The arc of the western Alps: a review and new kinematic model

The arc of the western Alps forms the western termination of the Alpine Chain. The E-W striking Austro-Italian-German and Swiss Alps turn into a N-S direction along the western margin of the Po plain, finally rotating back to an E-W strike along the Italian-French Mediterranean coast. The origin of this enigmatic shape was originally attributed to a variscan inheritance (Argand, 1916), but the vast majority of the present-day literature suggests that it results from the indentation of Adria during collision, as a result of a significant W-directed component of convergence. We briefly review previous interpretations and suggest a new kinematic model based on retrodeformation of syn-collisional shortening, on paleomagnetic results, on structural analysis of maps on the arc-scale, and on field-based structural investigations.

Retrodeformation of syn-collisional shortening around the arc of the Western Alps points to the existence of an arc of significant amplitude before the onset of collision. Paleomagnetic results from the External Zone (Dauphinois) suggest that most rotations around vertical axes only affect the Mesozoic cover above the Triassic, hence they do not provide an information on the kinematic of the entire crust. In the area of the Argentera Massif, where paleomagnetic data were derived from Permian beds, hence allowing to interpret rotations of the entire crustal block, it is shown that no significant rotations around vertical axes affected the area during Alpine orogeny. Structural analyses of maps indicate that the transition between the N-S and E-W striking parts of the arc in the External Zone is abrupt, taking place along the Var Valley. No progressive rotations of structures are observed there, instead N-S striking folds and thrusts appear to be interrupted by the E-W striking ones which continue all along the southern coast of France until the Pyrenees. In several localities, stratigraphic and structural evidences show that these E-W structures were initiated before the onset of Alpine collision, and amplified during Alpine collision.

Our field-based structural data and compiled ones point to the occurrence of a large-scale widely distributed system of sinistral shear zones, striking ENE-WSW, which affect the area north of the Argentera Massif including part of the Internal Zone. Such structure was often assumed to be the prime site accommodating the west-directed indentation of Adria. In spite of its significant extent, its newly mapped location within the Arc precludes such such a 1^st order kinematic role of this structure during collision.

To conclude, we suggest that the arc of the Internal Zone (Penninic Units) showing a progressive rotation of structures is not similarly observed in the External Zone, and we infer that this progressive, continuous curvature largely existed or formed during subduction. The arc of the western Alps as observed in the External Zone mainly reflects the existence of such a structure at the end of subduction and the transition between the Alps s.s. and the Pyrenean Chain, reactivated during Miocene time.