Timing of inversion of the Weald-Boulonnais Basin inferred from calcite U-Pb geochronology of brittle structures along the Opal Coast, northern France

Although far-field deformation related to the convergence between Eurasian and African plates has long been documented in central and western Europe, only few outcrops clearly document the corresponding compressive structures. In northern France, the Opal coast gives access to the Upper Jurassic strata of the north-eastern border of the Weald-Boulonnais inverted basin. They are affected by (1) E-W trending normal faults related to early and late phases of syn-rift extension, and by (2) E-W-trending folds and associated reverse faults very similar to those in southern England. Using calcite U-Pb geochronology, we show that folding and associated brittle structures formed during the Middle-Late Eocene to Oligocene period. U-Pb ages of compression-related structures span between 49.4+ or -3.6 Ma and 26.0+ or -2.0 Ma. Our data thus give an estimate of the period of basin inversion and uplift of the Weald-Boulonnais anticline, as well as that of the related denudation phases. At a larger scale, they emphasize the synchronicity of basin inversion with the most prominent phase of the Iberia-Europe convergence, mainly accommodated by shortening within the Pyrenean orogenic system. Other E-W to NW-SE regional folds in the Paris Basin are suggested to have formed during this period.