BUDGET OF CRUSTAL SHORTENING AND SUBDUCTION OF CONTINENTAL-CRUST IN THE ALPS

The stratigraphy, surface structure, and deep structure of the Alps permit, but do not require, that a significant amount of European continental crust was subducted into the mantle beneath Adria, the fragment of continental lithosphere now comprising much of Italy and the Adriatic Sea floor. The present thin crust in much of Europe suggests that the crust that was stretched in Mesozoic time to form the southern margin of Europe was initially thinner than normal. The stratigraphic history of sediment deposited on that margin before its collision with Adria suggests that the crustal basement on which much of this material was deposited had become particularly thin during the formation of the margin. Most of the crustal basement underlying the European margin and involved in the collision and the formation of the chain probably was thinner than 20 km, and much of it may have been thinner than 10 km. Profiles of the present deep structure of the Alps yield a range of possible values for the average cross-sectional area of crust involved in the collision that permit as much as 280 km of crustal shortening, if this crust was as thin as implied by the stratigraphy. Thus, although subduction of hundreds of kilometers of continental crust into the mantle beneath Adria could have occurred, the budget of crustal material involved in the formation of the Alps does not require such crustal subduction.