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Varios cuerpos ígneos alcalino-basálticos han sido identificados en la sección sedimentaria cretácica de la cuenca ecuatoriana oriente (≅ 110-80 ma). Los mismos están caracterizados por valores bajos en los radios LILE/HFSE y en los radios isotópicos de Sr y Nd, todos dentro del rango observado en las signaturas composicionales de basaltos de isla oceánica o punto caliente (“hot-spot”), consistentes con una fuente astenosférica. Sísmica de reflexión muestra que la ubicación de los distintos centros eruptivos es controlada por las estructuras pre-existentes del rift triásico-Jurásico invertido en un régimen transpresivo a partir del cretácico superior. edades radiométricas y el registro biostratigráfico de los sedimentos cretácicos sugieren una variación diacrónica de la posición geográfica de este volcanismo alcalino, ocurrido al menos a partir del albiano (40Ar/39K, 106±5Ma) en la parte septentrional de la Cuenca Oriente, y hasta el Campaniano (40Ar/39Ar, 84±2 Ma y 82±0,5 Ma) en la parte occidental-central (zona subandina).Se propone un mecanismo de “slab roll-back”, originado a partir de la cesación de los procesos de subducción en el Cretácico Inferior (≅130-120 Ma), como el modelo geodinámico más probable para explicar la generación de este magmatismo. El mismo está asociado con la migración lateral y vertical de material astenosférico hacia zonas previamente adelgazadas de la corteza o “thin-spots” litosféricos. Por lo tanto, el paleo-rift triásico-Jurásico ejerció un control fundamental en la generación de estos magmas alcalinos, facilitando la fusión parcial de una fuente astenosférica no afectada por procesos de subducción. Finalmente, la inversión tectónica del Cretácico Superior da lugar a la reactivación de las estructuras extensivas pre-existentes en un régimen transpresivo, lo que originó la migración del magmatismo hacia el sur-suroeste principalmente a lo largo del Corredor Sacha-Sushufindi de la Cuenca Oriente.

Summary Crustal structure of the continent–ocean transition off the Cote d'Ivoire–Ghana transform margin is determined by 2-D modelling of wide-angle seismic data and gravity data. The resulting models, together with geological data recently collected by drilling on the margin, are compared with available thermomechanical models of transform margins. In particular, we test the predicted effects on the transform margin crust of continental heating by the adjacent oceanic lithosphere. These effects include: (i) ductile flow of the lower continental crust either parallel to the transform direction or perpendicular to it, from the continent towards the ocean; and (ii) thermal uplift of the transform margin. In the region of the Cote d'Ivoire–Ghana transform margin, the seismic and gravity data indicate that the continental crust remains uniform in thickness as the continent–ocean boundary is approached. The interpreted models lack a transitional domain between continental and oceanic crusts, and abnormally thin oceanic crust is found along the margin. These results exclude large-scale ductile flow of lower continental crust during the margin formation. The geological data are, moreover, not consistent with thermal uplift of the margin during the period of contact between the transform margin and the spreading centre. Our geophysical and geological data thus infer only moderate heating of the transform continental margin by the adjacent oceanic lithosphere. Moderate heating of the Cote d'Ivoire–Ghana transform margin during margin formation implies that the thermal exchange between the transform margin and the adjacent spreading centre were lower than predicted by available thermal models. Two main factors may have contributed to reduce thermal exchanges across the continent–ocean boundary. The first factor is that the oceanic edge may have been colder than assumed by thermal models. An abnormally cold spreading axis along the transform margin when the oceanic crust was emplaced is supported by the atypical structure of the oceanic crust along the Cote d'Ivoire–Ghana transform margin. The oceanic crustal structure is similar to that found along oceanic fracture zones, where oceanic crust is’also’emplaced near a ridge–transform intersection. In both cases, abnormally low thermal gradients at the spreading centre along the transform margin may be explained by the cumulative effect of: (i) a cold edge effect of the adjacent continental lithosphere; (ii) accretion at the end of a ridge segment, away from the main magma supply point along the spreading axis; and (iii) an efficient upward cooling of the oceanic lithosphere owing to deep hydrothermal circulation within the faulted lithosphere. The second factor causing moderate thermal exchanges is the possibility of large amounts of water in oceanic lithospheric faults, which reduces the lateral thermal conduction in the lithosphere, since water is much less conductive than rock. During the margin formation, the oceanic edge thus seems to have acted as a thermal buffer between the cold continental margin and the hot spreading centre. We propose that this thermal buffer hypothesis may be generalized to most transform margins, implying a lower amount of continental heating than assumed by the available thermomechanical models.

