Deep crustal faults and the origin and long-term flank stability of Mt. Etna : first results from the CIRCEE cruise (Oct. 2013)

The relation between deep crustal faults and the origin of Mount Etna, the largest and most active volcano in Europe has long been suspected due to its unusual geodynamic location. Results from a new marine geophysical survey offshore Eastern Sicily reveal the detailed geometry (location, length, dip and orientation) of a two-branched 200-km long, lithospheric scale fault system, long sought for as being the cause of Mount Etna. Using high-resolution bathymetry and seismic profiling, we image a 60-km long, previously unidentified, NW trending fault with evidence of recent displacement at the seafloor, offsetting Holocene sediments. This newly identified fault connects NE of Catania, to a known 40-km long, offshore-onshore fault system dissecting the southeastern flank of Mount Etna, generally interpreted as purely gravitational collapse structures.peer-reviewe

Seabeam and seismic reflection imaging of the tectonic regime of the Andean continental margin off Peru (4°S to 10°S)

Suite à une campagne géophysique réalisée au large de la côte du Perou (croisière Seaperc du R/V "Jean Charcot", juillet 1986), les auteurs proposent une nouvelle interprétation des structures caractérisant la pente continentale de la région étudiée. D'autre part, ils considèrent que cette marge active est une marge active en extension ou bien une marge d'effondrement qui développe un complexe d'accrétion induit par les effondrements de la partie médiane de la pente

Nature et structure de l'isthme inter-américain, Panama (implication sur la reconstruction et l'évolution géodynamique de la plaque Caraïbe)

L'isthme de Panama se situe en bordure SW de la plaque Caraïbe, à la jonction de trois plaques lithosphériques: les plaques Amérique du Sud, Nazca et Cocos. Cet isthme est constitué de deux arcs volcaniques formant l'Amérique Centrale. Leurs présences reflètent une histoire complexe de convergence, en subduction. L'événement majeur de cette région correspond à la collision de l'Amérique Centrale contre l'Amérique du Sud entre 12-25 Ma. L'objectif de cette thèse est de documenter les déformations avant, pendant et après le processus d'accrétion continentale. Le but est de mieux comprendre comment un arc volcanique s'accrète sur une marge continentale pour reconstruire l'histoire géodynamique de cette région de 70 Ma jusqu'à nos jours. Cette thèse combine: - une étude sédimentologique et paléontologique, - une étude structurale à partir de données spatiales, géophysiques, et de terrain, - une étude thermochronologique (AFT), - et une étude interprétative sismique. Je propose ainsi une accrétion progressive et oblique de l'Amérique Centrale sur l'Amérique du Sud, s'initiant au sud de la région d'Istmina à partir de 40-37 Ma. La plaque Caraïbe, piégée entre l'arc volcanique et la marge continentale sud-américaine, disparaît progressivement sous l'Amérique du Sud. Vers 15 Ma, l'accrétion de l'arc dans la partie colombienne se termine. Au Panama, la convergence continentale se poursuit, mais le système s'inverse. Une nouvelle subduction s'initie : la plaque Caraïbe subducte sous l'isthme. Les déformations compressives engendrées par l'accrétion contrôlent la migration des masses sédimentaires et permettent la surrection progressive de l'isthme créant le pont inter-Amériques.The Panama Isthmus is located on the SW boundary of the Caribbean plate, at the junction of the South American, Nazca and Cocos plates. The isthmus is composed of two island arcs forming Central America. It formed by a complex history of plate subductions. The major tectonic event in this region is attributed to the accretion of Central America with South America between 12 and 25 Ma. The aim of this thesis is to document the deformation before, during and after the accretionary continental process. The main purpose is to better understand how a volcanic arc collides against a continental margin in order to reconstruct the tectonic history of this region since 70 Ma until today. This thesis combines: - a sedimentological and paleontological studies, - a structural study from spatial, geophysical and field work data, - a thermochronological study (AFT), - and an interpretative seismic study. I propose the initiation of progressive and oblique arc-continent collision during 40-37 Ma. The Caribbean plate, trapped between the arc and the continent, progressively disappeared beneath the South American continent. Around 15 Ma, the Colombian part of Central America was accreted and the convergence of Panama toward the continent progressed and produced a new subduction zone whereby the Caribbean plate subducted beneath the Panama Isthmus. Compressive deformations, caused by the collision, still actively control the migration of sedimentary masses, allowing the progressive emergence of the isthmus and forming the inter-American land bridge.NICE-Bibliotheque electronique (060889901) / SudocSudocFranceF

