The fluid escaspe processes that form both positive structures (mound) and negative structure (pockmark). Pockmarks that are found exclusively on the continental slope are the result of the ascent fluids that migrate through the Quaternary clastic succession. The morphology of these structures, still well-preserved, allow to suggest a recent formation. The chemical and isotopic signal of pore water extracted from sediment core, taken at one of these structures, mainly proved that pore water is slightly enriched in heavy isotopes with respect to Mediterranean seawater V-SMOW, while the distribution profiles of EC, ion concentration (Cl, SO4, Na, K, Mg, Ca), ion/Chloride ratios (Na/Cl, K/Cl, Ca/Cl, Mg/Cl and Alk/Cl) seem to indicate the existence of an external source of fluids and the occurrence of sediment-fluids interaction processes. A possible mechanism causing pore water freshening could be to be the destabilization of gas hydrates. Mounds, identified exclusively in the continental shelf, can be divided into two types: outcropping and buried. Only the latter can advance a hypothesis on the age of formation. Their age, between 20.5 and 5.5 ka, is inferred from the dislocation of the erosion surface, which is the base of the last depositional sequence (20.5 ka) and the maximum flooding surface (5.5 ka), which seals these structures, which is dated on the basis of the correlation with the curve of Fairbanks (1989). • The mass wasting processes that are highlighted by the presence of slide scar and related landslide deposits. In this area they are formed in response to three processes : 1) tectonic activity generating rockfalls, such as those observed in the offshore of Marettimo island and Monte Pellegrino, in the north western part of the Gulf of Castellammare where landslides affect the continental shelf, and in areas where fault escarpments are recognized as at the St. Vito High, in the western sector of Banco Scuso, in the south-western sector of the Marettimo island and in the eastern sector of the Gulf of Termini Imerese; 2) the fluid migration. Failure scars have been detected just below the pockmarks in the Palermo Gulf, between the Oreto and Eleuterio canyons, and in correspondence of the Solunto and S. Vito Highs; 3) retrogressive evolution of the canyons, e.g. Cofano, Mondello, Addaura, Arenella canyons. • Phenomena of gravity and channelized erosion that modify the slope continental, in which two types of canyons are recognized: 1) those that develop from the bottom upwards, facilitated by the highest steep of the slope, with retrogressive evolution (bottom-up), and that they develop by means of landslides, which have their origin in the lower part and that advance towards the upper slope (Cofano, Mondello, Addaura , Arenella and Eleuterio in part); 2) those that develop downward trend (top-down), whose evolution is typically linked to turbidite and hyperpycnal flows, e.g. the Oreto, Castellammare, Eleuterio partly and Capo Plaia canyons) . Submarine gullies are arranged along a preferential direction, which generally coincides with that of the tectonic features. Within some of these structures, pockmarks have been observed with eroded edges. Arranged in a parallel way and in the same direction of recognized tectonic features, are found both alignments of pockmark and deeper incisions. This evidence makes it possible to hypothesize their genetic mechanism. The aligned circular structures, formed in correspondence of tectonic features, coalesce creating the grooves, the latter could initially evolve into channels and later possibly into canyons. Based on the obtained results it is possible to consider the continental margin north of Sicily as an immature margin, in which the instability is strongly related to the tectonic activity. The faults, in addition to modifying the morpho-structure of the margin, are preferential way of fluids migration, as evidenced by the formation of the pockmarks. These facilitate the triggering of landslides, which in turn may determine the evolution of retrogressive canyon.
