Les "glaciers de marbre" de Patagonie, Chili. Un karst subpolaire océanique de la zone australe

Résumé En Fr

The " marble glaciers " of Diego de Almagro (Chilean Patagonia). Example of a subpolar karst in hyperhumid climate. The karst areas of Chilean Patagonia have remained virtually unknown until now because of their remoteness and very inhospitable climate. They are mainly located in two islands, Diego de Almagro and Madré de Dios, between latitude 52° and 50° South, with a subpolar and stormy climate "tempered" by heavy oceanic precipitations (7 m/year). In Diego de Almagro the Permian and Carboniferous limestones and dolomites have been transformed into marbles with lamprophyre dikes through contact metamorphism. Situated in the outer part of the archipelagoes, these long and narrow outcrops (0.5-2 km wide) are located between volcano-sedimentary formations of Upper Paleozoic (West) and the Mesozoic Patagonian batholit (East). The corallian paleoreefs are part of an accretionary prism of the Gondwana paleocontinent. The sutficial and underground karstification is one of the most spectacular ones in the world. The Karren (lapies) caused by the heavy rains can be 1-4 meter(s) wide and several hundred meters long for the solution runnels. Moreover, we can often observe solution karrens both due to rain and wind direction : flat karren (horizontal laminar flow), cascading ripples (sloping laminar flow) and profiled solution forms. The sutficial solution velocity is about 3 mm / 50 years (from old painting traces near the quarry of Guarello, Madré de Dios) ; and the lamprophyres dikes (Diego de Almagro) put in relief through corrosion indicate a 40-60 cm surficial solution since the melting of pleistocene glaciers.

Les karsts de la Patagonie chilienne se développent entre 50 et 52°S, sous un climat subpolaire hyperhumide (P = 7 m/an). Situées entre des dépôts volcano-sédimentaires et le Batholite Patagonien, les bandes carbonatées fortement écaillées correspondent à des paléorécifs coralliens du Permo-Carbonifère appartenant à un ancien prisme d'accrétion formé à la fin du Primaire en bordure de Gondwana. Aujourd'hui, la karstification est Tune des plus spectaculaires observées dans le monde. Les lapiés de ruissellement sont hypertrophiés en raison des pluies surabondantes et la violence du vent est à l'origine de formes particulières liées à un écoulement laminaire : lapiés plats, rides, cupules, formes profilées, etc. La vitesse de dissolution superficielle est de l'ordre de 3 mm en 50 ans (60 mm/millénaire) et les filons de lamprophyres mis en relief par la dissolution indiquent une ablation postglaciaire de 40 à 60 cm.