Origin of arc of the Western Alps

Abstract

The arc of the Western Alps started to develop during the main Alpine orogenic phase in Late Eocene to Early Oligocene time. This occurred as a side effect of the general north-south compression in the Central and Eastern Alps. Before this time no arc existed. A dominant east-west structural grain, expressed by the axes of the folds which originated during the Late Cretaceous, characterized the entire belt from the Dauphiné and Provence of southeastern France to the region which later became the Eastern Alps. The final and most powerful curvature was the result of an east-southeast to west-northwest compression caused by the motion of the Italian-Dinaric microplate along the right-lateral lnsubric line during Early Oligocene to Early Miocene time. The deformation produced in the Western Alps by these movements can best be explained by applying the slip-line field theory. Accordingly, the two systems of strike-slips faults observed in the Western Alps, in the Jura mountains and in the Dauphiné, are more or less parallel to the slip-lines created in the rigid plastic medium of the Western Alps by the motion of the northwestern prong of the Italian-Dinaric microplate, which acted as the indenter. It is thus postulated that by the recogniqd style of deformation the rock masses forming the arc of the Western Alps flowed toward north-northeast and south-southwest from an axis through the centre of the arc.