From spatio-temporal patterns of river incision rates to Quaternary uplift history of the Variscan Rhenish/Ardenne Massif (N Europe)

The Rhenish/Ardenne Massif (RAM) spreads across parts of Belgium, France, Germany and Luxemburg; it is one of the largest (>40,000 km²) and most “emblematic” Variscan structures located north of the Alps (think of the “Romantic Rhine”). Intraplate uplift affected the RAM during the Plio-Quaternary along with other Palaeozoic massifs located in the alpine foreland. However, its cause(s), shape and rates are still poorly understood and therefore remain debated (e.g., Demoulin & Hallot, 2009). This was, until recently, mainly due to a lack of reliable ages for uplift markers, such as the Quaternary terrace staircases along deeply incised valleys of the Rhine, Moselle and Meuse as well as their main intra-massif tributaries. Several studies based on numerical dating methods (i.e., in situ cosmogenic nuclides, electron spin resonance, luminescence…) have shed new light on these questions by assigning numerical age estimates on key levels of fluvial terraces (e.g., the so-called main terraces; Rixhon et al., 2011; Cordier et al., 2012) or cave levels related to phases of regional base-level stability (Rixhon et al., 2020). This contribution first compiles all chronological data produced over the last twenty years and critically assesses their reliability to infer massif-scale spatio-temporal patterns of river incision. Plio-Quaternary incision rates are accordingly reconstructed. A similar trend of increase is reported throughout the RAM with a peak of incision occurring during the Early or Middle Pleistocene and matching the massif-wide geomorphological marker materialised by the main terraces (and associated cave levels if any). However – and importantly – age control reveals a significant time lag (>250 ka) between the south-eastern and north-western RAM margins. The high incision rates onset is consistently older along the Rhine/Moselle and tributaries (e.g., the Sarre) than along the Meuse and tributaries (e.g., Ourthe). This key finding is well in line with Demoulin and Hallot’s (2009) hypothesis arguing for a wave of uplift migrating northward throughout the RAM. It also supports regional tectonic causes for uplift (i.e., late, upper-crustal stress transfer from the Alps to their foreland) rather than more local ones (i.e., mantle plume below the Eifel Massif). Age constraints along the river valleys draining the easternmost part of the RAM – so far absent – along with a global geodynamic modelling will represent further steps to better understand the evolution of the uplift history.   References: Cordier S. et al., 2012. Geomorphology, 165-166, 91-106. Demoulin, A., Hallot, E. 2009. Tectonophysics, 474, 696-708. Rixhon, G., et al. 2011. Quaternary Geochronology, 6, 273-284. Rixhon, G. et al. 2020. Geomorphology, 371, 107424.