Recent History (Quaternary)
Friday 20 November 2009 by Thomas Lebourg

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The second component of the landscape features is related to the Quaternary erosion of the valley. The latter includes both internal geodynamic factors (active tectonics) and external ones (climate). The Quaternary active tectonics of the area is mainly bound to the Argentera Massif uplift. Denudation velocities have been assessed on the basis of geochronological datings for the period 8MY till the present-day (S. Bogdanoff et al, 2000 ; F. Bigot-Cormier, 2002). The authors suggest velocities around 0.25 mm/year for the period [-8MY ; -3.5MY] then around 1 mm/year [-3.5MY ; present-day], although the present movements are still hypothetical. G. Musumeci et al, 2003 mention orographic anomalies (high concentration of slopes steeper than 70%) and anomalies in the drainage network (upper hydrographic catchments) which are linked to the expression of the long-term influence of the Argentera Massif uplift.

As described by M. Dubar and J.F. Stéphan, the Quaternary erosion is the sum of glacial period erosions plus interglacial erosions. The available data on the glacial history of the area mainly relate to the regional Würmian glacier. According to M. Julian, 1980 the Würmian glacier did not exceed 500 m thick. The thickness has been re-assessed greater than 600 m by M. Dubar et J.F. Stéphan. They also assume a maximum elevation not exceeding 1,800 m. Thus the Würmian glacier is qualified as a ’valley glacier’, that has left its track in the present landscapes of the upper Tinée valley. As described effectively by M. Julian and E. Anthony, 1996 the glacial and post-glacial erosions have structured the slopes of the valley into three parts:

- slope feet with very steep slopes (superior to 40°),
- an intermediate stage (with slopes comprised between 30 and 35°),
- a upper part with lower slopes (around 20°).

(a) Slope map of the upper Tinée valley area built with Surfer software and the database from the Institut Géographique National (IGN) - 1:25,000

(b) SW-NE topographic cross section of the La Clapière landslide. The average fields of occurrence for the morpho-structures (1: cracks, 2: counter slope scarp, 3: double crest) are set up according to the works by H. Jomard, 2006

That specific topographic ’inheritance’ induce the dispersion of the gravitational deformations affecting the slopes of the valley. The mapping works of the deformation indexes related to DSGSD conducted by H. Jomard, 2006 allow to determine average fields of occurrence for the morpho-structures (figure b). Cracks seem to occur mostly at mid-slope, whereas counter slope scarps generally are located in the upper parts .The double crests affect peaks. The lateral extension of the indexes can reach orders of magnitude in kilometers. Therefore the author puts forward two important axes:

- the 8 listed DSGSD gather 80% of the average and large extent rock movements,
- some morphological indexes (cracks and scarps) are superimposed on structural lineaments (N110°-140°),
- one of the DSGSD affecting the base of the upper Tinée is ’Colle Longue’ DSGSD. It spreads out from Isola village to the Saint Etienne de Tinée village, i.e. a 45 km2 area. Six rock average and large extent movements are associated.

Among those movements stands the La Clapière rock landslide. The still active landslide involves 60 millions m3.