Architecture et mécanismes du rift islandais dans la région du Vatnajökull

Iceland is an emerged part of the Mid-Atlantic Ridge caused by the superposition of the spreading ridge axis and the Icelandic hotspot. This peculiar situation gives rise of the two interactions between the tectonic and volcanic processes, intimately linked in such to a region. On the Earth s surface, these interactions are expressed by volcano-tectonic systems made of central volcanoes, long eruptive and non-eruptive fissures and faults. Although the volcano-tectonic environment has come to be well understood in the last three decades, the orientation and relative magnitude of the crustal stresses as well as fractures geometry in space and time is poorly documented. The central part of Iceland is located within the East Volcanic Zone where the youngest and most active volcanic systems are present. Among those systems, the Bardabunga-Veidivötn, the Gŕimsvötn-Laki and the Kverkhnjukar are especially interesting because at the surface thy highlight numerous fractures (reaching a few kilometers in length), important seismic activity is present (with occurrence of recent dykes); and their continuity in the off-rift zone allows assessment of the ancient brittle structures. It also contains the largest glacier in Europe : the Vatnajökull (recovering part of the geological structures). Finally, the apex of the Icelandic hotspot is underlying the crust. For reasons of glacier overlapping as well as tricky access in the field, the central part of Iceland is poorly documented in terms of tectonic analysis. The aim of this work is to characterize the stress field in space and time (from surface to depth) in order to better understands the tectonic and volcanic interactions. Using a collection of focal mechanisms (in the active rift zone) and of fault slip date (in the off-rift zone) the slip vector can be inverted for inferring the orientations and the relative magnitude of the stresses. At the surface photogrammetry techniques are used for inferring the fracturing geometry and mechanisms in the active rift zone. The presence of the glacier might induce a perturbation on the volcano-tectonic processes. To do so, we divided the central part of Iceland into three sub-zones : (i) the rift at the southwest part of the glacier, (ii) the rift at the northeast part of the glacier and (iii) the rift beneath the glacier. The comparison between the active rift and off-rift zones will allow the evaluation of the volcano-tectonic processes in space and time. This thesis is composed of three main parts. The first one is constituted of two chapters including (i) the state of the art of the geodynamics of Iceland and (ii) on methods used in this work. The second part is divided into three chapters corresponding to the different sub-zones mentioned above. The last part is a general discussion and synthesis from different analyses carried out in each area, and finally a conclusion.L émergence de l Islande résulte de la superposition du point chaud islandais et de la dorsale médio-atlantique. Ainsi, les processus tectoniques et volcaniques sont intimement liés dans cette région. Même si, dans ces dernières décennies, la compréhension des processus tectono-volcaniques a connu un progrès important, l orientation et les magnitudes relatives des contraintes crustales ainsi que la géométrie des structures restent peu étudiées. La Zone Volcanique Est, qui constitue la partie centrale de l île à laquelle se surimpose en partie le glacier Vatnajökull, offre la possibilité d étudier les interactions entre les processus tectoniques et les processus volcaniques, car elle présente les systèmes tectono-volcaniques les plus jeunes (3Ma), affectés de nombreuses failles kilométriques en surface, et les plus actifs en terme de micro sismicité et de volcanisme. Dans les zones de paléo-rifts la continuité avec ces systèmes tectono-volcaniques permet d accéder aux structures cassantes fin-pliocènes et pléistocènes exhumées par l érosion. Nous avons utilisé trois approches pour