La ricerca riguarda l’analisi degli elementi morfostrutturali, le strutture legate alle emissioni di fluidi ed i fenomeni di trasporto sia gravitativo che selettivo, in ambiente sommerso ed emerso, presenti nel settore del margine continentale nord-siciliano compreso tra i meridiani passanti per le isole Egadi e Cefalù, al fine di ottenere un modello evolutivo durante il Quaternario. In particolare sono stati studiati in dettaglio: a) gli elementi morfostrutturali, al fine di ottenere informazioni sull’evoluzione recente dell’area, concentrando l’attenzione sulle faglie recenti e attive, integrando anche le informazioni concernenti la sismicità dell’area, ricavati dai database online; b) le strutture legate alle emissioni di fluidi, che rappresentano una conseguenza dei processi superficiali e profondi all’interno del settore crostale indagato, implementate con lo studio geochimico dei fluidi estratti da carote di sedimento prelevate in corrispondenza di queste strutture; c) le forme prodotte dai fenomeni di trasporto gravitativo ed in particolare quelle innescate dalla migrazione dei fluidi, cercando di comprendere le relazioni che intercorrono tra i vari meccanismi e come questi contribuiscono all’evoluzione recente del margine. In questa ricerca, oltre ad un accurato lavoro bibliografico che ha permesso di approfondire le tematiche e di descrivere lo stato dell’arte delle stesse, sono stati utilizzati dati pregressi (acquisiti dal gruppo di Geologia Marina dell’Università di Palermo) e reperiti da database esistenti e fonti bibliografiche e dati di nuova acquisizione. In particolare tra il 2009 e il 2013 sono state effettuate diverse campagne oceanografiche (PUMA 09, MACS_2010, MASVE_2013) svolte nel settore compreso tra le isole Egadi e Cefalù (alle quali la scrivente ha partecipato), nell’ambito del progetto MaGIC (Marine Geohazards along the Italian Coasts), dove sono stati acquisiti dati morfobatimetici e di sismica a riflessione monocanale. Su questi dati sono state effettuate le procedure di editing e/o processing e l’interpretazione. I dati sismici multicanale acquisiti dall’ENI/AGIP e messi a disposizione in formato digitale dal Ministero dello Sviluppo Economico nell’ambito del progetto VIDEPI sono stati inseriti in un database georeferenziato e trasformati in formato seg-y per mezzo di software dedicati, al fine di ricostruire la superficie del riflettore M e verificare l’esistenza di faglie nei settori in cui erano stati individuati nel modello morfobatimetrico le scarpate di faglia, le incisioni, gli allineamenti di pockmark e i mound. I profili monocanale sono stati utilizzati per ricostruire le architetture deposizionali ed i caratteri strutturali della successione plio-quaternaria e per analizzare le strutture riconosciute nel modello morfobatimetrico. Inoltre sono state effettuate l’analisi granulometrica di una carota di sedimento prelevata nel Golfo di Palermo in corrispondenza di una struttura da risalita di fluidi (pockmark) alla profondità di 414 m e l’analisi geochimica dei fluidi estratti dalla stessa; quest’ultima al fine di caratterizzare chimicamente ed isotopicamente la fase fluida che occupa i pori dei sedimenti e di spiegarne l’origine. I dati ricavati da database online (http://www.idromare.it; http://www.ingv.it) e fonti bibliografiche sono in particolare: dati meteomarini (ricavati dalla rete ondametrica nazionale, boa di Capo Gallo), dati meteorologici e sulla circolazione delle correnti, elementi necessari per valutare i parametri relativi alla dinamica esogena in ambiente sottomarino e dati sismologici (localizzazione dei terremoti e soluzione dei meccanismi focali). In questo lavoro le strutture neotettoniche sono state individuate grazie all’interpretazione dei dati morfobatimetrici e dei dati di sismica a riflessione. Le faglie attive, in ambiente sottomarino, si riconoscono in base a dislocazioni del fondo mare o della coltre sedimentaria, causate da faglie che interessano anche strati più profondi; evidenza di faglie che controllano la sequenza tardo-quaternaria; dislocazioni del fondo mare o di sedimenti sub-superficiali, legate alla presenza di scivolamenti o slumping; dislocazioni del fondo mare o di strati più profondi in senso contrario rispetto al gradiente batimetrico; evidenze di morfologie sul fondo marino che testimoniano risalite di fluidi come pockmark, vulcani di fango, mound etc. L’allineamento degli epicentri dei terremoti con le strutture superficiali e sepolte riconosciute, ha permesso inoltre, di riconoscere