Since flat subduction of the Nazca Plate beneath Peru was first recognized in the 1970s and 1980s a satisfactory explanation has eluded researchers. We present evidence that a lost oceanic plateau (Inca Plateau) has subducted beneath northern Peru and propose that the combined buoyancy of Inca Plateau and Nazca Ridge in southern Peru supports a 1500 km long segment of the downgoing slab and shuts off arc volcanism. This conclusion is based on an analysis of the seismicity of the subducting Nazca Plate, the structure and geochemistry of the Marquesas Plateau as well as tectonic reconstructions of the Pacific¿Farallon spreading center 34 to 43 Ma. These restore three sub¿parallel Pacific oceanic plateaus; the Austral, Tuamotu and Marquesas, to two Farallon Plate counterparts; the Iquique and Nazca Ridges. Inca Plateau is apparently the sixth and missing piece in an ensemble of 'V-shaped' hotspot tracks formed at on-axis positions. We argue the mirror image of the Inca Plateau, the Marquesas Plateau, is an ancient edifice overprinted by recent volcanism, in disagreement with the widely accepted young (

Within the framework of the joint FAUST (French Australian Seismic: Transect) program, the Australian research vessel Rig-Seismic recorded a series of multichannel seismic profiles between the New Hebrides Arc and the Australian Margin. The very high quality of these profiles allows the examination of the continental or oceanic basement over the whole area surveyed. In basins beneath the eastern margin of the northern Lord Howe Rise and the New Caledonia Basin, where the crust is thinner, the Moho discontinuity is present at about 8.5 seconds two-way time (TWT) (about 12–13 km) on the profiles processed onboard. The data collected suggest that the sedimentary section within the basins includes syn-rift Cretaceous sediments that are not yet definitively identified. The Upper Eocene-Middle Oligocene subduction of the Australian plate beneath the Pacific plate is highlighted by thrust faults concentrated along the Lord Howe, Norfolk and Fairway Ridges. This episode probably corresponds to the period of convergent tectonism associated with the Upper Eocene obduction of the New Caledonian ophiolites.

We examine the early geological history of the southern Kerguelen Plateau (Indian Ocean) using ocean bottom seismometer ((ORS), multichannel seismic (MCS), and Ocean Drilling Program data. Velocity-depth models in the sedimentary Raggatt Basin are constrained by near-range OBS data (refractions and reflections, including multiples and converted shear waves) and migrated MCS data. The models elucidate the significance of Lower Cretaceous lava flows, Albian to Coniacian/Santonian terrestrial and terrigenous sediment, and Maastrichtian and Paleocene seismic sequences. The Albian/Aptian basaltic basement complex consists of an upper and a lower series. The upper series is characterized by average Vps of 4.6-4.7 km/s, an upward increase of intercalated terrestrial sediment and altered flowtops, and a concomitant decrease in flow thickness ; the lower series is marked by average Vps of 5.3-5.5 km/s, and contains thicker flows and less sediment. A volcanic center in the Raggatt Basin shows relatively low Vps (3.7-3.9 km/s for the upper series, 4.7-4.9 km/s for the lower series), and dipping reflections on the Raggatt Basin's flanks are also recorded as refractions. Terrestrial and terrigenous sediment of the Raggatt Basin, immediately overlying basement, is characterized by a seismic low-velocity layer with Vps ranging from 2.2-2.9 km/s and a thickness of greater than 1100 m in the central basin. Nearby source regions (e.g. Banzare Bank and other elevated areas south of the basin) account for a terrestrial and terrigenous sediment volume of approximately 12,500 km3, deposited over approximately 20 m.y. The depocenter of the Raggatt Basin began shifting in Santonian to early Maastrichtian time, and concluded by early late Paleocene time. (Résumé d'auteur)