caractériser la fracturation et l état de contraintes crustales : (1) les techniques de géomorphologie quantitatives issues de la photogrammétrie pour les traces de surface des failles et fissures en utilisant une collection de photographies aériennes pour reconstruire une mosaïque 2D et des modèles stéréoscopiques pour cartographier et évaluer le rejet de failles, accompagnées d une validation terrain dans des secteurs choisis ; (2) des techniques d inversion des vecteurs glissements des mécanismes au foyer pour reconstruire les champs de contraintes actuelles, incluant des tests de damier (checkerboard) avec des données synthétiques et de perturbations aléatoires pour évaluer la robustesse des inversions ; (3) des techniques d inversion des vecteurs glissements des paléo-failles pour établir l évolution de l état des contraintes sur le terme. Dans les zones de rift actives, les champs de contraintes actuelles se caractérisent par des régimes normaux et décrochant, associés à des rapports de forme élevés (S1 = S2). Le comportement sismotectonique se caractérise donc par des permutations de contraintes. En termes d orientation, deux directions principales ont été estimées : une direction d extension parallèle à la direction d ouverture du rift (globalement N105E, DeMets et al. 1990) et une autre déviée voire perpendiculaire à cette dernière. Ces directions déviées sont probablement liées à la présence de chambres magmatiques, que l on peut considérer comme des inclusions élastiques, et à l action de la suppression de fluides, qui doivent jouer un rôle de premier ordre sur le champ de contraintes local, là où les volcans actifs sont présents. L influence du glacier Vatnajökull sur le champ des contraintes actuel reste délicate à estimer. Certains auteurs avancent que celle-ci serait peu significative à l échelle de temps sismotectonique (20 ans). En surface, les mécanismes de fracturation ont pu être déduits à partir des systèmes volcaniques les plus récents dont Bardabunga-Veidivötn, Gŕimsvötn-Laki et Kverkhnjukar. Nos résultats montrent que la distribution des longueurs de failles en fonction des rejets suit une loi de puissance. Cette distribution met en lumière la croissance des failles par coalescence de fractures voisines, processus confirmé dans d autres segments du rift islandais. Les systèmes volcaniques s organisent généralement en larges structures de type graben. La forte régularité de la fracturation en surface et les modes de rupture déduits (tension en surface et cisaillement à 500 m de profondeur) permettent de déduire l orientation de la contrainte horizontale minimum, S3, qui est estimée en moyenne à N 130 E. Dans les zones hors rift, les paléo-failles montrent des régimes normaux et décrochant. Les directions de contraintes principales présentent deux directions de S3 : une direction cohérente avec l extension et une déviée par rapport à celle-ci. Ainsi, nous proposons que les paléo-contraintes déviées, de la même façon que pour le champ de contraintes actuel, pourraient être liées aux processus volcaniques anciens. La comparaison en termes d orientations de la contrainte horizontale minimum entre les zones de rift actives et hors rift montre une bonne cohérence avec l ouverture du rift. A la différence de ce qui a été observé pour les mécanismes de fracturation dans la croûte, les zones superficielles sont dépourvues des régimes décrochant. D ailleurs, les contraintes actuelles tendent à montrer un continuum entre les régimes décrochant et normal (S1 S2) alors que les paléo-contraintes semblent montrer des régimes purement décrochant et purement normal (avec un rapport de forme faible). Ces comportements différents peuvent s expliquer (1) par un biais méthodologique introduit par l observateur et (2) par la différence d échelle dans l espace et le temps différente (de quelques années à plusieurs millions d années).NICE-BU Sciences (060882101) / SudocSudocFranceF

Heat flow in the Sea of Marmara Central Basin: Possible implications for the tectonic evolution of the North Anatolian fault

