Style of Alpine tectonic deformation in the Castellane fold-and-thrust belt (SW Alps, France): Insights from balanced cross-sections Anthony Jourdon a , Yann Rolland a, ⁎, Carole Petit a , Nicolas Bellahsen b a Géoazur, UMR 7329, CNRS-UNS-UPMC-IRD, 250 Avenue Albert Einstein, 06560 Valbonne, France b ISTeP, UMR 7193, CNRS-UPMC, 4 Place Jussieu, 75252 Paris Cedex 5, France abstract article info Article history: Received 26 November 2013 Received in revised form 10 June 2014 Accepted 22 June 2014 Available online xxxx Keywords: Balanced cross-sections Fold and thrust belt Shortening Cover–basement decoupling Thick-skin tectonic Pyrenean and Alpine phases 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–S balanced cross-sections across the entire fold-and-thrust belt, in order to quantify the amount of horizontal shortening due to the Pyrenean and Alpine deformations. We then assess the role of the basement inherited structures 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 Cretaceous basement normal faults. We reconstructed three restoration stages: the oldest one highlights the geometry of Permian and Early Cretaceous extensional structures; the intermediate one after the Late Cretaceous evidences a first compressional episode linked to the Pyrenean compression; and the latest one in the Oligocene shows the first Alpine folds resulting from the southward thrusting of the internal units over the external ones along of the Penninic Frontal Thrust. Balanced cross-sections suggest very moderate crystalline basement–sedimentary cover decoupling. On the western and central cross-sections, the estimated amount of shortening ranges from 9.5 to 10 km (21%) whereas on the easternmost one shows ~5 km of shortening (9%). These shortening values are consistent with previously published estimates in the surrounding foreland subalpine chains. They highlight a decreasing value of Pyrenean shortening toward the east, while the Alpine shortening dominates and amplifies this first phase in a similar direction. We interpret this dominantly thick-skinned structural style as a possible consequence of the Neogene thermal weakening in the European passive margin above the Ligurian slab rollback. © 2014 Elsevier B.V. All rights reserved. 1. Introduction The foreland domains of convergent orogens are formed by fold- and-thrust belts, which comprise sedimentary layers overlying crystalline/hard-rock basements. Shortening in these domains is con- trolled by a regionally-dominated compressional strain state and occurs in different geodynamic contexts such as subduction accretionary wedges and collisional foreland thrust belts (e.g., Dahlen, 1990; Davis et al., 1983; Suppe, 1987). In foreland thrust belts, the deformation of the sedimentary cover is the outermost expression of deeper crustal shortening occurring inwards along intra-basement thrusts (e.g., Jackson, 1980). The width of the fold-and-thrust belt, i.e. the horizontal offset between the frontal foreland thrust and the first (i.e. outermost) intra-basement one, depends on the presence of a horizontal level of décollement that allows a mechanical decoupling between the upper and lower units. At first approximation, when this décollement exists and is efficient, foreland deformation is characterized by a thin-skin tec- tonic style (e.g., Affolter and Gratier, 2004). This model has for instance been generalized for the front of the Alps (e.g., Phillipe et al., 1998). On the other hand, when the décollement is inefficient or inexistent, the deformed cover remains globally coupled to its basement. This so- called thick-skin deformation style is particularly visible in the case of the Pyrenean compressional phase in Provence, where the cover re- mains partly coupled to its basement (Espurt et al., 2012; Roure and Colletta, 1996), or in the Alpine External Crystalline Massif (ECM) (Bellahsen et al., 2012; Burkhard and Sommaruga, 1998). In detail, the occurrence of thick- or thin-skin deformation styles is still an open question and in some cases, especially on ancient and previously thinned continental margins, both can occur simultaneously (e.g., Jackson, 1980). At the 1st order however, the mechanical resistance of the lithosphere seems to play a first-order role in the style of shortening, thin-skin styles being prominent in old and rigid cratonic lithospheres (Mouthereau et al., 2013). Inherited basement faults and their associat- ed sedimentary basins can also favor a thick-skin tectonic style, espe- cially when the (lower) crust is hot and ductile (Bellahsen et al., 2012; Nilfouroushan et al., 2013). Discriminating between thin- and thick- skin tectonic styles is therefore not straightforward, but is necessary to address important questions such as: how is a foreland thrust belt structured, and what are the crustal-scale settings which control the Tectonophysics xxx (2014) xxx–xxx ⁎ Corresponding author. E-mail address: rolland@geoazur.unice.fr (Y. Rolland). TECTO-126363; No of Pages 13 http://dx.doi.org/10.1016/j.tecto.2014.06.022 0040-1951/© 2014 Elsevier B.V. All rights reserved. Contents lists available at ScienceDirect Tectonophysics journal homepage: www.elsevier.com/locate/tecto Please cite this article as: Jourdon, A., et al., Style of Alpine tectonic deformation in the Castellane fold-and-thrust belt (SW Alps, France): Insights from balanced cross-sectio..., Tectonophysics (2014), http://dx.doi.org/10.1016/j.tecto.2014.06.022