The convergent Southern Kermadec margin is examined to study the structural development and mechanical parameters of the inner trench slope in response to the oblique subduction of the oceanic Hikurangi Plateau. Swath bathymetric data have been combined with other geophysical data to reveal that the South Kermadec inner trench wall is segmented into distinct upper, middle, and lower margins. The lower margin is a nongrowing, compressionally unstable accretionary wedge that is dissected by transcurrent faults. The wedge has been thinned and narrowed as a result of tectonic erosion. The middle margin is stable by comparison and separated from the lower margin by the 220-km-long Awanui Fault. This fault is a landward dipping, strike-slip, reverse fault. The middle upper margin, including East Cape Ridge (ECR), forms a deforming backstop with a sharp along-strike change in tectonic regime. South of 36°40'S, by transpression characterizes the ECR shear zone. North of 36°40'S, the ECR is marked by uplift, by extension and farther north by subsidence. It is suggested that subsidence and subsequent tectonic erosion of the margin north of 35°50'S, in the wake of Hikurangi Plateua subduction, have broken the link between once continuous segments of the ECR-Tonga Platform. The unstable nature of the southern Kermadec wedge under compression, together with a comparison of its taper characteristics with those of modern convergent margins, implies that the wedge deforms under a relatively high internal pore pressure but high effective basal friction regime. Strike-slip deformation in the wedge is attributable to a relatively strong interplate coupling related to the buoyancy and ruggedness of the Hikurangi Plateau. (Résumé d'auteur)

International audience; We propose to explain the origin of the double trend in seismicity of the Macas swarm in the Subandean Cordillera of Cutucú (Ecuador) and characterize the corresponding active deformation of that region. For that purpose, seismological and geological data have been used, with the deployment of a temporary seismological array, with geological field observations and image processing. We found that some earthquakes are aligned on a well known NNE SSW trend corresponding to the orientation of the nodal planes of the reverse focal mechanism of the Mw=7.0 1995 Macas earthquake as for its aftershocks. Nevertheless, many smaller events are aligned on an unexpected NNW SSE trend inside the Cutucú Cordillera. We interpret these two orientations of the Macas swarm as linked to Subandean basement thrusts inherited from the inversion tectonics of a NNE SSW trending Triassic Jurassic rift, which has been uplifted and partly extruded in the Cutucú Cordillera. The present partitioning of this part of the Subandean deformation is controlled by pre-existing NNE SSW to NNW SSE Triassic Jurassic normal faults that have been subsequently compressed transpressed and reactivated into reverse faults. Major boundary faults of the rift were NNE SSW oriented and correspond now to some main Subandean thrusts as confirms the focal mechanism of the 1995 main shock located on the eastern border (Morona frontal thrust) and the orientation of its aftershocks. In the Cutucú Cordillera, the double orientation of present swarm can be interpreted as the result of accommodation of deformation along NNW SSE pre-existing faults inside the inverted rift system, linked to the motion of the Morona frontal NNE SSW thrust.

Geochemistry, Geophysics, Geosystems, v. 5, p. Q02009, 2004. http://dx.doi.org/10.1029/2003GC000561; International audience; For a given site, many factors control the seismic risk. Earthquake magnitude, hypocentral distance, rupture mechanism, site effects and site vulnerability are among the most important. This article deals with one of these factors: the depth of the seismogenic zone, in the northern Ecuadorian subduction system, beneath a highly vulnerable site, the city of Esmeraldas and its industrial complex, the Ecuadorian oil refinery and shipping terminal. To address this problem, we analyzed data from a three weeks passive seismological experiment, conducted in the spring of 1998, using 13 Ocean Bottom Seismometers and 10 portable land-stations. A preliminary interpretation of wide-angle data obtained in the fall of 2000, in the Manta area, 100 km South of the study area, unambiguously indicates the presence of a velocity inversion in the Ecuadorian margin velocity structure. This velocity inversion is characterized by a shadow-zone of ∼1 s on the record-sections, and is interpreted as the result of a velocity contrast between the upper plate structure and the sedimentary and basaltic layer II of the subducted oceanic Nazca plate. One-dimensional velocity models are deduced from these wide-angle data and are used for earthquake location in the Esmeraldas area. This highly improved the hypocentral parameter determinations. The updip limit of the seismogenic zone is found at a depth of ∼12 km, 35 km eastward of the trench, and the depth of the seismogenic zone below the Esmeraldas city is found at ∼20 km. This shallow depth of the seismogenic zone dramatically increases the seismic hazard of the area.

