Multi-methodology characterisation  of low energy landslide : Example of Blamécourt (Vexin region, France)

Landslides are commonly investigated in mountainous regions characterized by steep slopes. In contrast, the low-plateau region of the French Vexin (Paris Basin) is shaped by slopes resulting from  ancient low-energy mass movements. The objective of this study is to describe the geometry and outcrops of an ancient landslide in order to obtain data to geologically characterize its dynamics and processes. In the French Vexin area, valleys are incised into a limestone plateau whose multilayered stratigraphy - comprising coarse limestone, fine sand and clay - controls the water table position. This water table can induce  seepage erosion within the sand layers  beneath  the limestone layers and can be considered as a predisposing factor. This leads to their fragmentation (rotational blocks) and/or their progressive dipping (i.e. cambering) towards the valley bottoms to adapt to the topography subjected to gravitational constraints. Recent studies conducted in a similar geological setting in the Champagne vineyards in France have improved our understanding of the links between these mass movements, substrate properties and hydrogeological conditions. However, the French Vexin region exhibits distinctive characteristics: the upper limestone layer is particularly thick and densely fractured, resulting in slope shapes that have never been studied before. A representative site in Blamécourt (Magny-en-Vexin, Val d’Oise) was investigated to develop methodology for characterizing slope processes and their geological context. The area includes  three disused quarries, multiple outcrops and a complex morphology. Field observations, high-resolution LiDAR, GIS mapping and electrical geophysical data were combined to analyse this complex landslide. Detailed morphological studies and characterization of geological structures in quarries beneath the plateau have revealed the state of the rock without the influence of the valley. The limestone blocks are fractured in two directions of tectonic origin, corresponding to the regional structural directions. From the plateau edge, a third structural trend aligned with the valley orientation is observed. These three structural directions persist downslope to the base of the slope, as confirmed by field observations and structural analysis. The limestone blocks covering the slope have therefore been affected by gravitational movements, whose structural boundaries result from the combined influence of inherited faults and newly formed structures.