Evaporites play a major role in the evolution of an orogenic wedge, modifying the shape and the deformation kinematics inside the wedge. Salt tectonics can occur at various stages, but early salt activity can create a structural inheritance which compartmentalizes the building of the sedimentary succession and the mechanical architecture of the orogenic wedge. Sub-Alpine fold-and-thrust belts commonly show evidence of inherited salt-related structures, which were mostly described as having been initiated during the Liassic Tethyan rifting. However, even though a few authors tried to introduce salt tectonics as a major factor in the Alpine history, most of the interpretations underestimate the phenomenon. In this paper, we show that the Digne Nappe area presents many salt-controlled structures within both the Digne and the Authon-Valavoire thrust sheets. Salt activity began during the Hettangian and continued through the whole Jurassic. At a larger scale, our observations show alignments of salt-controlled structures following NW-SE and NNE-SSW directions, as well as preferential locations at tectonic unit boundaries. Distribution of salt structures seems to follow a well-defined pattern directly inherited from the rifting, and this strongly influences deformation of the sub-Alpine domain.

The structure of the southwestern branch of the Alpine orogen is affected by the extensive Late Triassic evaporites. These evaporites have been involved in polyphased salt tectonics since the early Liassic, coeval with the Tethyan rifting, and are the décollement level for thrusts in the external parts during Alpine orogeny. The role of salt tectonics in this branch of the Alpine arc is re-evaluated in order to determine the relative importance of early deformation related to salt motion with respect to deformation related to main Alpine compressional events. This paper focuses on one structure identified as diapiric since the 1930’s: the Astoin diapir (Goguel 1939). Analysis of geological maps together with new field work have allowed to better define diapirism in the Upper Triassic evaporites outcrops around Astoin. Study of the diapir and the surrounding depocenters reveals a major involvement of salt in the structuration of the area, since the Liassic. Several salt ridges are linked to a main diapiric structure, explaining why we call it the “diapiric complex” of Astoin. Salt tectonics was initiated during the Liassic rifting, and a few locations show evidence of reactive diapirism whereas in others evidence of passive diapirism as early as the Liassic is seen. Passive diapirism continued during the post-rift stage of Alpine margin history in the Late Jurassic and Cretaceous when an allochthonous salt sheet was emplaced. Diapirism also occurred during the Oligocene while the Alpine foreland basin was developing in this part of the European margin of the Alps. Serial interpretative cross-sections have been drawn in order to illustrate the lateral variations of diapirism and structural style. Sequential evolutions for each cross-section are proposed to reconstruct the diapiric complex evolution through time. The Astoin diapir shows a complex structural framework with an important along-strike variation of diapiric activity. Most of the geometries are inherited from salt tectonics that occurred during extension, and in some places these early structures are overprinted by Alpine compressional structures.

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