Subduction of the Nazca plate beneath the Ecuador Colombia margin has produced four megathrust earthquakes during the last century. The 500 km long rupture zone of the 1906 (Mw = 8.8) event was partially reactivated by three thrust events, in 1942 (Mw = 7.8), 1958 (Mw = 7.7), and 1979 (Mw = 8.2), whose rupture zones abut one another. Multichannel seismic reflection and bathymetric data acquired during the SISTEUR cruise show evidence that the margin wedge is segmented by transverse crustal faults that potentially correlate with the limits of the earthquake coseismic slip zones. The Paleogene Neogene Jama Quininde and Esmeraldas crustal faults define a 200 km long margin crustal block that coincides with the 1942 earthquake rupture zone. Subduction of the buoyant Carnegie Ridge is inferred to partially lock the plate interface along central Ecuador. However, coseismic slip during the 1942 and 1906 earthquakes may have terminated against the subducted northern flank of the ridge. We report on a newly identified Manglares crustal fault that cuts transversally through the margin wedge and correlates with the limit between the 1958 and 1979 rupture zones. During the earthquake cycle the fault is associated with high stress concentration on the plate interface. An outer basement high, which bounds the margin seaward of the 1958 rupture zone, may act as a deformable buttress to seaward propagation of coseismic slip along a megathrust splay fault. Coseismic uplift of the basement high is interpreted as the cause for the 1958 tsunami. We propose a model of weak transverse faults which reduce coupling between adjacent margin segments, together with a splay fault and an asperity along the plate interface as controlling the seismogenic rupture of the 1958 earthquake.

El presente trabajo describe las características estructurales, estratigráficas y sísmicas de las unidades pre-Aptense en la Cuenca Oriente. La elaboración de columnas litológicas de pozos tales como: sacha Profundo, tambococha-1, etc., y la interpretación de secciones sísmicas permitieron redefinir las formaciones pre-Aptenses en el centro norte y Este de la cuenca. Una nueva formación fue definida en este estudio como Fm. tambococha (Jurásico Medio-Cretácico temprano) al este de la cuenca.El análisis tectono-sedimentario permitió caracterizar el contexto geodinámico de la Cuenca Oriente pre-Aptense. Se estableció el desarrollo de dos sub-cuencas de “back-arc” durante el Jurásico Medio a Cretácico temprano con estilos tectónicos diferentes. La primera conocida como Corredor sacha-shushufindi, controlada por fallas normales de gran profundidad y asociadas con un sistema de “Rift” previamente desarrollado durante el triásico y Jurásico inferior. La segunda, conocida como sistema Capirón-tiputini, con juegos de fallas normales de tipo lístrico conectadas sobre un nivel de despegue horizontal ubicado en el basamento.El sistema petrolífero pre-Aptense de la Cuenca Oriente es pobremente conocido. La Fm. santiago es una buena roca generadora, pero su variación y desarrollo lateral de facies en la cuenca no han sido aún definidos.

Desde fines del cretácico superior, la Cuenca oriente se ubica en posición de retro-Cuenca de antepaís relacionada con el crecimiento de una protocordillera de los andes. El tema de este artículo es mostrar la evolución de esa Cuenca de antepaís en términos de relaciones tectónica-sedimentación a lo largo del paleógeno y del neógeno. En base al análisis de facies sedimentarias, datos de subsuelo y mapas isópacos, varios son los indicios que muestran una deformación tectónica sinsedimentaria, como son:- a escala local, numerosas discordancias progresivas en el paleógeno;- a escala regional, ciclos de “loading/unloading” durante el paleógeno y el Neógeno.Todos esos procesos tectónicos tienen una firma particular en la sedimentación contemporánea de la Cuenca de antepaís. Cada modo de respuesta sedimentaria está ilustrado. Un modelo de evolución geodinámica es propuesto.

Contribution 303 of ‘Géosciences Azur.-- 14 pages, 5 figures, We present new multibeam bathymetric and multichannel seismic data on the Middle American Trench, between 17 and 19°N. These data show, north of the present tip of the East Pacific Rise, unsedimented seafloor with an abyssal hill fabric bare of sediments at the outer wall of the trench (18 3/4 0'N). Two multichannel seismic profiles orthogonal to the Middle American Trench display clear oceanic crust extending more than 20 km beneath the overriding plate. These seismic images suggest that the subducting oceanic crust along the Middle American Trench is younger than previously proposed. This result, therefore, implies a new location for the East Pacific Rise termination in relation to the trench. The subduction of this young oceanic crust off western Mexico provides new constraints on the deformation and magmatism of the western Mexican mainland. (C) 2000 Elsevier Science B.V. All rights reserved, This study was supported by the CICYT Spanish project ANT94-0182-C02-01/02, INSU-France, IRD, Spanish–France bilateral co-operation program (ref. HF1997-077). Additional funding was provided by agreement CSIC/CONACYT and by CIRIT (Project 1997SGR 00020)

