Abstract:
GYPSUM KARSTIC LANDFORMS IN VANOISE: the alpine and glacial valley of Vallon du Fruit-Gébroulaz (Alps, France) - In the inner part of the northern french Alps, the higher regions of the Vanoise offer outcrops of Triassic gypsum of which the surface and thickness vary. In the “vallon du Fruit”, the “Gébroulaz” glacier partially covers a long strip of gypsum, which reaches its highest point at the “Roc de la Soufrière” (2940 m). Rivers and springs in the “vallon du Fruit” are fed both by sub-aerial glacial outflows and by karstic underground flows, which are pro-glacial and sub-glacial. A very low chemical dissolution exists under the glacier and along the fast pro-glacial underground and sub-aerial flows, whereas the rate of karstic denudation is high in the margin of the glacier where it reaches 1500 mm/ky at around 2500m (1 ky = 1000 years). The formation and evolution of the dolines is rapid and occurs through underground sucking and dissolution once the area is deglaciated, thanks to underground active flows fed by the glacier and snow melting. Gypsum domes are uplifted under the effects of neotectonic movements and postglacial decompression brings about considerable superficial fissuring because of the elasticity of the rock. Over 10,000 years, the morphogenesis of these domes in the humid and cold climatic conditions of these high alpine mountains has transformed them into “perforated ladle” and domes. Small outcrops are changed into monoliths or gypsum inselberg. A model of the geomorphologic post-glacial evolution of these domes, over some 20,000 years, is proposed.