Late Quaternary incision rates in the Vésubie catchment area (Southern French Alps) from in situ‐produced 36 Cl cosmogenic nuclide dating: Tectonic and climatic implications

We have estimated recent river incision rates using the in situ‐produced 36 Cl cosmogenic nuclide concentrations. The target site consists of a ~25 m high vertical profile along a polished river cliff located in Jurassic limestones in the Vésubie catchment area, in the southern French Alps. The 36 Cl exposure ages of the sampled river polished surface range from 3 to 14 ka, i.e., after the Last Glacial Maximum. Our data suggest as a first approximation a linear age/height relationship and lead to a mean incision rate of 2.2 mm a −1 over the last 14 ka. More precisely, incision rates are characterized by two peaks reaching ~2 and 4–5 mm a −1 at 4–5 ka and 11–12 ka, respectively, separated by a period experiencing a lower incision rate (~1 mm a −1 ). A chi‐plot of the river longitudinal profile suggests that on the long term, the river is close to equilibrium conditions with a concavity index of 0.475. The evolution of the Vésubie River longitudinal profile over a time period of 2 Ma based on the stream power law of river incision was then modeled with varying erodibility coefficients and uplift rates ranging from 0.5 to 2 mm a −1 . The best fitting models yield erodibility coefficient values ranging from 2.5 to 9.0 × 10 −6 m −0.475 a −1 for the considered uplift rates. For long‐term uplift rates lower than 2 mm a −1 , an increase of the erodibility coefficient during the last 16 ka, with two peaks at 11–12 and 4–5 ka, is necessary to precisely match the observed incision rates and is interpreted as resulting from recent climatic changes. These variations do not strongly affect the general shape of the river profile and suggest that the measured short‐term incision rate is dominated by a climatic signal, which does not preclude the possible role of tectonic uplift.