DEM and GIS analysis of the stream gradient index to evaluate effects of tectonics: The Normandy intraplate area (NW France) Marianne Font ⁎, Daniel Amorese, Jean-Louis Lagarde Laboratoire Morphodynamique Continentale et Côtière (M2C), UMR CNRS 6143, Université de Caen, Caen, F-14000, France abstract article info Article history: Received 9 April 2009 Received in revised form 12 March 2010 Accepted 17 March 2010 Available online 21 March 2010 Keywords: Morphometry Stream gradient index DEM GIS Intraplate zone Tectonic uplift Computer-based geomorphometry using a DEM (Digital Elevation Model) allows the analysis of the three- dimensional properties of landscape. This methodology is particularly useful in an intraplate region like western Europe where the simple visual inspection of the topography cannot resolve the evolutionary trends of landforms. In these domains, the morphologies of the topographic surface may be controlled mainly by climate under a low rate of tectonic deformation. Among the geomorphometric parameters, the stream length index (SL) has been used to characterize fluvial systems in relation to tectonics movements. This work develops an algorithm to derive and map the SL index using a DEM and GIS, to investigate its spatial variations in a broad area. The algorithm is applied to a zone of weak intraplate deformation: the coastal lowlands of Normandy (France). The obtained spatial distributions of SL point to anomalous zones with high SL values. These zones are adjacent to mapped fault scarps and characterized by changes in flow direction. A Kruskal–Wallis test shows that the bedrock lithology has no impact on the SL value. Therefore, the SL variations can be related mainly to a differential uplift due to Quaternary tectonic forcing. Quaternary sea level fluctuations may also be responsible for high SL values in a part of the coastal lowland. © 2010 Elsevier B.V. All rights reserved. 1. Introduction In typical intraplate area (where the strain rate b 10 −10 y −1 ; Gordon, 1998), the usual geomorphometric parameters (Keller and Rockwell, 1984) may not highlight recent uplifts induced by a weak tectonic activity. In these areas, landforms tend to prevailing smooth topographies and the low uplift rates (Lagarde et al., 2000; Cloetingh et al., 2006) cannot induce conspicuous fault scarps and marked changes in slope gradient along rivers (knickpoints). In the intraplate area of NW France (Normandy), standard parameters of watersheds elevation, slope gradient and hypsometry give evidences of topographic anomalies (Baize et al., 1998; Font, 2002). To take the analysis further, we analyze a specific morpholog- ical parameter: the stream length gradient index (SL). The SL index describes the morphology of a stream network using the distribution of the topographic gradients along rivers. This index was first used to reflect stream power or differential rock erodibility (Hack, 1973). Because rivers are highly responsive landforms, they may register geomorphological phenomena better than hillslopes (Hesterberg et al., 2000; Duvall et al., 2004; Pearce et al., 2004; Carretier et al., 2006). Because SL is very sensitive to changes in channel slope, it is also used to evaluate possible effects of tectonic activity and rock resistance on topography (Keller and Pinter, 1996). Usually, SL is used as an indicator of uplifted zones in active geodynamical settings (Merritts and Vincent, 1989; Chen et al., 2003; Harkins et al., 2005; Troiani and Della Seta, 2008). In areas with low uplift rates, streams are characterized by lower SL values while streams flowing in areas with high to intermediate uplift rates show higher SL values. In this work, we test the relevance of SL in zones of weak intraplate deformation. Our methodology is based on the processing of both a Digital Elevation Model (DEM) and vectorized streams. It is applied to the coastal lowlands of Normandy (France). This case study aims to better define the usefulness of SL analysis in detecting the impact of differential uplift on drainage systems in an intraplate region. It does not aim to show that SL is an unequivocal criterion of tectonic forcing, but to show that mapping SL using a DEM, instead of using topographic profiles, helps to understand landforms in relation to tectonics. It should be noted that in the study area the pattern of SL may also be influenced by sea level variations during the Quaternary period. The SL index may help to compare the effects of tectonic forcing and eustatic changes on topography. 2. Normandy test zone 2.1. Geological setting and Plio-Quaternary evolution of Normandy Normandy can be divided into two geological zones. Western Normandy is a part of the Armorican Massif, comprising extensive outcrops of the Proterozoic and Palaeozoic basement, whereas Eastern Normandy belongs to the margin of the Paris Basin (Fig. 1). The post- Geomorphology 119 (2010) 172–180 ⁎ Corresponding author. Tel.: + 33 231 565 718; fax: + 33 231 565 757. E-mail address: marianne.font@unicaen.fr (M. Font). 0169-555X/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.geomorph.2010.03.017 Contents lists available at ScienceDirect Geomorphology journal homepage: www.elsevier.com/locate/geomorph