Earth Sciences DEM-Based Morphotectonics Analysis of Western Ligurian Alps

Scientiﬁca Acta 2, No. 2, 44 – 47 (2008)

Earth Sciences DEM-based morphotectonics analysis of Western Ligurian Alps

Davide Zizioli Dipartimento di Scienze della Terra, Università di Pavia, Via Ferrata, 1, 27100 Pavia, Italy [email protected]

Development and interpretation of morphometric maps are important tools in studies related to neotectonics and geomorphology; Geographic Information System (GIS) allows speed and precision to this process. This work presents a methodology to evaluate commonly morphometric indices used in geomorphology to estimate the inﬂuences on the drainage evolution by the structure.

1 Introduction

The Western Ligurian Alps shows a peculiar drainage network related to the geological events that marked this area since the Pliocenic ingression to date. From long-time, several researchers have studied the drainage evolution history of this area, with the purpose to understand how and how much its character is due to heritage ("the structure") and/or to neotectonics. Of particular relevance, the GIS approaches permit the analysis of several scale Digital Elevation Model (DEM) and the "enhancement" of neotectonics lineaments. This zone is in fact affected by numerous systems of faults, whose the Breil Sospel Monaco (BSM) line, and the Saorge-Taggia (ST) line, represent the principal expressions. The Saorge -Taggia system consists in a rather complex right strike slip bundle of faults, with orientation NW-SE, that is extended from the Ligurian Sea to the intersection northwards with the BSM, considered a left strike slip. Focal mechanisms which represent the earthquake of Ventimiglia of Aprils 21, 1995 (intensity 4.7), announce the presence of an inverse fault, oriented NW-SE, belongs to a system of faults, set to SE and subparallel to the Saorge-Taggia, identiﬁable probably with the Ospedaletti-Olivetta San Michele line, even denominated in literature as Sanremo fault. In this work we show some results given by the analyses of parameters extracted from digital elevation model, that give us the opportunity to develop and interpret morphometric maps. Principal morphometric indices evaluated are: slope, aspect, surface roughness, swath proﬁles, lineament and drainage density, isobase, hydraulic gradient and drainage fractal dimension. Here we want to show the chosen study approach, but we think that clearly emerges from preliminary results that drainage network established its course along the main structures. Its evolution however, especially in recent era, is strongly related to the different lithological competence and, secondly, to the brittle tectonics.

2 Geological setting

In the Souht–Eastern part of the Argentera Crystalline massif, several Piemont–Ligurian tectonic units outcrop, positioned like a fan which widens towards the coast and with its apex pointing Nord (Fig. 1). The highest and the most extensive of these units is The S. Remo–Monte Saccarello, whose Flysch sequence is believed to be deposited in the Piemont–Ligurian Ocean: along its eastern margin for a brief tract northwards, this unit directly overlays Briançonnais type sequences, while the rest lies on another Piemont Flysch, called Moglio–Testico unit. Locally, the relationships of superposition with the Moglio–Testico are inverted due to post–nappe deformations. Along its western margin the San Remo–Monte Saccarello unit cover instead the Ventimiglia Sandstone, stratigraphic top of the Dauphinois–Provençal foreland series.

Fig. 1: Tectonic sketch map of the Maritime Alps and adjoining areas [3]. 1 = main Pliocene basins in Western Liguria; 2 = tectonic lines (V-V = Villalvernia-Varzi; O-L = Ottone-Levanto); 3 = meso-alpine fold axes; 4 = neo-alpine buried fold axes; 5 = Oligocene grabens (Ba = Bagnasco; Ca = Cadibona; S.G. = Santa Giustina; Sa = Sassello); 6 = normal faults affecting the Ligurian Sea ﬂoor; 7 = post-Tortonian buried overthrust; 8 = macroseismic epicenters. 5. S-V. = Sestri-Voltaggio Zone the contact with the Dauphinois–Provençal happens by means of the interposition of a complex series of slices of different origin [1,2].

3 Methodologies

Analyses were carried out using an original five meters resolution DEM derived from vector maps and applying several automatic routines to extract morphometric parameters developed by the author. To better characterize the original surface for apply an hydrogeomorphic analysis, the original DEM was first filtered with a 3x3 low pass algorithm, secondly converted to points and then reinterpolated at 10 meters resolution, using regularized splines with tension. This preparatory steps were necessary to remove noise and some spurious peaks in elevation due to the contours lines derived nature. Slope and aspect were calculated with a 3x3 neighborhood operator. Surface roughness was computed to provide an objective quantitative measure of topographic heterogeneity; this method [4] is useful for morphological characterization since it is related with the shape of landforms and not its elevation. Thus, tectonically tilted areas have their expression shown, while it could be masked in a hypsometric map, as consequence of altimetrical variations. According to Riley [5] it was computed calculating the sum change in elevation between a grid cell and its eight neighbour grid cells. Drainage networks were extracted using curvature criterion [6]; for each major basin were derived the spatial distribution of the number of stream reaches (Strahler order [7]), of [8] Horton’s parameters variations, of the total length of the stream network and were computed bifurcatio ratio, drainage density and hierarchical anomalies. The isobase method [9], concerns about re- lations between stream channel order and topography. The stream order refers to the relative position of

