The use of the Stream Length–Gradient index in morphotectonic analysis of small catchments: A case study from Central Italy

Abstract

The aim of this work is to test an integrated quantitative geomorphic approach based on Stream Length–Gradient Index (SL Index) analysis within the small catchment of the Tarugo River in the Northern Marchean Apennines (Central Italy). The Tarugo River basin, 92 km² wide, strikes SW–NE across the Marchean Ridge and the western sector of Marchean Piedmont. The area is characterized by a series of NW–SE trending, NE verging thrust folds affecting the Meso–Cenozoic bedrock that pass upward from dominantly carbonate to dominantly terrigenous rocks. The area investigated has been affected since the Pliocene by extensional tectonics, accompanying a regional uplift decreasing in rate north-eastwards. In this work the SL Index analysis has been integrated with the spatial analysis of the Amplitude of relief (A_r) and compared to geological and geomorphological field data. Results of this work indicate that the SL Index is a valid tool to detect the long wavelength structural effect on topography as well as the incipient local response to regional processes (i.e. regional uplift) that is often undetectable by other morphotectonic parameters. In contrast, the SL Index analysis seems not be a valid tool for discriminating the local lithological influence from the tectonic one.

Introduction

Some geomorphic indices have been developed to identify areas experiencing rapid tectonic deformation (Keller, 1986, Keller and Pinter, 1996 and references therein). Geomorphic indices are particularly useful in tectonic studies because they provide a rapid evaluation of large areas and can be obtained easily from topographic maps or aerial photos (Strahler, 1952). Moreover, in recent decades, the increasing usefulness of GIS software has made it easier to undertake quick and detailed processing of data.

Recently, in morphotectonic studies traditional geomorphic analysis has been integrated with morphometric analysis of landforms and with geostatistical topographic analysis (Keller et al., 1982, Mayer, 1990, Cox, 1994, Merritts et al., 1994, Lupia Palmieri et al., 1995, Lupia Palmieri et al., 2001, Currado and Fredi, 2000, Pike, 2002, Della Seta, 2004, Della Seta et al., 2004). The Stream Length–Gradient Index (SL Index) is one of the quantitative geomorphic parameters included in morphotectonic investigation (Hack, 1973). In tectonically active regions, and/or at the regional scale of analysis, the SL Index can be a useful tool to detect tectonic displacements (Keller and Pinter, 1996, Chen et al., 2003 and references therein; Zovoili et al., 2004). Nevertheless, the effectiveness of the parameter in detecting local active structures has not been confirmed for small catchments and/or in regions where tectonic activity is less intense (Chen et al., 2003 and references therein; Verrios et al., 2004). In small river basins the contribution of the lithological effect to anomalous values of the SL Index seems indistinguishable from the tectonic one.

In this work SL Index analysis was performed for the small Tarugo River drainage basin, located on the Adriatic flank of Apennine chain, in Central Italy. This case study was chosen with the aim of better defining the usefulness of SL Index analysis in detecting the impact of neotectonics on small river-valley development. The longitudinal profile and SL Index analyses were coupled with the geostatistical analysis of the Amplitude of relief (A_r), which proved to be a significant morphotectonic parameter (Ahnert, 1970, Ciccacci et al., 1988, Centamore et al., 1996, Della Seta et al., 2004).