Evidence of synextension tilting and doming during final exhumation from analysis of multistage faults (Queyras Schiste lustrés, Western Alps).
Résumé
During the Neogene, the internal arc of the Western Alps underwent extension behind its inverted Paleogene frontal thrust while shortening affected the external arc. In the core of the internal arc, doming of eclogite-bearing gneissic nappes has formed the Dora–Maira massif. The blueschist-bearing Schistes lustrés of Queyras, which overlie the western flank of the Dora–Maira dome, allow observation of how the Neogene extension developed in ductile to brittle conditions. During the evolution from ‘chocolate tablet' boudinage to cross-trend normal faulting, the extension kept a tendency to be multidirectional. The analysis of variably oriented fault families allows considering them as globally associated with the same long-lived tectonic regime. Focusing on faults bearing several generations of slickenlines and using an adapted right dihedra analysis, we propose that faulting accompanied the westwards tilting of the eastern Queyras structure. This tilting is consistent with the doming of the Dora–Maira massif, itself associated with tectonic denudation along its western flank. Therefore, two processes operated simultaneously during final exhumation: regional thinning through widespread multitrend normal faulting and more localised tectonic denudation and doming. Both are consistent with a Neogene regime dominated by vertical compression. The role of an ascending deep indenter below the internal arc remains to be documented.