Chronological constraints on processes leading to large active landslides

Cosmic ray exposure dating of gravitational scarps along a slope nesting a presently active landslide (La Clapière landslide, Tinée valley, Argentera massif, SE France) reveal successive periods of gravitational destabilization. An initial gravitational destabilization at 10.3 ± 0.5 10Be ka occurs more than 3000 yr after the end of the last deglaciation of the Tinée valley, dated in the studied area from “stoss-and-lee” topography sampled along the Tinée and its tributary Rabuons valleys. A second gravitational destabilization is well constrained at 7.1 ± 0.5 10Be ka through the dating of two gravitational scarps. Although coincidental with the so-called “climatic optimum”, other local or regional triggering factors may be invoked. A third evidenced gravitational destabilization occurred 2.3 ± 0.5 10Be ka ago. This study thus demonstrates that in situ produced cosmogenic 10Be allows establishing the chronology of gravitational destabilization leading to present major active landslides. However, in the present case, deciphering between the various possible triggering factors will necessitate similar studies on other landslides inside the alpine arc.