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The Congo Basin (CB) was initiated during a Neoproterozoic intracratonic rifting process across the Congo Shield, possibly related to the break-up of the Rodinia Supercontinent. The Neoproterozoic history of the CB remains poorly constrained because it is largely concealed by Phanerozoic sequences and is thus accessible only by drilling and geophysical investigations. Neoproterozoic basins peripheral to the CB, however, share first-order similarities in their stratigraphic successions: an initial coarse clastic sequence followed by a thick carbonate sequences and terminating by a dominantly siliciclastic sequence. The subsequent transition to the Lower Paleozoic is influenced by regional Pan-African deformation and molasses-like foreland basin sediment-fill. The Neoproterozoic stratigraphy and structure beneath the CB can be tested also using data from two fully cored stratigraphic wells (Samba: 2,038 m and Dekese: 1,836 m), and two exploration wells (Gilson-1: 4,563 m and Mbandaka-1: 4,350 m), seismic refraction and reflection profiles, gravimetric and magnetic data. Synthesizing this data, we show that the CB shares a similar early evolution as its flanking basins in that they all initiated in extensional tectonic environments and were subsequently affected by compressional deformation during late Neoproterozoic to Early Cambrian Pan-African orogenesis.

Resume Cette etude est basee sur l'interpretation d'un profil sismique tire dans le pas de Calais, avec la realisation d'un modele gravimetrique et magnetique. Situe entre deux fronts de deformation majeurs, la deformation caledonienne du massif du Brabant, au nord et la deformation varisque, soulignee par la faille du Midi au sud, le detroit est dans une position cle. Le profil sismique montre la superposition de plusieurs bassins, dont l'evolution est controlee par des failles profondes compressives, heritees de l'âge Paleozoique inferieur. On y reconnait aussi deux phases de deformation majeures post-paleozoiques, l'extension jurassique et l'inversion tectonique post-Cretace. Ces phases sont similaires a celles observees dans les bassins du Weald, du Wessex et dans ceux du Sud de la mer du Nord. Elles sont cependant plus faiblement marquees. Le modele geophysique montre, quant a lui, la geometrie des series profondes ordovicienne et silurienne, non accessibles en sismique. Deux hypotheses sont proposees. La premiere est la continuite du massif du Brabant, sous le detroit, qui est une vision nouvelle par rapport aux limites anterieurement proposees. La deuxieme concerne l'existence d'un pluton granitique probablement responsable de la remontee du Silurien (kilometre 27 sur le profil). De tels plutons ont deja ete proposes dans le Sud et dans l'Est de l'Angleterre ainsi qu'en Belgique. Les similitudes de geometrie, de structure et la reconnaissance de certains episodes majeurs suggerent donc que l'histoire geologique du detroit est comparable a celle des regions connexes. Pour citer cet article : B. Minguely et al., C. R. Geoscience 337 (2005).

