Gravimetry contribution in the Boulonnais and Artois areas (France)

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 Dele pine, 1921]. Extensive coal exploration added substantially to the general understanding of the geology of the region but as outcrop is poor, many que stions remain. Gravity methods used in the analysis of geological structures have had a lo ng and successful history in helping to study the earthy crust for scientif ic and applied objectives. Regional gravity data are particularly useful in mapping geographic distribution and configuration of density contrast of r ocks. The previous gravity research [C.F.P er al., 1965] shows the main tre nds of the structure. In most cases the regional Bouguer gravity hides the relationship between t he 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 characte ristics that are diagnostic for the geology. Because of their simple mathem atical forms, most potential field filters are in the spectral domain. It i s advisable to transform the original unfiltered field to the spectral doma in, apply the filter, then transform the filtered map back to the spatial d omain for use in the interpretation. Several spectrally filtered versions o f the original gravity map are used in this regional interpretation. In the case of the Boulonnais the most useful filters have been the horizon tal component and the first vertical derivative. In the first instance comp uting 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 r ocks. 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 horizo ntal gradient. In the case of a low dipping contact maxima stay close to th e contact, but are displaced down dip. In the second instance the first ver tical derivative acts as a booster for the short wavelength: this attenuate s or destroys the effect of the regional field. The resulting map shows a b etter structure because in complex areas they give a better definition of t he different bodies by separating their effects. In the case of the Boulonn ais the first vertical derivative allows us to distinguish the depressed re gion from the uplifted one. The structural evolution of the Boulonnais-Artois area includes two main ex tensional events in the late Palaeozoic-early Cretaceous interval and an in version in mid-late Palaeocene time. The new gravity data in combination wi th recent field and published data have provided a new insight into the str ucture of the Boulonnais-Artois area and a new interpretation is proposed. - Fault patterns are oriented 110N and 040N in the Boulonnais and 140N in A rtois areas. - The linkage between the faults shows a relay geometry with transfer zones [cf. Morley et al., 1990 and Peacock and Sanderson, 1994]. The best exampl e 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 sy stem. - Linkage between Boulonnais and Artois fault is not well constrained. An i mportant discontinuity between the two regions is apparent. This model underlines the importance of overlapping fault tips with the gen eration of transfer zones. These structures are also known in the Wessex an d Weald basins [Stoneley, 1982; Chadwick. 1993] where heritage and inversio n are significant.