The Central Basin in the Sea of Marmara is a syntectonic basin related to the evolution of the North Anatolian fault. A well-dated (ca. 15.5-16 ka) homogenite sediment can be used as a marker in three-dimensional depth model calculations, allowing a precise determination of the seafloor subsidence rates during the Holocene. A steady-state model based on the propagation of the rates downward through the basin fill provides a good correlation with the deeper seismic reflection imagery for the past 250 ka but indicates variation of subsidence pattern for older ages. Heat flow measured at the seafloor is affected by sedimentation blanketing effects. Heat flow and subsidence data can only be reconciled if the Central Basin depocenter migrated northward with time. According to that scenario, subsidence and deposition started earlier (ca. 5-3.5 Ma) in the southern subbasin, and an acceleration of subsidence in the northern subbasin occurred at ca. 2.5-1.5 Ma. These results allow us to propose that a southern fault system distinct from the Main Marmara fault is responsible for the southern onset of the subsidence. Changes in the fault network and slip rates are implied during the last 2.5-1.5 Ma despite no apparent change since 250 ka

Site Effects in Port‐au‐Prince (Haiti) from the Analysis of Spectral Ratio and Numerical Simulations

To provide better insight into seismic ground motion in the Port-au- Prince metropolitan area, we investigate site effects at 12 seismological stations by analyzing 78 earthquakes with magnitude smaller than 5 that occurred between 2010 and 2013. Horizontal-to-vertical spectral ratio on earthquake recordings and a standard spectral ratio were applied to the seismic data.We also propose a simplified lithostratigraphic map and use available geotechnical and geophysical data to construct representative soil columns in the vicinity of each station that allow us to compute numerical transfer functions using 1D simulations. At most of the studied sites, spectral ratios are characterized by weak-motion amplification at frequencies above 5 Hz, in good agreement with the numerical transfer functions. A mismatch between the observed amplifications and simulated response at lower frequencies shows that the considered soil columns could be missing a deeper velocity contrast. Furthermore, strong amplification between 2 and 10 Hz linked to local topographic features is found at one station located in the south of the city, and substantial amplification below 5 Hz is detected near the coastline, which we attribute to deep and soft sediments as well as the presence of surface waves.We conclude that for most investigated sites in Port-au- Prince, seismic amplifications due to site effects are highly variable but seem not to be important at high frequencies. At some specific locations, however, they could strongly enhance the low-frequency content of the seismic ground shaking. Although our analysis does not consider nonlinear effects, we thus conclude that, apart from sites close to the coast, sediment-induced amplification probably had only a minor impact on the level of strong ground motion, and was not the main reason for the high level of damage in Port-au-Prince.Published1298-13154T. Sismologia, geofisica e geologia per l'ingegneria sismicaJCR Journa

Tectonic expression of an active slab tear from high-resolution seismic and bathymetric data offshore Sicily (Ionian Sea)

Subduction of a narrow slab of oceanic lithosphere beneath a tightly curved orogenic arc requires the presence of at least one lithospheric scale tear fault. While the Calabrian subduction beneath southern Italy is considered to be the type example of this geodynamic setting, the geometry, kinematics and surface expression of the associated lateral, slab tear fault offshore eastern Sicily remain controversial. Results from a new marine geophysical survey conducted in the Ionian Sea, using high-resolution bathymetry and seismic profiling reveal active faulting at the seafloor within a 140 km long, two-branched fault system near Alfeo Seamount. The previously unidentified 60 km long NW trending North Alfeo Fault system shows primarily strike-slip kinematics as indicated by the morphology and steep-dipping transpressional and transtensional faults. Available earthquake focal mechanisms indicate dextral strike-slip motion along this fault segment. The 80 km long SSE trending South Alfeo fault system is expressed by one or two steeply dipping normal faults, bounding the western side of a 500+ m thick, 5 km wide, elongate, syntectonic Plio-Quaternary sedimentary basin. Both branches of the fault system are mechanically capable of generating magnitude 6-7 earthquakes like those that struck eastern Sicily in 1169, 1542, and 1693

Partitioning of deformation along a reactivated rifted margin: example of the northern Ligurian margin.

