Abstract
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.
Highlights
In the Western Alps, the Late Triassic (Carnian–Norian) evaporites are known to be the major décollement level of the external alpine fold-and-thrust belt (Fry, 1989; Gidon and Pairis, 1992; Lickorish and Ford, 1998)
In map view, when all the salt structures and the associated basins are restored to their original locations in the Jurassic with respect to the Astoin main diapir main (Fig. 20), we see the main feeder of the diapiric complex in the east (Fig. 20), and an allochtonous salt sheet emplaced during the passive margin stage (Fig. 16)
The study of the Astoin diapir reveals that it formed a plurikilometric diapiric complex where the Astoin diapir constituted the main diapir to which are connected several salt ridges
Summary
In the Western Alps, the Late Triassic (Carnian–Norian) evaporites are known to be the major décollement level of the external alpine fold-and-thrust belt (Fry, 1989; Gidon and Pairis, 1992; Lickorish and Ford, 1998). From the very early stages of the Alpine history, there is evidence of salt tectonics which has created an important structural inheritance (Goguel, 1939; Lapparent, 1940; Graciansky et al, 1986; Dardeau and Graciansky, 1990; Dardeau et al, 1990; Graham et al, 2012; Célini et al, 2020). In this paper remarkable outcrops allow us to describe the 3D lateral variability of the most prominent salt-controlled structure of the Digne area, the Astoin diapir and its surrounding sedimentary basins (Goguel, 1939; Arnaud et al, 1977; Gidon, 1997; Célini et al, 2020). This study is mainly based on the geological maps at 1/50 000 and published works (Arlhac et al, 1983; Gidon et al, 1991a) together with our own field observations and comparisons with other regions of the world
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