Hercynian high-altitude phenomena in the French Massif Central: tectonic implications

Stratified slope deposits, diamictite and sedimentary structures such as water-escape craters, ghosts of ice crystals and solifluction flowage in the Stephanian-Autunian of the French Massif Central are related to high-altitude periglacial effects. The clay mineral assemblage, dominated by the association illite-chlorite-irregular interlayed clays and the slight weathering of feldspathic clasts and granite/gneiss pebbles favour the prevalence of a cold to temperate climate at this time rather than a hydrolyzing climate of humid equatorial or tropical type. The flora and entomofauna also indicate a seasonally varying climate with cold winters favouring the development of periglacial phenomena at high altitude. In view of the equatorial paleogeographic position of the Hercynian belt, this type of climate could exist only at altitudes higher than 4500 m. The tectonic implications of these altitudes are examined through a comparison with the present-day Himalayas and Tibetan plateau.