A 250 km long NE-SW lithospheric transect spanning the 40 km wide island of La Réunion and its submarine edifice is derived from lines of air gun shots at sea on either side, along the assumed hotspot trace. Seismic records were obtained from an array spanning the whole transect and including sea bottom and land receivers, providing a system of reversed and overlapping observations. Low seismic velocity, and hence density, is found on average for the whole edifice above the oceanic plate. We attribute high-velocity anomalies within the edifice to an intrusive core confined under the central northern quarter of the island-crossing segment. Unexpectedly, the main seismic interfaces, top and bottom of the prevolcanic crust, do not show significant flexural downwarping under the island. In addition, clear multipathing in the recorded wave field requires the presence of a body with a seismic velocity intermediate between the prevolcanic crustal material and the normal mantle. This lithospheric structure provides the first example where underplating occurs beneath an active volcanic island, suggesting a genetic relationship. The underplated body could represent residues of the evolution of primary picritic melts that yield erupted basalts. Evidence for reflectors deeper in the lithosphere may indicate further related heterogeneity. In the plate/hotspot model commonly assumed, the structural variation along the transect could be interpreted as a variation with time of the amount and physical state of underplated material. Copyright 1999 by the American Geophysical Union., Support by the authors’ institutions and by the European Community Volcanic Risk programme under contract EV5V-CT92-0188 is acknowledged. IFRTP and IFREMER are thanked for the technical and financial support to the cruise MD-76 REUSIS. UMR 6526 (Ge´osciences Azur) contribution 214.

A network of 60 seismographs was deployed across the Andes at approximately 23.5°S. The array was centered in the backarc; atop the Puna high plateau in NW Argentina. P and S arrival times of 426 intermediate depth earthquakes were inverted for 1-D velocity structure and hypocentral coordinates. Average velocities and v sub p/v sub s in the crust are low. Average mantle velocities are high but difficult to interpret because of the presence of a fast velocity slab at depth. Although the hypocenters sharply define a 35° dipping Benioff zone, seismicity in the slab is not continuous. The spatial clustering of earthquakes is thought to reflect inherited heterogeneties of the subducted oceanic lithosphere. Additionally, 57 crustal earthquakes were located. Seismicity concentrates in the fold and thrust belt of the foreland and Eastern Cordillera, and along and south of the El Toro-Olacapato-Calama Lineament (TOCL). Focal mechanisms of two earthquakes at this structure exhibit left lateral strike-slip mechanisms similar to the suggested kinematics of the TOCL. We believe that the Puna north of the TOCL behaves like a rigid block with little internal deformation, whereas the area south of the TOCL is weaker and currently deforming. (Résumé d'auteur)

Wide-angle seismic lines recorded by ocean bottom and land seismometers provide a pseudo three-dimensional investigation of the crust and upper mantle structure around the volcanically active hotspot island of La Réunion. The submarine part of the edifice has fairly low seismic velocities, without evidence for intrusives. An upper unit with a velocity-depth gradient is interpreted as made of material erupted subaerially then transported and compacted downslope. Between this unit and the top of the oceanic plate, imaged by normal incidence seismic reflection, a more homogeneous unit indicated by shadow zones on several wide-angle sections may correspond to lavas of a different nature, extruded underwater in the earlier phase of volcanism. Coincident wide angle and normal incidence reflections document that the oceanic plate is not generally downwarping toward the island but doming instead toward its southeastern part, with limited evidence for some intracrustal intrusion. Deeper in the lithosphere, the presence of a layer of intermediate velocity between the crust and mantle is firmly established. It is interpreted as resulting from the advection of hotspot magmatic products, possibly partially molten, and of a composition for which the crust is a density barrier. The extensive wide-angle coverage constrains the extent of this body. It does not show the elongated shape expected from plate drift above a steady hotspot supply. Alternative propositions can hence be considered, for example, that La Réunion is caused by a solitary wave of hotspot material or by a young hotspot. The size of the underplate, 140 km wide and up to 3 km thick, corresponds to less than half the volume of the edifice on top of the plate. Copyright 1999 by the American Geophysical Union.

