Unravelling the drainage divide migration of a slow deforming mountain range: Insights from morphometry, hilltop curvature and cosmogenic nuclides in the Vosges Mountains (NE France)

The Vosges massif, a mid-altitude mountain range in northeast France, stretches 250 km north of the Alps, displaying significant north-south and east-west topographic, geological, and geomorphological gradients. Despite the region's low deformation rates and moderate seismicity, dominated by strike-slip mechanisms, the Vosges still maintain significant topographic relief, raising questions about the forces sustaining the massif’s topography.

The evolution of drainage divides offers insights into the interplay of tectonic, erosional, and climatic processes shaping mountain landscapes. This study investigates the dynamics of the main drainage divide of the Vosges through a comprehensive analysis integrating morphometric indices, denudation rates derived from cosmogenic nuclide concentrations (¹⁰Be and ²⁶Al isotopes) in small catchments, and hilltop curvature as a proxy for denudation rates. By analysing small-scale, second-order basins near the divide, we reduce complexities associated with sediment routing, lithological heterogeneities, and large-scale geomorphic integration. In addition, by using a calibrated relationship between hilltop curvature and the local denudation rates, we provide a region-wide quantification of the divide migration rate unravelling the processes driving its ongoing dynamics.

The results reveal that the main drainage divide is systematically migrating westward, away from the Rhine River valley. In the southern Vosges, transient geomorphic processes, including knickpoints and episodic river captures, drive rapid adjustments intertwined with a background of slow and gradual divide migration. Conversely, the northern Vosges exhibit lower relief and subdued topographic gradients, where weaker erosional contrasts result in negligible divide mobility despite comparable cosmogenic nuclide-derived denudation rates.

These findings emphasize that even in low-deformation settings, subtle and transient processes can maintain relief and reconfigure drainage networks. Mid-altitude, slowly deforming landscapes like the Vosges are not necessarily stable; instead, they continue to evolve through a balance of gradual erosional processes and episodic drainage readjustments, sustaining relief over geological timescales.