The northern Ligurian margin, of Oligo-Miocene age, is currently undergoing compression related to microplate motions and/or to gravity spreading of the Alpine chain located immediately north of it. Active thrust faults and folds have previously been identified below the margin, together with a global uplift of the continental edge, since at least the Messinian. The seismicity that goes with the present-day margin contraction (e.g. Mw 6.9, 1887/02/23) extends to the axis of the adjacent oceanic basin (e.g. ML 6.0, 1963/07/19; ML 5.4, 2011/07/07). However, we do not know of any recent or active crustal contractional structure within this oceanic domain. In this study, we use new 12-channel high-resolution seismic data (FABLES seismic cruise, 2012, R/V Tethys II) in order to image the sedimentary cover of the Ligurian oceanic basin, up to 3km below the seabed, including the Plio-Quaternary and the Messinian sediment down to the bottom of the Messinian salt layer. Because the Messinian event is well dated (5.96-5.32 Ma) and well identified in the seismic data, it forms a clear marker that we use to characterize the recent deformation related to both mobile salt motion and crustal tectonics. About 50 km south of the margin offshore of Italy, we identify huge and complex salt walls that elongate SW-NE. Such salt walls, which cannot be explained by salt tectonics only, are interpreted as evidence of deepseated crustal deformation. They form en echelon structures that are well expressed in the seabed morphology, and do not correspond to any significant vertical throw at the base of the salt layer. This suggests that within the deep basin, mainly strike-slip faulting accommodates long-term crustal deformation. It thus offers a contrast with the margin where deformation is mainly marked by shortening and reverse faulting, with vertical throws of several hundred meters. This discrepancy in the tectonic styles between the margin and the adjacent oceanic basin suggests some partitioning of the deformation. It may result from the difference in the topographic gradient of the main crustal interfaces between the steep margin and the adjacent oceanic domain, and/or to different mechanical behaviours of the adjacent lithospheric domains

From oblique subduction to intra-continental transpression : structures of the southern Kermadec-Hikurangi margin from multibeam bathymetry, side-scan sonar and seismic reflection

The southern Kermadec-Hikurangi convergent margin, east of New Zealand, accommodates the oblique subduction of the oceanic Hikurangi Plateau at rates of 4-5 cm/yr. Swath bathymetry and sidescan data, together with seismic reflection and geopotential data obtained during the GEODYNZ-SUD cruise, showed major changes in tectonic style along the margin. The changes reflect the size and abundance of seamounts on the subducting plateau, the presence and thickness of trench-fill turbidites, and the change to increasing obliquity and intracontinental transpression towards the south. In this paper, we provide evidence that faulting with a significant strike-slip component is widespread along the entire 1000 km margin. Subduction of the northeastern scarp or the Hikurangi Plateau is marked by an offset in the Kermadec Trench and adjacent margin, and by a major NW-trending tear fault in the scarp. To the south, the southern Kermadec Trench is devoid of turbidite fill and the adjacent margin is characterized by an up to 1200 m high scarp that locally separates apparent clockwise rotated blocks on the upper slope from strike-slip faults and mass wasting on the lower slope. The northern Hikurangi Trough has at least 1 km of trench-fill but its adjacent margin is characterized by tectonic erosion. The toe of the margin is indented by 10-25 km for more than 200 km, and thus is inferred to be the result of repeated impacts of the large seamounts that are abundant on the northern Hikurangi Plateau. The two most recent impacts have left major indentations in the margin. The central Hikurangi margin is characterized by development of a wide accretionary wedge on the lower slope, and by transpression of presubduction passive margin sediments on the upper slope. Shortening across the wedge together with a component of strike-slip motion on the upper slope supports an interpretation of some strain partitioning. The southern Hikurangi margin is a narrow, mainly compressive belt along a very oblique, apparently locked subduction zone. (Résumé d'auteur