Des études de terrain appuyées par l'interprétation de profiles sismiques nous ont permis de réaliser une coupe du bord Est de la Cordillère Orientale de Colombie intégrant le massif de Quetame et la zone subandine des foothills du Guavio. Cette étude met en évidence l'importance du régime transpressif lors des phases orogéniques Andines responsables du soulèvement de la Cordillère Orientale. (Résumé d'auteur)

The structure of the submerged part of La Réunion hotspot island is determined by a grid of multichannel seismic reflection profiles. The submarine part of the edifice appears as a poorly stratified wedge of material lying above a significant thickness of preexisting sediments and the oceanic basement. The dense data coverage has allowed us to derive contour maps of the top of the basement and of the base of the volcaniclastic edifice, further constrained by coincident wide-angle profiles. The resulting isobath maps reveal new, unsuspected features that could not be deduced from observation along a single seismic line since the geometry of these horizons varies significantly from one radial profile to the next. Both maps show a large degree of heterogeneity in the topography, with no axial nor cylindrical symmetry, indicating that plate flexure is not dominant. A slight depression toward the island is observed only in the southern area, ahead of the hotspot trace. The lack of angular unconformity in the volcano-sedimentary pile that covers the oceanic basement firmly establishes the lack of significant vertical movement and flexure. The base of the edifice is roughly domed, centered on the island, with several topographic highs or lows superimposed. The submarine apron appears as a composite constructional body, spreading by slumping of its flanks. Superficial lenses of laterally transported material are observed on the seismic data south of the island, not only to the east of the active Piton de la Fournaise volcano. Oceanic sediments trapped beneath the apron seem undeformed. (Résumé d'auteur)

Not available, La caractérisation 3D quantitative de réservoir par une propriété géologique est un domaine en plein essor dont le but est de produire des modèles les plus fiables et précis possible de distribution spatiale de différents paramètres de réservoir comme la porosité ou les faciès lithologiques. Cette thèse propose, dans le cadre du projet GOCAD, une méthodologie basée sur les approches statistiques multivariables permettant d'incorporer les données sismiques dans l'estimation des propriétés géologiques à partir des données de puits. Dans un premier temps, nous présentons la méthode de reconnaissance des formes sismiques non-supervisée dont l'originalité est la possibilité d'étudier le voisinage local géométrique de chaque point sismique. Cet aspect discret et volumique de l'analyse permet d'exploiter d'avantage le potentiel des données sismiques. L’algorithme proposé fait appel à l'analyse en composantes principales afin d'éliminer l'information sismique non-cohérente, et à la méthode de centres dynamiques pour générer les classes sismiques à partir d'un ensemble de points sismiques dans l'espace des composantes principales. Dans un second temps, différentes méthodes de calibrage géologique des classes sismiques sont proposées. Mais d'abord nous introduisons une étape intermédiaire entre la classification des données sismiques et leur interprétation géologique qui permet d'orienter d'une manière non-linéaire les classes sismiques vers la propriété de réservoir. Cela consiste à quantifier la correspondance entre les classes sismiques et les faciès géologiques et à regrouper les classes sismiques en fonction de cette correspondance. Les méthodes d'interprétation, quant à elles, reposent sur les techniques statistiques de calage des classes sismiques aux diagraphies de puits et permettent aussi de prendre en compte l'information géométrique sur les couches stratigraphiques, lorsque cette information est disponible. La variable issue de ce traitement des données sismiques peut être ensuite utilisée comme une variable secondaire dans des méthodes d'estimation ou de simulation stochastique. Trois de ces méthodes (cokrigeage colocalisé, réseaux de neurones et D.S.I.) sont présentées et appliquées dans la dernière partie de ce travail. Cette méthodologie est appliquée sur deux cas réels 3D, suivi par la modélisation 3D géométrique des réservoirs en utilisant des grilles irrégulières, déformées suivant la géométrie des horizons stratigraphiques.

SUMMARY The Crozet Bank, the easternmost region of the Crozet Plateau (austral Indian Ocean), is capped by two groups of islands which form the Crozet Archipelago (Terres Australes and Antarctiques Francaises). A N-S-trending 2 km deep basin, the Indivat Basin, extends between the two groups of islands and bisects the Crozet Bank into two separate domains. The interpretation of the KeOBS8 seismic refraction profile shot during the KeOBS/MD66 cruise (January-February 1991) over the eastern Crozet Plateau was carried out by ray tracing and the computation of synthetic seismograms. This interpretation leads to a rather complex seismic structure and outlines a pro- nounced eastward crustal thinning from 16.5 to 10 km at the expense of layer 2. The thinning of the crust is abrupt east of the Indivat Basin. Unlike below the Hawaiian Islands and the Marquesas Islands, no underplated mantle material underlies the crust of the Crozet Bank. Moreover, this deep seismic sounding has further revealed that a high-velocity feature outcropping the seabed 30 km east of Ile aux Cochons could be a major structural feature, deeply rooted within the lower crust. The occurrence of this feature (a submarine volcano?) is associated with a mantle dyke causing a steep shallowing of the lower crustal interfaces. Gravity modelling was performed along line KeOBS8, with a density structure deduced from the seismic section, to model free-air anomalies derived from altimetry data. This modelling confirms that the Indivat Basin, underlined by a gravity low running roughly N-S between the two groups of islands, is a major structural boundary. As the model generates medium-wavelength anomalies of adequate amplitude, it also confirms that the volcano, located west of the Indivat Basin, is a deeply rooted feature. The Crozet Bank clearly appears as a plume-aVected structure, which may have originated from a deep thermal anomaly within the lithosphere. More recent volcanic episodes, related to a still active plume activity under the Crozet Bank, could have uplifted upper-mantle material and caused the emplacement of the newly discovered feature and of the western group of islands.

