Cenozoic tectonics of the Western Approaches Channel basins and its control of local drainage systems

The geology of the Channel Western Approaches is a key to understand the post-rift evolution of the NW European continental margin in relation with the Europe/Africa collision. Despite considerable evidence of Tertiary tectonic inversion throughout the Channel basin, the structures and amplitudes of the tectonic movements remain poorly documented across the French sector of the Western Approaches. The effect of the tectonic inversion for the evolution of the "Channel River", the major system that flowed into the English Channel during the Plio-Quaternary eustatic lowstands, also needs to be clarified. Its drainage basin was larger than the present-day English Channel and constituted the source of terrigenous fluxes of the Armorican and Celtic deep sea fans. A lack of high-resolution seismic data motivated the implementation of the GEOMOC and GEOBREST cruises, whose main results are presented in this paper. The new observations highlight the diachronism and the contrast in amplitudes of the deformations involved in the inversion of the French Western Approaches. The tectonic inversion can be described in two stages: a paroxysmal Paleogene stage including two episodes, Eocene (probably Ypresian) and Oligocene, and a more moderate Neogene stage subdivided into Miocene and Pliocene episodes, driven by the reactivation of the same faults. The deformations along the North Iroise fault (NIF) located at the termination of the Medio-Manche fault produced forced folds in the sedimentary cover above the deeper faults. The tectonic inversion generated uplift of about 700 m of the mid-continental shelf south of the NIF. The isochron map of the reflectors bounding the identified seismic sequences clearly demonstrates a major structural control on the geometry of the Neogene deposits. First, the uplift of the eastern part of the Iroise basin during the upper Miocene favoured the onset of a broad submarine delta system that developed towards the subsiding NW outer shelf. The later evolution of the 'palaeovalley' network corresponding to the western termination of the "Channel River" exhibits a 'bayonet' pattern marked by a zigzagging pattern of valleys, with alternating segments orientated N040 degrees E and N070 degrees E, controlled by Neogene faulting. The palaeovalley network could have begun during Reurevian or Pre-Tiglian sea-level lowstands, which exposed the entire shelf below the shelf edge. The amplitude of the sea-level fall is assumed to have been magnified by uplift of the Iroise basin, followed by later tilting of the outer shelf, as observed in many other examples documented along the North Atlantic margins.