le faglie sismogenetiche, mentre i meccanismi focali hanno consentito di ricostruire la cinematica delle faglie e il campo regionale degli sforzi. Le strutture da risalita di fluidi sono state riconosciute attraverso l’interpretazione del modello morfobatimetrico, il riconoscimento delle zone di Wipeout (sbiancamento), l’alta riflettività (backscatter), la geometria dei riflettori, le variazioni di velocità e l’inversione di fase nei profili sismici a riflessione. Esse sono state distinte in strutture in rilievo (mound) e strutture depresse (pockmark). I pockmark sono stati parametrizzati in funzione di: diametro, altezza, profondità, distanza dalla linea di costa e dal ciglio della piattaforma, vicinanza dalle faglie e dalle frane e orientazione quando si presentano allineati. Le strutture da trasporto gravitativo che includono canyon e frane sottomarine sono state studiate attraverso l’interpretazione dei dati morfobatimetrici, effettuando analisi morfometriche, come la pendenza dei versanti, l’indice di sinuosità, la lunghezza del thalweg e l’esposizione rispetto alle correnti sottomarine, la stima dei volumi dei sedimenti mobilizzati, fattori chiave per la comprensione della dinamica dei pendii. In particolare queste strutture sono state analizzate per valutare la loro interazione sia con i processi legati alla risalita di fluidi che con quelli legati alla tettonica. Inoltre è stato analizzato l’andamento dei profili longitudinali dei canyon sottomarini e messo a confronto con quelli di canyon (Carlsbad, Oceanside, East Break) presenti in altri contesti geodinamici, per valutare dalla forma del profilo (concava o convessa) e dalle architetture deposizionali il tipo di margine e il modello geologico evolutivo. L’evoluzione del margine continentale nord-siciliano durante il Quaternario, ricostruita attraverso l’analisi e l’interpretazione di dati di diversa tipologia, risulta essere legata: • alla deposizione della successione quaternaria, che è organizzata in sequenze deposizionali legate all’interazione tra tettonica, sedimentazione ed oscillazioni eustatiche. Essa è costituita da depositi clastico-terrigeni pleistocenici immergenti verso mare, troncati nel settore di piattaforma dalla superficie di erosione, formatasi durante l’ultima oscillazione glacio-eustatica, sulla quale poggiano i corpi sedimentari della sequenza deposizionale tardo-quaternaria. • Alla tettonica recente che agisce prevalentemente secondo due sistemi: 1) un sistema distensivo e/o transtensivo orientato NO-SE, che crea una morfologia del fondo mare articolata, nella quale si riconosce un’alternanza di alti e bassi strutturali che individuano nel loro complesso una serie di blocchi tiltati; 2) un sistema compressivo o transpressivo orientato ENE-OSO, evidenziato dalla presenza di faglie inverse ad alto angolo e da sovrascorrimenti come ad esempio nel settore della Barra, in accordo con quelli trovati nel settore emerso. Gli stessi trend sono stati riconosciuti nel settore onshore, dove si osserva la dislocazione dei depositi sin e post tirreniani, e la differente quota a cui sono stati ritrovati i terrazzi recenti. Lo studio integrato dei dati morfo-strutturali e sismologici ha permesso di evidenziare i rapporti tra i meccanismi focali dei terremoti e le principali strutture tettoniche riconosciute nell’area di studio, risultando entrambi coerenti con un modello tettonico compressivo con trend NO-SE. Inoltre i cluster degli epicentri dei terremoti del 2002 (6/9/02 M 5.6), del 2010-2013, localizzati a nord del Golfo di Palermo e allineati con la faglia che borda l’alto morfologico della Barra in direzione ENE- OSO, ed i cluster allineati in direzione NO-SE come quello del terremoto del 1998 e il recente (13/04/2012) localizzato sulla scarpata nella zona antistante Sferracavallo (M 4,2), allineati con le strutture tettoniche, che si riconoscono nell’offshore dei Monti di Palermo, le cui soluzioni focali rivelano meccanismi compressivi, permettono di definire questi due sistemi come sistemi attivi e soprattutto sismogenetici. • Ai fenomeni di risalita di fluidi che si manifestano attraverso strutture sia in rilievo (mound) che depresse (pockmark). I pockmark, che si impostano esclusivamente sulla scarpata continentale sono il risultato della risalita di fluidi che migrano attraverso la successione clastica quaternaria. L’espressione morfologica ancora ben conservata di queste strutture, che non presentano evidenze di erosione (come normalmente riportato in letteratura), permette di ipotizzare