Testing the tectonic significance of some parameters of longitudinal river profiles: the case of the Ardenne (Belgium, NW Europe)

The longitudinal profiles of 24 rivers of 3rd to 5th order have been analyzed in order to recognize the regional tectonic differentiation pattern of the Ardenne, a Paleozoic massif of NW Europe. Data collected exclusively from 1/25,000 topographic maps were used to define a set of parameters related to the geometry of the profile and proxies for river bedload and drainage area. Validation of the proxy defined for the bedload/discharge ratio demonstrates its potential use for inter-river comparison based on its value at the river mouth; it is however inadequate to estimate the ratio changes along a given stream course. This proxy has been computed to estimate the portion accounted for by hydrodynamic factors in the river bed profiling. Thus, outliers to the correlation function linking it to the river bed gradient have been considered as undergoing other influences, principally of lithologic or tectonic origin. Most indicative of the computed profile parameters are those related to the concavity of the profile. Four more or less regional longitudinal profile types are recognized which are partly determined by differential tectonic movement. The combined analysis of all the parameters confirms the current uplift of the NE Ardenne and points to a subsidence of the Namur area with respect to its SW and SE surroundings, Whether periglacial or tectonic in origin, the Upper Pleistocene lowering of a local base-level has also been identified in the Famenne. Another conclusion of this study is that in order to evidence tectonic influences and to avoid any misinterpretation in a region of complex geologic and geomorphic history and moderate tectonic activity, it is essential to consider a number of parameters and to consider their significance within the framework of detailed geologic setting and morphogenetic history. (C) 1998 Elsevier Science B.V. All rights reserved.