Spatial differences in late Quaternary structural style and deformation rates indicate a complex pattern of strain partitioning within the transition area from oblique subduction beneath southern North Island to oblique continental collision in northern South Island, New Zealand. The late Quaternary structure of the offshore southern Hikurangi margin is determined here using seismic reflection profiles, MR1 side-scan sonar swath images, EM12Dual multibeam swath bathymetry and backscatter images, and sediment core and dredge samples. Where oblique (50°) subduction of the oceanic Hikurangi Plateau is occurring beneath southern Wairarapa region, the steep, dissected, upper margin consists of a wedge of rocks that are being deformed by strike-slip and reverse faults. Beneath the lower margin an accretionary wedge of Quaternary age has developed rapidly by frontal accretion of a thick succession of turbidites on the Pacific Plate. Estimates of late Quaternary geological strain rates have been made on two margin-perpendicular transects across southern North Island and the offshore margin to budget the predicted NUVEL-1A plate motion. More than 33–55 % of the total plate motion is expressed in the offshore part of the upper plate above the subduction detachment in the central Hikurangi margin between 40°S and 41°S. This offshore deformation could potentially include margin-parallel fault slip of up to 14–15 mm/yr. Across the upper plate, strain is partitioned into zones of folding and thrust faulting and possibly more than one zone of strike-slip faulting, but the partitioning may be incomplete with oblique-slip thrusting an important component of the deformation. The wide distribution of deformation within the upper plate partially reflects strong coupling between the subducting oceanic plateau and the leading edge of the overriding plate. This coupling increases toward the southwest, where the continental part of the subduction zone beneath Marlborough may now be inactive or very strongly coupled. Toward the southwest the transpressive deformation style of the upper Wairarapa margin continues into the upper Marlborough margin and onshore into the strike-slip Marlborough Fault System of northern South Island. In contrast, the accretionary wedge narrows to the southwest and terminates in southeastern Cook Strait, approximately above the transition from subducted oceanic crust to subducted continental crust of the Pacific Plate. Farther southwest beneath the lower Marlborough margin, low-strain folding and thrust faulting is occurring, but the style of deformation is not typical of the accretionary wedge. This shortening results either from a small amount of subduction slip beneath Marlborough and/or from contractional strain partitioned to the southeast of the continental transpression zone. Whereas the initial onset of continental collision occurs in Marlborough, the surface expression of deformation of the Pacific Plate occurs south of 42°30′S. Normal faulting of the northwestern margin of the continental Chatham Rise possibly reflects flexural extension of the Pacific Plate crust at the southern end of the subduction zone, whereas folding and thrust faulting of the north Canterbury coastal region results from NW-SE contraction of the delaminated upper crust entering the Southern Alps collision zone.

Pendant l'intervalle Crétacé supérieur-Quaternaire, le bassin Oriente en Equateur a été affecté par une succession de déformations compressives. L'objectif de ce mémoire est de préciser la cinématique des déformations successives qui ont affecté le bassin Oriente depuis le Crétacé supérieur à partir d'une étude structurale et sismique du champ pétrolier Shushufindi. L'étude de profils sismiques montre que les failles situées sur le champ de Shushufindi correspondent à une structure en fleur positive réactivant et inversant des failles normales anté-Crétacées. Des plis par propagation de faille de grande longueur d'onde et d'axe N-S à NNW-SSE sont associées à la structure en fleur. Par ailleurs, quatre phases compressives successives pour le crétacé supérieur et le Tertiaire sont mises en évidences par le pointé des réflecteurs sismiques. En opérant un déplissage des structures du champ, nous sommes arrivés à des variations de surface trop faibles pour pouvoir obtenir une carte du champ de déplacement fini associé à la déformation. Cependant ce déplissage a permis d'obtenir des ellipses de déformation finie pour la zone considérée. Ces ellipses indiquent une direction de raccourcissement ENE-WSW impliquant un jeu compressif sur les failles principales au nord du champ Shushufindi et un jeu compressif avec une légère composante décrochante dextre sur les failles au sud du champ de Shushufundi. L'analyse fine tridimensionnelle de plusieurs surfaces isobathes montre une série de plis situés en échelons le long des failles principales et compatibles avec un jeu décrochant dextre. La représentation tridimensionnelle des plans de faille suggère quand à elle un jeu compressif dominant sur les plans de failles principales. Ainsi, l'ensemble des données structurales et sismiques mettent en évidence pour la déformation Crétacé supérieur-Tertiaire un régime transpressif où le jeu compressif est fortement dominant par rapport au jeu décrochant dextre. (Résumé d'auteur)