una genesi recente. La loro distribuzione areale coincide con gli alti del substrato messiniano. Il segnale chimico ed isotopico delle acque interstiziali, estratte dalla carota di sedimento, prelevata in corrispondenza di una di queste strutture, mostra prevalentemente un arricchimento degli isotopi pesanti rispetto allo standard di riferimento V-SMOW, e piccole variazioni nelle concentrazioni dei cloruri e del sodio, che risultano il 10% inferiori, rispetto a quelle delle acque del Mediterraneo. Questi valori, sono compatibili con la presenza di un fluido a salinità inferiore, rispetto all’acqua di mare, ed arricchito in isotopi pesanti, per cui è possibile ipotizzare la presenza di fluidi, provenienti dalla disidratazione dei gas idrati. I mound, individuati esclusivamente nella piattaforma continentale, si presentano in due tipologie: affioranti e sepolti. Solo su questi ultimi è possibile avanzare un’ipotesi sull’età di formazione. La loro età, compresa tra 20.5 e 5.5 ka, viene indicata, dalla presenza della dislocazione della superficie di erosione, che limita alla base l’ultima sequenza deposizionale (20.5 ka) e dalla superficie di maximum flooding (5.5 ka), che sigilla queste strutture, e che viene datata sulla base della correlazione con la curva di Fairbanks (1989). • Ai fenomeni franosi che vengono evidenziati dalla presenza di nicchie di frana e depositi di accumulo. In quest’area essi si sviluppano in risposta a tre processi: 1) attività tettonica, generando frane di crollo, come quelle che si osservano nell’offshore nord-orientale di Marettimo e Monte Pellegrino, dove si riconoscono gli accumuli costituiti da blocchi di dimensioni decametriche, nel settore nord occidentale del Golfo di Castellammare, dove i fenomeni franosi interessano la piattaforma continentale, e nelle aree dove si riconoscono alte scarpate di faglia, come in corrispondenza dell’Alto di S. Vito, nel settore occidentale di Banco Scuso, nel versante sud-occidentale dell’isola di Marettimo e nel settore orientale del Golfo di Termini; 2) alla risalita di fluidi. Le nicchie di frana sono state trovate a valle dei pockmark nel Golfo di Palermo, nel settore compreso tra i canyon Oreto ed Eleuterio, in corrispondenza di Monte Solunto e nell’Alto di S.Vito; 3) evoluzione retrogressiva dei canyon, come nel caso dei canyon Cofano, Mondello, Addaura, Arenella e in parte Eleuterio. • Ai fenomeni di erosione gravitativa e da incisioni canalizzate che modificano la morfologia della scarpata, sulla quale sono stati riconosciuti due tipologie di canyon: 1) quelli che si sviluppano dal basso verso l’alto, agevolati dalla più alta inclinazione della scarpata, con evoluzione retrogressiva (bottom-up), e cioè che si sviluppano per mezzo di fenomeni franosi, che hanno origine nella parte inferiore, e che avanzano verso la scarpata superiore (Cofano, Mondello, Addaura, Arenella ed Eleuterio in parte); 2) quelli che si sviluppano dall’alto verso il basso (evoluzione top-down), la cui evoluzione è tipicamente legata ai processi torbiditici, innescati dall’attività dei sistemi fluviali presenti a monte (Oreto, Castellammare, Capo Plaia ed Eleuterio in parte). Le incisioni canalizzate, tranne nel settore del Golfo di Castellammare, si dispongono lungo una direzione, che generalmente coincide con quella dei lineamenti tettonici. All’interno di alcune di queste incisioni, sono stati osservati i pockmark, che presentano orli erosi. Disposti parallelamente, e in direzione dei trend tettonici riconosciuti, si ritrovano sia allineamenti di pockmark, che incisioni più profonde. Questa evidenza permette di ipotizzare il loro meccanismo genetico. Le strutture circolari allineate, formatesi in corrispondenza di lineamenti tettonici, coalescono creando i solchi, questi sia per effetto di risalite continue, che per effetto della dinamica esogena, possono svilupparsi, evolvendo prima in canali e poi forse in canyon. Sulla base dei risultati ottenuti è possibile considerare il margine continentale nord siciliano come un margine immaturo, nel quale l’instabilità è fortemente legata all’attività tettonica. Le faglie, oltre a modificare l’articolazione del margine, facilitano la risalita dei fluidi, evidenziata dalla formazione dei pockmark. Questi ultimi agevolano a luoghi l’innesco dei fenomeni franosi, che a loro volta possono determinare l’evoluzione retrogressiva dei canyon.
Pennino, . (2014). PROCESSI ATTIVI NEL MARGINE CONTINENTALE DELLA SICILIA NORD-OCCIDENTALE: NEOTETTONICA, RISALITA DI FLUIDI E FENOMENI GRAVITATIVI.
