Along-strike variations of salt-related structural style in the Western Alps

The Upper Triassic evaporites of Western Europe, also known as the ‘Keuper’, are well-known and have been mostly considered as an efficient décollement level for the thrusts of the external fold-and-thrust belts. Numerous recent studies aimed to reappraise their role, and especially the role of salt tectonics, in the formation of several mountain belts such as the Pyrenees, the Betics, Provence, and the Alps.

The Western Alps represent a good laboratory to study the role of salt in shaping a mountain belt because it contains areas with (1) no evaporites, (2) evaporites involved as an efficient décollement level during orogeny and (3) evaporites mobilised in salt tectonics since the Lias rifting. We propose here, based on literature and our recent works regarding salt tectonics in the SW Alps, to present and discuss the different salt-related structural styles observed along-strike the Western Alps. A focus will be done on the SW Alps where evaporites influence their structure during the whole Alpine history from rifting until collision. They were mobilised by the Lias rifting through reactive diapirism. Salt tectonics carried on during the post-rift period by passive diapirism, controlled by sediment loading. A few structures were reactivated during the Oligocene and in places evaporites influenced the structure of the subalpine chains until the Mio-Pliocene.

Our study shows that evaporites strongly influence the structure of a mountain belt at different scales and that along-strike variations of structural style are observed along the strike of the Western Alps depending on the presence, the amount or the absence of evaporites.