A seismic refraction survey in the Kerguelen isles, southern Indian Ocean

Two 65 km long on land refraction lines, with shot detonated at sea, were implemented in the Courbet Peninsula, eastern region of the Kerguelen archipelago (Terres Australes et Antarctiques Françaises, southern Indian Ocean). The first profile P1, oriented NE SW, runs from Cap Ratmanoff toward Mont Ross, and the second one, P2, from Pointe Suzanne, SE of the Courbet Peninsula, through the Val Studer. Data from both profiles were supplemented by using the Bouguer gravity anomaly map not previously studied. The data combined standard travel time interpretation, wide angle reflected wave study, synthetic seismograms and S wave analysis. The velocity depth behaviour shows that the mean crustal thickness ranges from 14 to 17 km only, varying with the locality. The structure of the crust beneath Kerguelen resembles those observed beneath aseismic ridges (i.e. the Crozet Rise and the Madagascar Ridge). Average velocity of 5.5 km s 1 on both lines is in the range of those determined for oceanic layer 2, which is there 8 9 km thick. Average velocity of 6.6 km s 1 is in the range of velocities within oceanic layer 3. The transition to mantle velocity, 7.95 km s 1 is best modelled by a positive velocity gradient within the crust. This feature is similar to that observed on structures generated near spreading centres. These new data combined with geological and geochemical investigations in the archipelago support an oceanic origin for the Kerguelen Heard Ridge, and also Broken Ridge. Both structures were joined 40 Myr ago, as shown by the magnetic anomaly pattern. Refraction studies invalidate the assumption that isostatic compensation is achieved by a 23 km deep crustal root, but substantiate the contribution of a low velocity mantle. Gravity data are consistent with refraction studies. No crustal root appears to be present beneath Mont Ross. The best match between gravity and refraction data in the Val Studer is obtained by assuming the presence of a shallow intrusive body, related to the fault and dyke system, and Montagnes Vertes plutonic intrusive complex nearby

The northern New Hebrides back-arc troughs : history and relation with the North Fiji basin

The New Hebrides back arc troughs (southwest Pacific) are located between the New Hebrides trench arc system and the active North Fiji marginal basin. They are restricted to the southern and northern segments of the arc and were generally related to effects of the Indo Australian subducting plate (rolling back and/or subduction of the d'Entrecasteaux ridge). A detailed bathymetric and magnetic survey over the northern back arc troughs is used to propose a new model for the origin of the New Hebrides back arc troughs. The northern troughs extend over a width of 60 km and are composed of N S trending grabens and horsts, discontinuous along strike and associated with volcanism. The troughs are disrupted southward at 13° 30'S, where the Hazel Holme fracture zone intercepts the New Hebrides island arc. The E W trending Hazel Holme fracture zone is an extensional feature bisecting the North Fiji basin. In its western end, the Hazel Hohne fracture zone is composed of a succession of horsts and grabens striking N90 ° N100 ° E. Geometrical and structural relationships between the back arc troughs and the Hazel Holme fracture zone suggest that both these extensional features result from the same process and are closely linked. The northern troughs western end of the Hazel Holme fracture zone region is dominated by N130° 135°E trending magnetic lineations typical of oceanic crust. These lineations are oblique to the horsts and grabens systems, and are characteristic of the old North Fiji basin oceanic crust. Consequently we conclude that the northern back arc troughs are partly developed on the North Fiji basin oceanic basement and that extensional tectonic processes postdate the oldest North Fiji basin oceanic crust. Morphological and structural evidence suggests that both the back arc troughs and the Hazel Holme fracture zone are recent, still active and result from NE SW extensional tectonics. Because other tectonic features throughout the North Fiji basin are related to the same stress field, it is inferred that such a NE SW extension could be a large scale deformation affecting the North Fiji basin. It is proposed that the back arc troughs are primarily related to this recent extension within the North Fiji basin, but their locations along the arc are also influenced by the subduction of the d'Entrecasteaux ridge which produces, south of 13°30'S, nearly E W trending compression and prevents the formation of troughs. Possibly, these recent extensional tectonic processes result from a major reorganization in the spreading process of the North Fiji basin, and could be as young as 0.6-0.7 Ma

Внутренние резервы для спасения Украины

В статье рассматриваются проблемы выхода Украины из социально-экономического кризиса. Основные пути выхода из него - это не заимствования зарубежных кредитов, а поиск внутренних резервов для спасения страны. В Украине имеются реальные потенциальные возможности решить кризисные проблемы без помощи из Запада.У статті розглядаються проблеми виходу України із соціально-економічної кризи. Основні напрямки вихо- ду із неї — це не запозичення закордонних кредитів, а пошук внутрішніх резервів для спасіння країни. В Укра- їні є всі реальні потенційні можливості для вирішення кризових проблем без допомоги із Заходу.The paper considers the problems of Ukraine’s way out of socio-economic crisis. Main ways out of crisis are to search for internal reserves for saving the country, instead of borrowing foreign loans. Ukraine has real potentialities to overcome the crisis without the assistance from the West

Structure profonde de la transition arc insulaire-bassin marginal dans le Nord des Nouvelles-Hébrides (Vanuatu, Pacifique sud-ouest)

Trois profils de sismique réfraction ont été réalisés dans la zone arrière-arc des Nouvelles-Hébrides. Les modèles obtenus sur la terminaison occidentale du bassin Nord-Fidjien et dans l'axe des fossés arrière-arc présentent une loi de vitesse typique d'une croûte océanique récente mais recouverte par une couverture sédimentaire épaisse. Au contraire, le modèle obtenu à l'ouest des fossés met en evidence une croûte épaissie d'arc insulaire. La structure profonde et la signature magnétique de cette zone montrent que les fossés arrière- arc sont installés sur la croûte océanique du bassin Nord-Fidjien. Le passage entre l'arc et le domaine océanique se fait brutalement sur une distance inférieure à 50 km

Kerguelen plateau : a volcanic passive margin fragment ?

The 21-25-km-thick crust of the southern Kerguelen Plateau consists of three units: (1) a ≤ 2.3-km-thick sedimentary cover; (2) a 3-6-km-thick basaltic layer with velocities ranging from 4.5 to 6.2 km/s; and (3) a 15-17-km-thick lower crust with velocities from 6.6 to 6.9 km/s, including a 3-6-km-thick transition zone located at the base of the crust. The low-velocity transition zone has an average velocity of 6.7 km/s and exhibits several internal wide-angle reflections. The velocity-depth structure of the crust differs significantly from that of other hotspot-related oceanic plateaus and suggests that the southern Kerguelen Plateau may be a fragment of a volcanic passive margin composed of a thinned continental crust overlain by basalt flows

The new « Hippocampe » OBS and the French OBS pool

The study of continental margins, subduction zones and oceanic basins as well as the quantitative assessment of seismic hazard near densely populated coastal areas request the deployment of a large number of Ocean Bottom Seismometers (OBS) during period of several weeks for active tomography, up to several months for passive experiment