The boundary between the Helvetic and the Penninic (=Briançonnais) Zones has long been recognized as a major fault (“Penninic Front”) in the Western Alps. A narrow oceanic domain has been postulated at least along part of this boundary (the Valaisan Basin). However, the information provided by the pre‐Triassic basement has not been fully exploited and will be discussed here in detail. The igneous and metamorphic history of the pre‐Triassic basement shows significant differences between the External Massifs from the Helvetic Zone, with abundant Late Carboniferous granites, and the basement of the Briançonnais Zone, including the Internal Massifs (Dora‐Maira, Gran Paradiso, and Monte Rosa), devoid of Carboniferous granites. A major coal‐bearing basin, the “Zone Houillère,” opened along this boundary. This limnic intramontane basin has never been properly investigated. The Zone Houillère is not comparable with the external, paralic, flexural, basins on both sides of the Variscan belt but shows similarities with the Saar‐Saale Basin. Like the latter, we interpret the Zone Houillère as a transtensional basin opened along a major, crustal‐scale, fault zone, namely, the East Variscan Shear Zone. The Permian magmatism and sedimentation displays contrasting distributions, being absent or very localized in the Helvetic Zone, and widespread in the Penninic Zone. The above data indicate that the structural inheritance from the Variscan belt plays a major role in defining the future location of the Valaisan Basin, that is, the boundary between the European paleomargin and the Briançonnais microcontinent.

International audienceThis study reviews and synthesizes the presentknowledge on the Sesia–Dent Blanche nappes, the highesttectonic elements in the Western Alps (Switzerland andItaly), which comprise pieces of pre-Alpine basement andMesozoic cover. All of the available data are integrated in acrustal-scale kinematic model with the aim to reconstructthe Alpine tectono-metamorphic evolution of the Sesia–Dent Blanche nappes. Although major uncertainties remainin the pre-Alpine geometry, the basement and coversequences of the Sesia–Dent Blanche nappes are seen aspart of a thinned continental crust derived from the Adriaticmargin. The earliest stages of the Alpine evolution areinterpreted as recording late Cretaceous subduction of theAdria-derived Sesia–Dent Blanche nappes below theSouth-Alpine domain. During this subduction, severalsheets of crustal material were stacked and separated byshear zones that rework remnants of their Mesozoic cover.The recently described Roisan-Cignana Shear Zone of theDent Blanche Tectonic System represents such a shearzone, indicating that the Sesia–Dent Blanche nappes representa stack of several individual nappes. During thesubsequent subduction of the Piemonte–Liguria Oceanlarge-scale folding of the nappe stack (including the Roisan-Cignana Shear Zone) took place under greenschistfacies conditions, which indicates partial exhumation of theDent Blanche Tectonic System. The entrance of the Brianc¸onnais micro-continent within the subduction zone ledto a drastic change in the deformation pattern of the Alpinebelt, with rapid exhumation of the eclogite-facies ophiolitebearingunits and thrust propagation towards the foreland.Slab breakoff probably was responsible for allowing partialmelting in the mantle and Oligocene intrusions into themost internal parts of the Sesia–Dent Blanche nappes.Finally, indentation of the Adriatic plate into the orogenicwedge resulted in the formation of the Vanzone back-fold,which marks the end of the pervasive ductile deformationwithin the Sesia–Dent Blanche nappes during the earliestMiocene

SUMMARY We provide a high-resolution image of the Ivrea Geophysical Body (IGB) in the Western Alps with new gravity data and 3-D density modelling, integrated with surface geological observations and laboratory analyses of rock properties. The IGB is a sliver of Adriatic lower lithosphere that is located at shallow depths along the inner arc of the Western Alps, and associated with dense rocks that are exposed in the Ivrea-Verbano Zone (IVZ). The IGB is known for its high seismic velocity anomaly at shallow crustal depths and a pronounced positive gravity anomaly. Here, we investigate the IGB at a finer spatial scale, merging geophysical and geological observations. We compile existing gravity data and we add 207 new relative gravity measurements, approaching an optimal spatial coverage of 1 data point per 4–9 km2 across the IVZ. A compilation of tectonic maps and rock laboratory analyses together with a mineral properties database is used to produce a novel surface rock-density map of the IVZ. The density map is incorporated into the gravity anomaly computation routine, from which we defined the Niggli gravity anomaly. This accounts for Bouguer Plate and terrain correction, both considering the in situ surface rock densities, deviating from the 2670 kg m–3 value commonly used in such computations. We then develop a 3-D single-interface crustal density model, which represents the density distribution of the IGB, including the above Niggli-correction. We retrieve an optimal fit to the observations by using a 400 kg m–3 density contrast across the model interface, which reaches as shallow as 1 km depth below sea level. The model sensitivity tests suggest that the ∼300–500 kg m–3 density contrast range is still plausible, and consequently locates the shallowest parts of the interface at 0 km and at 2 km depth below sea level, for the lowest and the highest density contrast, respectively. The former model requires a sharp density discontinuity, the latter may feature a vertical transition of densities on the order of few kilometres. Compared with previous studies, the model geometry reaches shallower depths and suggests that the width of the anomaly is larger, ∼20 km in west–east direction and steeply E–SE dipping. Regarding the possible rock types composing the IGB, both regional geology and standard background crustal structure considerations are taken into account. These exclude both felsic rocks and high-pressure metamorphic rocks as suitable candidates, and point towards ultramafic or mantle peridotite type rocks composing the bulk of the IGB.