The Reinga basin northwest of the North Island of New Zealand was initially formed by crustal extension in Cretaceous time. Gravity models suggest up to 35-40% crustal thinning. The seismic stratigraphy of the basin is continuous with that of the offshore western North Island, where reflectors are well constrained by oil exploration data. In the Reinga Basin, there are two Cretaceous sequences above an older Mesozoic basement. The lower sequence is apparently terrestrial and may include both pre-rift and syn-rift subsequences ; the upper is a rift-filling marine sequence. These are overlain by Paleocene and Eocene blanket sequences that were laid down during a period of relative tectonic quiescence consistent with cooling subsidence, continued submergence, a northeast-facing continental shelf, and absence of a significant active plate boundary. A strong regional reflector, caused by a combined unconformity and Oligocene condensed sequence, separates the paleogene and Neogene sequences. The Neogene sequences record sedimentary infill from several source directions, not only from the New Zealand landmass, but from the north and west as well. Near the Northland coast, sediment accumulated in clastic wedges and ponded sub-basins from the Miocene to the present day. Along the flanking ridges to the northwest, similar deposition occurred in the Early and Middle Miocene but changed in the Late Miocene to sedimentation in drifts flanked by scours. This change reflects the end of tectonism, a diminishing clastic sediment supply, and the establishment of a throughgoing oceanic current regime as the marginal ridges submerged. This pattern of sedimentation persists today ; Post-Cretaceous volcanism occurred in two parts of the basin. In the central southeastern part, volcanic bodies in the ?Oligocene to Early Miocene sequences could be a northwestern extension of the Northland volcanic arc... (D'après résumé d'auteur)

Dans cet article, les caractéristiques morphologique et tectonique du système des Tonga-Kermadec sont reconsidérées afin de déterminer les effets possibles du passage de la ride de Louisville. Les résultats obtenus dans la zone d'interaction actuelle, lors de la campagne Seapso 5 du N/O Jean Charcot sont présentés afin de préciser les mécanismes tectoniques régissant la subduction de la ride. L'ensemble de ces données permet de proposer un schéma illustrant les effets de la subduction de la ride oblique de Louisville sur la marge des Tonga-Kermadec

The New Hebrides back-arc troughs are restricted to the southern and northern rear of the arc. Lack of trough in the central part is related to the compression induced by the collision of the d'Entrecasteaux ridge and the New Hebrides arc. The southern troughs are 50-60 km wide and are composed of one or two grabens striking N130-140 degree , while the northern troughs extend over 70 km wide and are composed of a succession of N-S trending grabens, horsts and half-grabens. Although the location and the main trend (N150 degree ) of the troughs indicate that they are largely guided by the subduction, the direction of extension (N40-45 degree ) is clearly oblique to the volcanic line and thus does not seem related to the subduction process. Formation of the New Hebrides back-arc results from extensional tectonics induced by a major reorganization at 3 Ma of the seafloor spreading of the North Fiji basin, which appears since this time to be developed independently of the subduction process.

Results from refraction experiments have been combined with local earthquake data to produce a preliminary model for the structure of the New Hebrides arc-trench system. Maximum crustal thickness of about 28 km occurs beneath the island ridge. The crust is unusually thick (about 10 km) in the zone seaward (westward) of the trench, where the velocity of the deepest layer is 7.0 km/s. The crustal thickness of the Fiji plateau, east of the New Hebrides ridge, varies between 5 and 7 km. The velocities of th materials that form the inner trench wall are similar to those found in the island ridge. A Zone of relatively high velocity (4.1 km/s) is encountered at a depth of about 1 km beneath the arc-trench slope. This zone thickness toward the trench and is overlain by material with velocity typical of surficial sediments (approx. =2 km/s). This may be an accretionary deposit, or it may be of volcanic origin.