Style of Alpine tectonic deformation in the Castellane fold-and-thrust belt, (SW Alps, France): Insights from balanced cross-sections

Description

This study proposes a reappraisal of the role of the basement tectonics in the structuration of the Alpine foreland,across the Castellane fold-and-thrust belt located in southwestern Alps. We construct three 30 km length N–Sbalanced cross-sections across the entire fold-and-thrust belt, in order to quantify the amount of horizontalshortening due to the Pyrenean and Alpine deformations. We then assess the role of the basement inheritedstructures during the compressional phases which resulted in the exhumation of the Argentera–Mercantour Ex-ternal Crystalline Massif and the Barrot Dome. The construction of these balanced cross-sections suggests a dom-inant thick-skinned deformation style, which includes the reactivation of inherited Permian and Early Cretaceousbasement normal faults. We reconstructed three restoration stages: the oldest one highlights the geometry ofPermian and Early Cretaceous extensional structures; the intermediate one after the Late Cretaceous evidencesafirst compressional episode linked to the Pyrenean compression; and the latest one in the Oligocene showsthefirst Alpine folds resulting from the southward thrusting of the internal units over the external ones alongof the Penninic Frontal Thrust. Balanced cross-sections suggest very moderate crystalline basement–sedimentarycover decoupling.On the westernand central cross-sections, the estimatedamountof shorteningranges from 9.5to 10 km (21%) whereas on the easternmost one shows ~5 km of shortening (9%). These shortening values areconsistent with previously published estimates in the surrounding foreland subalpine chains. They highlight adecreasing value of Pyrenean shortening toward the east, while the Alpine shortening dominates and amplifiesthisfirst phase in a similar direction. We interpret this dominantly thick-skinned structural style as a possibleconsequenceofthe Neogenethermal weakening inthe European passivemarginabovethe Ligurian slab rollback.

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