Investigating seismotectonic activity in northern France from LiDAR, palaeosismological trench and OSL/C-14 dating : new results along the Artois and Mélantois structures

Northern France presents mostly a low level of seismic hazard according to the French national seismic hazard map. Despite low instrumental seismicity rates, strong and unfrequent historical earthquakes occurred, with for instance the M~6, 1580 Strait of Dover earthquake, or the M~5, 1896 Lens-Arras earthquake, whose seismogenic sources are presumably the Sangatte and the Marqueffles Faults, respectively. Both belong to the NW-SE-directed Weald-Boulonnais-Artois structure. Moreover, the Haubourdin Fault (also named Lille-Hazebrouck Fault), at the hinge of the Mélantois anticline, and bordering the southern edge of the Lille Metropolis (1.2 millions inhabitants), is considered as potentially active during Quaternary times. All these above-mentioned faults are linked to deep Paleozoic structures in the basement that formed along the northern front of the Variscan orogeny, and that were regularly reactivated during the Mesozoic and Cenozoic. To investigate and document the possible neotectonic activity of the Artois structure and the Haubourdin Fault, and therefore improve seismic hazard assessment in northern France, we used a pluridisciplinary approach based on the analysis of 1) LiDAR dataset, 2) paleoseismological trenching, 3) OSL/¹⁴C dating, and 4) sub-surface geophysical survey.

Along the Artois structure, we focused on the locality of Harnes within Lens city suburbs. A preventive archeological work unraveled a clear sub-surface deformation feature that we analyzed through several ~2m-deep trenches. Field investigations indicated that the fault presents a regular N130° strike, which is consistent with surface and subsurface regional structures, and a 25-30° southwestward dip. Reverse throw along the fault were measured to about 15-20 cm thanks to a clearly displaced coal-rich horizon, sampled for C-14 dating. Interpretation of the data is complex since the site is located in a region where glacio-tectonic processes, severe First World War bombing and subsidence due to underground mining are documented.

Along the Haubourdin Fault, our analysis of high resolution LiDAR data highlighted two topographic scarps aligned along a N110°E trend, but that do not match with the fault trace extracted from the geological map. This new fault trace is confirmed by subsurface geophysical survey (electric resistivity tomography and mapping). Both scarps present contrasted uplifted blocks since the southern block is uplifted (by several meters) for the western branch, whereas the northern block is uplifted (by 1-2 m) for the eastern branch. All these new field mapping results call for a substantial revision of the fault trace in the region together with a consideration of its segmentation. Finally, two new OSL datings has been obtained from a sandy-clay layer unconformably sealing some Late Paleocene deformation in the southern Lille suburb (i.e., Villeneuve d’Ascq). It gives the first minimum age in the literature for the deformation of the Mélantois northern limb.