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SUMMARY Altimetric measurements of sea-surface heights have long been used to locate uncharted marine features. Newly available data sets (Geosat-GM, ERS-1 and Topex/Poseidon), now allow the topography of submarine volcanoes to be accurately restored and a method based upon inverse modelling is presented in this paper. This method allows a joint assimilation of altimetric data and shipborne profiles of bathymetry and/or gravity anomalies when available. Furthermore, uncertainties can be computed together with the topography, which constitutes a noticeable improvement since these parameters are still missing in the usual bathymetric data bases. Tests are performed using simulated data, in order to characterize the errors which may or must occur with the modelled topography of a previously unknown volcano, in particular the errors due to data uncertainties and the lack of precise knowledge of sensitive parameters that are used in the modelling. These errors can be maintained under the 100 m level (rms) for altimetric data with a measurement noise of 5 cm (rms) or less, which is achieved for Geosat, ERS-1 and Topex/Poseidon altimeters. An example of inversion of Seasat data is presented in the case of a seamount in French Polynesia.

Summary The methods of magnetotelluric (MT) and differential geomagnetic soundings (DGS) have been applied to study the electromagnetic response of the structure of the Senegal sedimentary basin. The measurements at 10 sites were carried out along a profile running perpendicularly across a zone of north-south flexures and faults over the period range 10-1000 s. Variation of periods up to 10000 s were obtained at two sites. The anomalous geomagnetic variation field across the major fault zone is characterized by an increase in the magnetic eastward component. Two-dimensional modelling of the apparent resistivities and phases reveals a highly anomalous upper crustal structure involving a low-resistivity zone in the central part of the basin (less than 30 Ωm). To satisfy the additional long-period data at two sites, a deep conductor is also required at a depth below 300 km with a resistivity less than 10 Ωm. The crustal discontinuity located ∼ 200 km off the coast is possibly related to the opening of the Atlantic Ocean. The difficulty in resolving the question of the nature of the crust in this region results in part from the limitations of the methods used at present for this determination.

Summary. Stable natural remanent magnetization (NRM) in the Jersey Volcanics and in a single rhyolite dyke was probably acquired during the Cambrian before folding of the volcanics in the Cadomian Orogeny. After dip correction, the volcanics yield a palaeomagnetic pole at 323" E, 52" N (dp = 33", dm=35"). In Jersey dolerite dykes three groups of stable NRM directions are recognized, with palaeomagnetic poles at 248' E, 26" N (dp = lo", dm = 20°), 339" E, 1" S (dp = 9", dm = 12"), and 336" E, 31" S (dp = 5", dm = 9"). Comparison with the European apparent polar wander path implies that stable NRM in these groups was acquired respectively during Late Precambrian or early Cambrian, Siluro-Devonian and middle Carboniferous time. The stable NRM of the Jersey lamprophyre dykes yields a palaeomagnetic pole at 322" E, 16" N (dp = 31°, dm = 38") and is probably of Silurian or Devonian age. These palaeomagnetic poles and other new data determined by the author for the Armorican Massif can be fitted to a common apparent polar wander path for Europe, and this implies that the basement of Lower Palaeozoic Europe extended from the Baltic Shield at least as far south as the Armorican Massif. The Hercynian Orogeny in these parts of Europe was therefore probably intracratonic. This polar wander path implies that in early Cambrian time the pole did not move significantly relative to Europe, but that this was followed by a large middle to late Cambrian polar shift which corresponded to rapid drift of Europe across the South Pole.

Summary. Magnetotelluric soundings (MT) were conducted along the 14th parallel on the eastern border of the West African craton in the Republic of Niger. This profile includes eight sites an average of 30 km apart. We determined the regional structure of electric conductivity and its relation to the various geological belts covered. This study took place within a 15–500 s period band. Two-dimensional modelling suggests that, in the sites located on the sedimentary basin and the mobile belt, there exists a conductive layer in the upper mantle at a depth of 80 km. Within the craton we were unable to prove the existence of this conductive layer. There also exists another conductive layer at the crust—upper mantle boundary at a depth of 30 km, but this seems to disappear in the cratonic belt. A significant electric discontinuity is present between the mobile belt and the sedimentary basin, due to a variation in resistivity in the substratum and a thickening in the surface cover.

Received 1983 January 14 ; in original form 1982 June 25; International audience; Samples of the keratophyre tuffs of Tréguier (640 ± 12 Myr) of the Upper Brioverian series, and the microgranite dykes of Loguivy (548 ± 24 Myr) have been collected from the north coast of Brittany, France. The samples were progressively demagnetized with AF and, predominantly, thermal methods. Blocking temperatures, coercivities and acquisition of Isothermal Remanent Magnetization (IRM) indicate that the keratophyre remanence is variably carried in magnetite, hematite, or occasionally in both. After removal of a soft viscous component, two stable palaeomagnetic directions were found, one in magnetite, the other in hematite. The characteristic magnetite direction (Tb= 300-580°C) is a pre-folding primary Thermoremanent Magnetization (TRM) which after structural correction has a mean declination/inclination of 217.9°/-30.4°. The younger hematite component has a mean in situ direction of 320.4°/+20.3°.The microgranite Natural Remanent Magnetization (NRM) intensity distribution is distinctly bimodal, forming high and low (≤ 1.6 × A m-1) intensity groups. Upon demagnetization, a single stable remanence (in magnetite) was removed between 300 and 580°C or 20 and 100 mT. Two characteristic directions were found to be statistically related to the intensity groups at the 99 per cent confidence level. Low-intensity samples have a mean direction of 201.9°/+2.3° and probably resulted from a Late Devonian - Early Carboniferous local magnetization. High intensity microgranites have a mean direction of 200°/+50.8° and this remanence is shown to be a primary TRM. Palaeopoles obtained from the two keratophyre magnetizations (magnetite 44.7°N, 116.4°E; hematite 39.5°N, 233.6°E), as well as the high intensity microgranite remanence (5.8°S, 339.9°E), show good agreement with similar age poles from the apparent polar wander path for the Armorican Massif. Comparison of these data to recent palaeomagnetic data from southern England and Wales gives additional evidence to link that region with the Armorica plate as proposed by Van der Voo and Hagstrum et al.