Thickening of the oceanic layer in the Pacific Ocean

A statistical analysis of published seismic refraction results in the Pacific Ocean confirms the existence of a progressive thickening of the oceanic layer with age, following an inverse exponential law. There is no strong indication in the Pacific data of an increase in the thickness of layer 2 with decreasing spreading rate.Contribution No. 93 of the Département Scientifique of the Centre Océanologique de Bretagne, B.P. 337, 29200, Brest, France.     

Gravity Anomaly and Tectonics in the Northern Part of the North Fiji Basin - Preliminary Result of the 1994 NOFI Cruise under the NEW STARMER Programme

A surface ship gravity survey was carried out in the northern part of the North Fiji Basin during the NOFI cruise by the R/V l'Atalante in August-September, 1994. The two ridges inside the study area, the South Pandora Ridge and the Tripartite Ridge, present different structures and states of isostatic equilibrium in terms of gravity anomaly and its tectonic implications. The former is supported by a restoring force of an imaginary elastic plate in the crust and the latter by the Airy type isostasy. These characteristics can be derived from the difference in magmatic activity, as influenced by the difference in lithospheric structure. The latter is characterised by greater active magmatism and hydrothermalism underneath the ridge than the former. Such a difference in the magmatic activity and the horizontal scale of the shallow subsurface structure is derived from the difference in the stiffness or viscosity of the lithosphere beneath the two ridges.     

Mechanical and thermal isostatic response of the Del Cano Rise and Crozet Bank (southern Indian Ocean) from altimetry data

An analysis of the relationship between the bathymetry and the gravity field has been carried out both in the frequency domain (admittance technique) and in the space domain over the Del Cano Rise and the Crozet Bank (southern Indian Ocean). A bathymetric map, contoured from original shipboard data and a geoid anomaly map, computed from a merged set of SEASAT and GEOS3 data were used. The inversion of admittances for water depth, crustal thickness, crustal density and elastic plate thickness shows that the isostatic response of the lithosphere supporting the Del Cano Rise differs very much from that of the one situated beneath the Crozet Bank. The Del Cano Rise appears to have been formed near the active Southwest Indian Ridge before Early Eocene and is locally supported by a thickened crust. Shallow thermal processes contribute to the present topography of the Crozet Bank, emplaced further away from the Southeast Indian Ridge on older crust.     

Active oceanic spreading in the northern North Fiji Basin: Results of the NOFI cruise of R/V L'Atalante (newstarmer project)

The South Pandora and the Tripartite Ridges are active spreading centers located in the northern part of the North Fiji Basin. These spreading centers were surveyed over a distance of 750 km during the NOFI cruise of R/V L'Atalante (August–September 1994) which was conducted in the frame of the french-japanese Newstarmer cooperation project. SIMRAD EM12-dual full coverage swath bathymetric and imagery data as well as airgun 6-channel seismic, magnetics and gravity profiles were recorded along and offaxis from 170°40 E to 178° E. Dredging and piston coring were also performed along and off-axis. The axial domain of the South Pandora Ridge is divided into 5 first-order segments characterized by contrasted morphologies. The average width of the active domain is 20 km and corresponds either to bathymetric highs or to deep elongated grabens. The bathymetric highs are volcanic constructions, locally faulted and rifted, which can obstruct totally the axial valley. The grabens show the typical morphology of slow spreading axes, with two steep walls flanking a deep axial valley. Elongated lateral ridges may be present on both sides of the grabens. Numerous volcanoes, up to several kilometers in diameter, occur on both flanks of the South Pandora Ridge. The Tripartite Ridge consists of three main segments showing a sigmoid shape. Major changes in the direction of the active zones are observed at the segment discontinuities. These discontinuities show various geometrical patterns which suggest complex transform relay zones. Preliminary analysis of seismic reflection profiles suggest that the Tripartite Ridge is a very young feature which propagates into an older oceanic domain characterized by a significant sedimentary cover. By contrast, a very thin to absent sedimentary cover is observed about 100 km on both flanks of the South Pandora Ridge active axis. The magnetic anomaly profiles give evidence of long and continuous lineations, parallel to the South Pandora Ridge spreading axis. According to our preliminary interpretation, the spreading rate would have been very low (8 km/m.y. half rate) during the last 7 Ma. The South Pandora and Tripartite Ridges exhibit characteristics typical of active oceanic ridges: (1) a segmented pattern, with segments ranging from 80 to 100 km in length; (2) an axial tectonic and volcanic zone, 10 to 20 km wide; (3) well-organized magnetic lineations, parallel to the active axis; (4) clear signature on the free-air gravity anomaly map. However, no typical transform fault is observed; instead, complex relay zones are separating first-order segments.     

Absolute and relative plate motions and hypotheses on the origin of five aseismic ridges in the Indian Ocean

Paleopositions of the African, Indian and Antarctic plates are used as an independant set of data to test various hypotheses, derived from geophysical data, on the origin, emplacement mechanism and evolution of five aseismic structures of the Western and Southern Indian Ocean (Crozet and Kerguelen plateaus, Marion Dufresne, Léna and Ob seamount chain, Madagascar and Mascarene ridges).Except for a continental fragment of very limited extent bearing the Seychelles Islands, these structures, together with the Madagascar and Mascarene ridges, are probably due to anomalous volcanic episodes at —or near— active plate boundaries, rather than to intraplate emplacement. Their creation processes are therefore mainly related to the relative motions between the Antarctic, African and Indian plates. The volcanic episodes are synchronous with major changes in the relative motions between these plates and thus substantiate the role played by frequent irregularities in the kinematic pattern of the Western and Southern Indian Ocean, such as ridge jumps, asymmetric spreading and rapid variations in spreading velocity or direction. Finally, we think that thermal anomalies, located not far from active spreading boundaries, may have played a role in the physical processes creating the aseismic ridges.     

Slow-ridge/hotspot interactions from global gravity, seismic tomography and 87Sr/86Sr isotope data

Among the mantle hotspots present under oceanic areas, a large number are located on—or close to—active oceanic ridges. This is especially true in the slow-spreading Atlantic and Indian oceans. The recent availability of worldwide gravity grids and the increasing coverage of geochemical data sets along active spreading centres allow a fruitful comparison of these data with global geoid and seismic tomography models, and allow one to study interactions between mantle plumes and active slow-spreading ridges. The observed correlations allow us to draw preliminary conclusions on the general links between surficial processes, which shape the detailed morphology of the ridge axes, and deeper processes, active in the upper mantle below the ridge axial domains as a whole. The interactions are first studied at the scale of the Atlantic (the Mid-Atlantic Ridge from Iceland to Bouvet Island) from the correlation between the zero-age free-air gravity anomaly, which reflects the zero-age depth of the ridge axis, and Sr isotopic ratios of ridge axis basalts. The study is then extended to a more global scale (the slow ridges from Iceland to the Gulf of Aden) by including geoid and upper-mantle tomography models. The interactions appear complex, ranging from the effect of large and very productive plumes, almost totally overprinting the long-wavelength segmentation pattern of the ridge, to that of weaker hotspots, barely marking some of the observables in the ridge axial domain. Intermediate cases are observed, in which hotspots of medium activity (or whose activity has gradually decreased) located at some distance from the ridge axis produce geophysical or geochemical signals whose variation along the axis can be correlated with the geometry of the plume head in the upper mantle. Such observations tend to preclude the use of a single hotspot/ridge interaction model and stress the need for additional observations in various plume/ridge configurations.