© 2008 Università degli Studi di Pavia 46 Scientifica Acta 2, No. 2 (2008) stream segments in a drainage basin network; within such basin, streams of similar orders relate to similar geological events and are of similar geological age. The main goal of this method is to be able to identify areas with possible tectonic influence even within lithological uniform domains. Isobase line were con- structed intersecting drainage networks (2nd and 3rd order) with 10 meters spacing contour lines derived from DEM; isobase map derived from spatially interpolation of isobase lines using tension splines. In an attempt to determine areas with similar hydraulic behaviour, a map of hydraulic gradient was elaborated, according to the proposition of Grohman [10]. This parameter is evaluated for each 2nd order stream as the ratio of the altimetric difference between head and mouth with the plan length; the value is attributed to the mid-portion of the stream. Hydraulic gradient and longitudinal river profiles were represented in the same graphs so that quickly point out possible anomalies. Lineament analyses were approached both by traditional methods (geological photogrammetry) and modern digital methods (digital stereo and digital anaglyphs of TERRA Aster satellite images) but the main results were given by the analyses of shaded relief maps. This approach has advantages over the use of satellite imagery, since it is possible to set up the position (azimuth and inclination) of scene illumination, thus emphasize the several existing lineament orientations, due the enhancement of directions perpendicular to lighting, in spite of parallels ones. Using the method proposed by Belisario [11], several azimuthal transects were performed, allowing hypothe- sising the kinematics of the main tectonic elements within the area. Finally Fractal Dimension of river networks was delineated to evaluate the influence of lineaments on river channels and to establish rivers freedom degree. To this aim author has developed a Fractal dimension calculator, that works directly in GIS environment, using the Box Counting Method. That consists in place grids over the river network and then the number of cells intersecting the river is counted. This process is repeated using a smaller cell size and is continued until it becomes unable to be estimated. Once these values are calculated they are converted to log10 and plotted. The slope of the best fit line for the plot of log10 of cell count against log10 of 1/ cell size is the Fractal Dimension fd.

4 Discussion and results

The main morphometric parameters (bifurcatio ratio, hierarchical anomalies, shape factors) allowed to classify most of the hydrographic basins located in the study area as low hierarchized. All in all fractal di- mensions set up to low values, frequently close to 1.2, typical of networks strongly inﬂuenced by tectonic or controlled by abrupt lithological changes. Lowest founded values are indicative of rectangular, sub-parallel and trellis channel geometries. Lineaments analysis allowed recognition of 1228 lines corresponding to 900 Kilometers length. Most of this lines are oriented NW-SE, quite parallel to the most important regional system that affect the area. It’s possible to recognize another organized system, prevalently oriented NE-SW. Transect analysis applied to 1st and 2nd stream order channels and developed perpendicular to lineament systems, globally shows a right strike slip kinematics for NW-SE lines and a left strike slip for lines oriented NE-SW. Surface roughness, longitudinal proﬁle, isobase maps and hydraulic gradient let us to discriminate different area affected by tilting in recent period. In particular, at regional scale, the application of this methodologies to hydrographic basins located in Western Ligurian Alps allowed to classify this area in two different morphologic region. One characterized in the whole by nearly-level rugged surface, the other more articulate and rugged. The boundary between these regions seems to be due to a geolithological change for the Northern and Eastern parts (S. Remo–Monte Saccarello unit more resistant than Ventimiglia Flysh), whereas in the South-Western there is a structural boundary corresponding to the Saorge-Taggia right strike slip system. At local scale they allowed to recognized tectonically tilted area, mostly located in the lower Nervia River Valley, near Ventimiglia and in the upper Argentina Valley.

5 Conclusions From a methodological point of view, we’ve developed several routines that let easily to obtain most of commonly used morphometric parameters in hydrographic analyses working directly in GIS environment. The application of this delineated methodologies in the study area, has highlighted a strong inﬂuence of ancient structures on the establishment of hydrographic networks. Besides we can assert that some of these ancient structures are still actives at the present, as demonstrated by 1st and 2nd stream order channel rotation, especially clear in the lower Argentina valley near Arma di Taggia.

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