PROCESSI ATTIVI NEL MARGINE CONTINENTALE DELLA SICILIA NORD-OCCIDENTALE: NEOTETTONICA, RISALITA DI FLUIDI E FENOMENI GRAVITATIVI
PENNINO, Valentina
2014-02-25
Abstract
In obtaining an evolutionary model of the northern Sicilian continental margin during the Quaternary, this research is concerned with the analysis of morphostructural elements, fluid escape structures, and selective and mass movement transport mechanisms, both in offshore and onshore settings, in the sector spanning between the Egadi Islands and Cefalù meridians. In particular the following have been studied in detail: a) morphostructural elements, in order to gather information pertaining to the recent evolution of the area, focusing on the recent and active faults, also integrating seismicity data of the region, taken from online databases; b) structures linked to the fluid escape, subsequent to shallow and deep processes within the investigated crustal sector, integrated with data from geochemical analysis of pore water derived from core sediment samples; c) structures produced by mass movement mechanisms and in particular those triggered by fluid migration, seeking to highlight the relationships between different mechanisms and their contribution to the recent evolution of the margin. In addition to a meticulous bibliographic search which allowed for an in-depth analysis and an up-to-date account on the subjects, pre-existing data has been used (provided by the Marine Geology group of the University of Palermo), as well as those derived from established databases and bibliographic sources, and overall newly acquired. Regarding the latter, morpho-bathymetric and single-channel seismic reflection data was collected during several oceanographic cruises between 2009 and 2013 (PUMA 09, MACS_2010, MASVE_2013) carried out in the area between the Egadi islands and Cefalù (in which the writer participated), in the context of the MaGIC project (Marine Geohazards along the Italian Coasts). The data collected was edited and/or processed accordingly before being interpreted. The multi-channel seismic reflection data acquired by ENI/AGIP and provided in digital format by the Italian Ministry of Economic Development in the context of the VIDEPI project has been inserted into a georeferred database and converted into seg-y format via a dedicated software, so as to reconstruct the surface of the M-reflector and verify the actual existence of faults in the sectors where fault escarpments, incisions, pockmark alignments and mounds have been identified through the morpho-bathymetric model. The single-channel profiles have been utilized to reconstruct the depositional architecture and the structural characteristics of the Plio-Quaternary succession, also allowing for an analysis of the structures of the morpho-bathymetric model. In addition, on a sediment core sample collected in proximity to a fluid escape structure (pockmark) in the Gulf of Palermo, at 414 m b.s.l., granulometric analysis as well as pore-water analysis have been performed; the latter aimed to chemical and isotopic characterization of the fluid phase occupying pores so as to determine its origin. The specific data obtained from online databases (http://www.idromare.it; http://www.ingv.it) and from literature sources is: meteo-marine, meteorological and sea surface current data; elements necessary to evaluate the parameters relative to submarine dynamics and seismological data (location and focal mechanisms of earthquakes). In this study neotectonic elements have been identified through interpretation of morpho-bathymetric and seismic data. Active faults, in a submarine environment, are recognized as displacements of the seafloor or shallow sedimentary layers by faults that clearly displace deeper geological strata at the same localities; evidence of fault control on late Quaternary sedimentation; displacement of the seabed or near-surface sediments where evidence of large-scale land-sliding or slumping occurs; displacement of the seafloor and deeper strata in the opposite sense to the local bathymetric gradient; evidence of morphologies on the seabed as pockmark and mound. The alignment of the earthquake epicenters with recognized seabottom and buried structures, allowed to recognize seismogenic faults, whilst focal mechanisms have permitted the reconstruction of the fault kinematics and the regional stress field. The fluid escape structures have been recognized thanks to interpretation of the morpho-bathymetric model, presence of wipeout zones, backscatter, reflector geometry, change of velocity and phase inversion in seismic reflection profiles. They have been distinguished as mounds or pockmarks. The latter have been characterized according to the following parameters: diameter, height, depth, distance from coastline and shelf edge, proximity to faults and landslides. Mass wasting structures including canyons and submarine slope failures have been studied through morpho-bathymetric analysis, conducting morphometric analyses, such as slope steepness and aspect, sinuosity index and thalweg length, estimation of volume of sediments mobilized, all key factors for understanding slope dynamics. In particular, these structures have been analyzed for their interaction with the processes related both to the ascent of fluids as well as to those related to tectonics. In addition, the longitudinal profiles of submarine canyons were compared to those present in other geodynamical settings (Carlsbad, Oceanside, East Break), so as to make inferences about the margin type and evolutionary geological model from the physical features (concave or convex) and depositional architecture. The evolution of the north-Sicilian continental margin during the Quaternary, reconstructed through analysis and interpretation of different