DRAINAGE EROSION AND CONCAVE LANDFORM OF TIJUCA GNEISSIC MASSIF, STATE OF RIO DE JANEIRO, BRAZIL, WITH THE HELP OF SUMMIT LEVEL AND BASE LEVEL TECHNIQUE BASED ON ASTER GDEM

This paper presents the state of drainage erosion and three-dimensional concavity of Tijuca gneissic massif, Municipal District of Rio de Janeiro, Brazil, by means of geomorphologic analyses based on the ASTER GDEM. The massif has an extension of 15 x 10 km and relative height of 1000 m. The summit level maps show absence of marginal scarp and limited area of virtual plateau. The massif is divided into Tijuca Forest and Carioca Range by Alto da Boa Vista valley zone, which is wider than 2 km. Small rocky hills form five parallel chains with N60°E orientation. The base level maps present the drainages wider than 1 km striking to N45°E, which is different from the hill orientation. The narrow and short drainages at the highest range zone constitute a radial system. The areas of Tijuca Forest Range, Carioca Range, and Gavea Rock have high relief amount with the risk of large-scale landslide. They are characterised by subvertical gneiss exposures. The altitude distribution histogram shows an Eiffel Tower-like pattern, indicating concave three-dimensional landform of the massif. The volume-normalised three-dimensional concavity index (TCI) and macro concavity index (MCI) are, respectively, 0.54 and -0.5, which are significantly larger than those of the felsic alkaline massifs. The concave landform of Tijuca massif is attributed to erosive vulnerability of the gneiss because of low mechanical strength, gneissic texture, and low weathering passivity effects. Keyword: Tijuca massif, summit level, base level, TCI, MCI, GDEM.