The Variscan basement of the Aiguilles-Rouges massif (Western Alps) exposes the Servoz syncline which consists of a metavolcano-sedimentary sequence composed of (i) a volcanic unit of unknown age and origin, (ii) Early Carboniferous sedimentary series affected by the Variscan orogeny and intruded by the Montées-Pélissier pluton, and (iii) a Late Carboniferous late-orogenic sedimentary sequence. We combined field investigations, Raman Spectroscopy on Carbonaceous Material geothermometry, and LA-ICPMS U-Th-Pb geochronology on zircon in order to reappraise the sedimentary sequence of the Servoz syncline. Our results allow us to identify three distinct sedimentary formations (F1, F2 and F3). The F1 formation is composed of metagreywackes, bimodal volcanic and magmatic rocks formed during basin opening at an early rifting stage (370–350 Ma) within a back-arc geodynamic setting. This extensional regime was responsible for a high thermal event recorded by a ca. 115 °C/km apparent geothermal gradient. Local anatexis of the basement rocks under the basin is dated at 351 ± 5 Ma. Basin inversion occurred between 350 and 330 Ma in response to oblique collision, with the development of large-scale dextral shear zones and syn-kinematic 340–330 Ma granite intrusions. Subsequent dextral transtension was responsible for the opening of a pull-apart basin between ca. 330 and 310 Ma with the deposition of the F2 phyllite formation that was later deformed by the ongoing dextral transcurrent Variscan tectonics at temperatures between 200 and 350 °C. Finally, the F3 terrigenous sedimentary rocks deposited at ca. 310–290 Ma in a late-orogenic extensional basin. The Alpine-related tectonic event overprinted all the temperatures below 350 °C. Although similar basins have been recognized in other External Crystalline Massifs of the Alps, the Servoz syncline is the first example that allows a major part of the polyphase tectonic evolution, since the early stages of the Devonian, to be recognized. Comparison with similar back-arc basins from the French Central massif, the Vosges massif and the Bohemian massif suggests that the External Crystalline Massifs initially belonged to the Moldanubian hinterlands of the Variscan belt.