types of data, appears linked to: • the deposition of the Quaternary succession, which is organized in depositional sequences associated with the interaction between tectonic, sedimentation and eustatic oscillations. It is made up of seaward dipping clastic and terrigenous Pleistocene deposits, truncated by an erosional surface formed during the last glacio-eustatic oscillation, on which lie the late Quaternary depositional sequence. • the recent tectonic that operates mainly by two fault systems: 1) a NW-SE trending extensional and/or transtensional system, which produces the recent articulation of the seabed, and 2) an ENE-WSW trending compressive system, highlighted by high-angle reverse faults and thrusts in accordance to those found onshore. The integrated study of the morpho-structural and seismological data has highlighted the relationship between the focal mechanisms of earthquakes and major tectonic structures recognized in the study area, both being consistent with a compressional tectonic model with NW-SE trend. Moreover, the earthquake clusters of 2002 (M 5.6 06/09/02), 2010 to 2013, located in the northern Gulf of Palermo and aligned with the ENE -WSW trending fault recognized in “La Barra High”, and the clusters aligning in NW-SE direction such as that of the earthquake of 1998 and the recent one (13/04/2012) located in the offshore Sferracavallo upper continental slope (M 4.2), aligned with the tectonic structures, which are recognized in the offshore Palermo Salient, whose focal solutions reveal mechanisms compression, which allow to define these two systems as active and especially seismogenic systems. • The fluid escaspe processes that form both positive structures (mound) and negative structure (pockmark). Pockmarks that are found exclusively on the continental slope are the result of the ascent fluids that migrate through the Quaternary clastic succession. The morphology of these structures, still well-preserved, allow to suggest a recent formation. The chemical and isotopic signal of pore water extracted from sediment core, taken at one of these structures, mainly proved that pore water is slightly enriched in heavy isotopes with respect to Mediterranean seawater V-SMOW, while the distribution profiles of EC, ion concentration (Cl, SO4, Na, K, Mg, Ca), ion/Chloride ratios (Na/Cl, K/Cl, Ca/Cl, Mg/Cl and Alk/Cl) seem to indicate the existence of an external source of fluids and the occurrence of sediment-fluids interaction processes. A possible mechanism causing pore water freshening could be to be the destabilization of gas hydrates. Mounds, identified exclusively in the continental shelf, can be divided into two types: outcropping and buried. Only the latter can advance a hypothesis on the age of formation. Their age, between 20.5 and 5.5 ka, is inferred from the dislocation of the erosion surface, which is the base of the last depositional sequence (20.5 ka) and the maximum flooding surface (5.5 ka), which seals these structures, which is dated on the basis of the correlation with the curve of Fairbanks (1989). • The mass wasting processes that are highlighted by the presence of slide scar and related landslide deposits. In this area they are formed in response to three processes : 1) tectonic activity generating rockfalls, such as those observed in the offshore of Marettimo island and Monte Pellegrino, in the north western part of the Gulf of Castellammare where landslides affect the continental shelf, and in areas where fault escarpments are recognized as at the St. Vito High, in the western sector of Banco Scuso, in the south-western sector of the Marettimo island and in the eastern sector of the Gulf of Termini Imerese; 2) the fluid migration. Failure scars have been detected just below the pockmarks in the Palermo Gulf, between the Oreto and Eleuterio canyons, and in correspondence of the Solunto and S. Vito Highs; 3) retrogressive evolution of the canyons, e.g. Cofano, Mondello, Addaura, Arenella canyons. • Phenomena of gravity and channelized erosion that modify the slope continental, in which two types of canyons are recognized: 1) those that develop from the bottom upwards, facilitated by the highest steep of the slope, with retrogressive evolution (bottom-up), and that they develop by means of landslides, which have their origin in the lower part and that advance towards the upper slope (Cofano, Mondello, Addaura , Arenella and Eleuterio in part); 2) those that develop downward trend (top-down), whose evolution is typically linked to turbidite and hyperpycnal flows, e.g. the Oreto, Castellammare, Eleuterio partly and Capo Plaia canyons) . Submarine gullies are arranged along a preferential direction, which generally coincides with that of the tectonic features. Within some of these structures, pockmarks have been observed with eroded edges. Arranged in a parallel way and in the same direction of recognized tectonic features, are found both alignments of pockmark and deeper incisions. This evidence makes it possible to hypothesize their genetic mechanism. The aligned circular structures, formed in correspondence of tectonic features, coalesce creating the grooves, the latter could initially evolve into channels and later possibly into canyons. Based on the obtained results it is possible to consider the continental margin north of Sicily as an immature margin, in which the instability is strongly related to the tectonic activity. The faults, in addition to modifying the morpho-structure of the margin, are preferential way of fluids migration, as evidenced by the formation of the pockmarks. These facilitate the triggering of landslides, which in turn may determine the evolution of retrogressive canyon.File | Dimensione | Formato | |
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