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 a nalyzed in order to recognize the regional tectonic differentiation pa ttern of the Ardenne, a Paleozoic massif of NW Europe. Data collected exclusively from 1/25,000 topographic maps were used to define a set o f parameters related to the geometry of the profile and proxies for ri ver bedload and drainage area. Validation of the proxy defined for the bedload/discharge ratio demonstrates its potential use for inter-rive r comparison based on its value at the river mouth; it is however inad equate to estimate the ratio changes along a given stream course. This proxy has been computed to estimate the portion accounted for by hydr odynamic factors in the river bed profiling. Thus, outliers to the cor relation function linking it to the river bed gradient have been consi dered as undergoing other influences, principally of lithologic or tec tonic origin. Most indicative of the computed profile parameters are t hose related to the concavity of the profile. Four more or less region al longitudinal profile types are recognized which are partly determin ed by differential tectonic movement. The combined analysis of all the parameters confirms the current uplift of the NE Ardenne and points t o a subsidence of the Namur area with respect to its SW and SE surroun dings, Whether periglacial or tectonic in origin, the Upper Pleistocen e lowering of a local base-level has also been identified in the Famen ne. Another conclusion of this study is that in order to evidence tect onic influences and to avoid any misinterpretation in a region of comp lex 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 mo rphogenetic history. (C) 1998 Elsevier Science B.V. All rights reserve d.