The geology of the Boulonnais has been well studied since the early part of the last century [Gosselet and Bertaut, 1873; Olry, 1904; Pruvost and Delepine, 1921]. Extensive coal exploration added substantially to the general understanding of the geology of the region but as outcrop is poor, many questions remain. Gravity methods used in the analysis of geological structures have had a long and successful history in helping to study the earth's crust for scientific and applied objectives. Regional gravity data are particularly useful in mapping geographic distribution and configuration of density contrast of rocks. Previous gravity research shows the main trends of the structure. In most cases the regional Bouguer gravity hides the relationship between the geology and the shape of the anomaly caused by the perturbing body. New information can be obtained by filtering the maps. The purpose of filtering a map is to remove unwanted characteristics and enhance desirable characteristics that are diagnostic for the geology. Because of their simple mathematical forms, most potential field filters are in the spectral domain. It is advisable to transform the original unfiltered field to the spectral domain, apply the filter, then transform the filtered map back to the spatial domain for use in the interpretation. Several spectrally filtered versions of the original gravity map are used in this regional interpretation. In the case of the Boulonnais the most useful filters have been the horizontal component and the first vertical derivative. In the first instance computing the horizontal gradients of the gravity field permits us to localise the limit of the blocks and then the fault positions. The gravimetric field above a vertical contact of rock with different density shows a low on the side of the low density rocks and a high on the side of the high density rocks. The inflection point is located just on the contact of the two types of rocks. This contact can be outlined by locating the maxima of the horizontal gradient. In the case of a low dipping contact maxima stay close to the contact, but are displaced down dip. In the second instance the first vertical derivative acts as a booster for the short wavelength; this attenuates or destroys the effect of the regional field. The resulting map shows a better structure because in complex areas they give a better definition of the different bodies by separating their effects. In the case of the Boulonnais the first vertical derivative allows us to distinguish the depressed region from the uplifted one. The structural evolution of the Boulonnais-Artois area includes two main extensional events in the late Palaeozoic-early Cretaceous interval and an inversion in mid-late Palaeocene time. The new gravity data in combination with recent field and published data have provided a new insight into the structure of the Boulonnais-Artois area and a new interpretation is proposed. -- Fault patterns are oriented 110N and 040N in the Boulonnais and 140N in Artois areas. -- The linkage between the faults shows a relay geometry with transfer zones [cf. Morley et al., 1990 and Pea-cock and Sanderson, 1994]. The best example is located between Sangatte (near the tunnel) and Landrethun faults where overlapping synthetic faults with a relay ramp are imaged. -- There is no major continuous fault zone but a complex en echelon fault system. -- Linkage between Boulonnais and Artois fault is not well constrained. An important discontinuity between the two regions is apparent. This model underlines the importance of overlapping fault tips with the generation of transfer zones. These structures are also known in the Wessex and Weald basins [Stoneley, 1982; Chadwick, 1993] where heritage and inversion are significant.

Within the context of the western Paris Basin, considered to be a region experiencing slight uplift during Neogene and Quaternary, the ESR dating of Quaternary deposits, together with those of Neogene age, the morphology and available geophysical data suggest that the Somme valley is an actively subsiding sector (tectonic shortening between two active inversions). The dissymmetry of the valley tributaries and of the deposits preservation is controlled by a flexuring above the hidden Somme fault. The record of neotectonic events and the geophysical data demonstrate its activity and elucidate the recent history of the Paris Basin.

The Dover Strait is open during the Paleogene, closed from the Upper Eocene and reopens from the middle Quaternary; but Pliocene foramifers assemblages are identical on both sides of the strait. The re-analysis of the Messinian shore line of the Monts de Flandres (Diestian at the Noires Mottes) in the prolongation of the Landrethun Flexural zone, of the Slack Formation, and the discovery of a new formation (Wimille Formation, inside the Boulonnais), all scealled by a goethitic pan, attest of an opening of the Dover Strait, controlled by the Messinian tectonical crisis and by the evolution of the Western Channel and its paleovalley system. Tectonical, geomorphological and climatic implications of this opening are discussed within the Western European context.

Low motion rates, typical of intraplate settings, make it particularly difficult to isolate a tectonic signal in vertical displacements of the ground dominated by near-surface influences. Since the spring 2003, we have performed half-yearly GPS and gravimetric campaigns in NE Ardenne in order to evaluate the seasonal changes imposed to the ground height by groundwater variations. The GPS height data show an excellent negative correlation with a proxy for groundwater variations, based on rainfall in the 6 months before the survey, that allows a reliable correction of the measured height changes. During the 2003–2005 time span, the seasonal groundwater-dependent height changes have amounted to a maximum 7.5 mm. The gravimetric campaigns were able to detect reliably only gravity changes larger than 10 μgal, which corresponds to the upper limit of the gravity changes associated with the proposed groundwater-GPS model. No conclusive result may therefore be derived from the gravity observations.