DIRECT EVIDENCE FOR COMPOSITION OF LOWER CRUST AND MOHO

Geophysical investigations in the Ivrea-Verbano zone of the Southern Alps provide evidence that in this area a crustal chip has been detached from the mantle and bent upward to the surface during the Alpine orogenesis. Therefore, a petrographie section of the Ivrea zone which can be studied at the surface probably corresponds to a depth profile of the upper and lower crust including the crust-mantle transition zone. It leads to the following picture: the essential part of the crust consists of gneiss and granite which in their deeper parts are highly metamorphosed, dehydrated, and by mobilisation differentiated. With increasing depth it follows a mixture zone of 5-10 km vertical extension with lenticular dioritic and basic bodies. The true gabbroidic layer of the lower crust must be fairly thin. The transition toward the peridotitic mantle is indicated by ultrabasic enclosures. This petrographic model agrees surprisingly well with crustal models derived from geophysical data north of the Alps. © 1969.