Thesis of Aboubacar Dibousse

Defense of thesis Aboubaccar Dibousse - laboratoire LOG

Tectonics-Sedimentation Interactions in the Boulonnais and Dieppe-Hampshire basins (Eastern Channel): Integrated Land-Sea Approach

Abstract :

The Eastern Channel is a complex geological area. Its current morphology results from major tectonic deformations during the Mesozoic and early Cenozoic, including extension and subsidence coeval with the opening of the North Atlantic Ocean, as well as compression involving basin inversion related to the Iberian-Eurasian convergence. The primary objective of this thesis is to refine the geometry of the deformations in the bedrock of the Eastern Channel, while evaluating their relationship at the land-sea interface along the northern coast of France. This thesis focuses on the inverted Boulonnais basin and particularly, of its southern and northern boundaries.
The work relies on new structural surveys onshore, along the coast and the analysis of new high-resolution seismic data (Sparker type), complemented by lower resolution old industrial seismic data. The seismic data analysis allowed for a detailed description of the Meso-Cenozoic stratigraphic column along the Eastern Channel's border, providing further details on the spatial distribution of the Jurassic and Cretaceous series. An attempt was made to correlate the mapped seismic units with the known series onshore. The most significant stratigraphical surfaces (unconformities, erosional surfaces) were identified in the series. Structural analysis of the bedrock reveals that the Jurassic series of Kimmeridgian-Tithonian age as well as the Wealden series (Lower Cretaceous) are affected and controlled by major normal faults (E-W and N120) dipping to the south, along with antithetic faults, which have smaller displacements and dip to the north, conferring a general asymmetric half-graben structure to the basin. Several markers indicate the tectonic inversion of the Boulonnais basin, both onshore, along the coast and in the offshore domain. This inversion phase is characterized by a general parallelism between the pre-existing normal faults, locally reactivated as reverse faults, and the main anticline folds associated with them. Some major faults in the basin (Slack-Epitre, Wimereux-Belle, Nord-Bassurelle) show lateral attenuation and transfer onto parallel faults. This phenomenon is also observed for longitudinal fold axes, such as the Crèche anticline, transferred southward into the Boulogne anticline offshore. This general configuration of structural relay, between land and sea, is exemplified by the Sangatte fault, which is linked onshore to the Cap Gris-Nez–Landrethun fault system through two transfer faults with a N020-030 orientation (Wissant-Les Quénocs zone). The deformed area of Cap Gris-Nez, which stands out from the rest of the study area due to the concentration of an extensive fault network, results from the inversion process of a major normal fault (the Courte-Dune fault), coeval with the formation of the basin. This compressional reactivation results in the development of asymmetric anticlines, buttressing against an inverted fault zone, thereby characterizing a general model of "buttress folds" against a normal fault. On the southern border of the basin, the inversion of the Nord-Bassurelle fault zone and its satellites induces the formation of forced folds into the Mid-Upper Cretaceous discordant cover, directly above the reverse fault. Inverted faults rarely propagate into the Upper Cretaceous and Cenozoic cover, where deformation is essentially accommodated by folding of the cover series.

Keywords : Tectonic,Eastern Channel,Boulonnais,seismic reflection