Solid Earth, 12, 237–251, 2021 https://doi.org/10.5194/se-12-237-2021 © Author(s) 2021. This work is distributed under the Creative Commons Attribution 4.0 License. Extensional reactivation of the Penninic frontal thrust 3 Myr ago as evidenced by U–Pb dating on calcite in fault zone cataclasite Antonin Bilau1,2, Yann Rolland1,2, Stéphane Schwartz2, Nicolas Godeau3, Abel Guihou3, Pierre Deschamps3, Benjamin Brigaud4, Aurélie Noret4, Thierry Dumont2, and Cécile Gautheron4 1EDYTEM, Université Savoie Mont Blanc, CNRS, UMR 5204, 73370 Le Bourget-du-Lac, France 2ISTerre, Université Grenoble Alpes, Univ. Savoie Mont Blanc, CNRS, IRD, IFSTTAR, 38000 Grenoble, France 3Aix-Marseille Université, CNRS, IRD, INRAE, Collège de France, CEREGE, 13545 Aix-en-Provence, France 4GEOPS, CNRS, Université Paris-Saclay, 91405 Orsay, France Correspondence: Antonin Bilau ([email protected]) and Yann Rolland ([email protected]) Received: 10 July 2020 – Discussion started: 2 September 2020 Revised: 8 December 2020 – Accepted: 11 December 2020 – Published: 28 January 2021 Abstract. In the Western Alps, the Penninic frontal thrust tion of the HDFS seismogenic zone, in agreement with the (PFT) is the main crustal-scale tectonic structure of the belt. present-day seismic activity. This thrust transported the high-pressure metamorphosed in- ternal units over the non-metamorphosed European margin during the Oligocene (34–29 Ma). Following the propaga- tion of the compression toward the European foreland, the 1 Introduction PFT was later reactivated as an extensional detachment as- sociated with the development of the High Durance exten- Dating of major tectonic inversions in orogens is gener- sional fault system (HDFS). This inversion of tectonic dis- ally achieved by indirect and relative dating, but rarely by placement along a major tectonic structure has been widely the direct dating of fault-related minerals using absolute emphasized as an example of extensional collapse of a thick- geochronometers. For instance, tectonic cycles are defined ened collisional orogen. However, the inception age of the worldwide by the sediment unconformities or by exhuma- extensional inversion remains unconstrained. Here, for the tion ages through thermochronological investigation. How- first time, we provide chronological constraints on the exten- ever, the recent progress in U–Pb dating of carbonate using sional motion of an exhumed zone of the PFT by applying U– high-resolution laser ablation analyses (Roberts et al., 2020) Pb dating on secondary calcites from a fault zone cataclasite. allows us to directly date minerals formed during fault ac- The calcite cement and veins of the cataclasite formed after tivity and thus to establish the age of tectonic phases by the main fault slip event, at 3.6 ± 0.4–3.4 ± 0.6 Ma. Cross- absolute radiometric dates (Ring and Gerdes, 2016; Good- cutting calcite veins featuring the last fault activity are dated fellow et al., 2017; Beaudoin et al., 2018). This method is at 2.6 ± 0.3–2.3 ± 0.3 Ma. δ13C and δ18O fluid signatures de- especially well suited to disentangle the successive tectonic rived from these secondary calcites suggest fluid percola- motions along a given tectonic structure. U–Pb dating can tion from deep-seated reservoir at the scale of the Western be coupled to stable isotopic analysis to infer the nature of Alps. Our data provide evidence that the PFT extensional fluids through time, which may give insights into the scale reactivation initiated at least ∼ 3.5 Myr ago with a reactiva- of fluid circulations and thus the scale of the active tectonic tion phase at ∼ 2.5 Ma. This reactivation may result from the structure and changes in the stress regime (e.g. Beaudoin et westward propagation of the compressional deformation to- al., 2015; Rossi and Rolland, 2014). In the Western Alps, ward the external Alps, combined with the exhumation of the Penninic frontal thrust, or PFT, represents a major thrust external crystalline massifs. In this context, the exhumation structure at lithospheric scale (e.g. Tardy et al., 1990; Mug- of the dated normal faults is linked to the eastward transla- nier et al., 1993; Zhao et al., 2015) that accommodated the main collisional phase during the Palaeogene–Neogene (e.g. Published by Copernicus Publications on behalf of the European Geosciences Union. 238 A. Bilau et al.: PFT extensional reactivation dated by U-Pb calcite Ceriani et al., 2001; Ceriani and Schmid, 2004). Later on, this Maira massif, which was later transported as a tectonic nappe thrust was reactivated as a normal fault, and the extensional during the collision (Duchêne et al., 1997). PFT activation deformation is still ongoing (Sue and Tricart, 1999; Tricart and underthrusting of external crystalline massifs are indica- et al., 2006; Sue et al., 2007). This transition from compres- tors of the transition from subduction to continental collision sion to extension in a collisional chain has been diversely in- in the internal zones, between 44 and 36 Ma (e.g. Beltrando terpreted to reflect slab break-off, crustal overcompensation et al., 2009). This transition is marked by shear zone devel- or post-glacial and erosion-induced isostatic rebound (e.g. opment in greenschist facies conditions and recrystallization Champagnac et al., 2007; Sternai et al., 2019). However, un- during burial of the Alpine external zone in the PFT footwall til now, no direct dating of the tectonic shift from compres- compartment (Rossi et al., 2005; Sanchez et al., 2011; Bel- sion to extension on the PFT has been obtained, which leads lahsen et al., 2014). The early ductile PFT activity is dated to many possible geodynamic scenarios. At the present day, at 34–29 Ma by 40Ar=39Ar dating of syn-kinematic phengite a large range of ages for this transition have been hypothe- from shear zones in the Pelvoux and Mont Blanc external sized, from ∼ 12–5 Ma (Tricart et al., 2006) to only a few tens crystalline massifs (Seward and Mancktelow, 1994; Rolland of thousands of years (Larroque et al., 2009), which shows et al., 2008; Simon-Labric et al., 2009; Bellanger et al., 2015; the lack of direct dating of brittle deformation (Bertrand and Bertrand and Sue, 2017) and by U–Pb on allanite (Cenki-Tok Sue, 2017). In this study, we applied the laser ablation U– et al., 2014). The age of the PFT hanging wall tectonic mo- Pb dating method on secondary calcites from a cataclasite tion and joint erosion is highlighted by the exhumation of fault zone that testify to the extensional deformation of an the Briançonnais units constrained by apatite fission tracks exhumed palaeo-normal fault during the PFT inversion. (AFTs) at 26–24 Ma (Tricart, 1984; Tricart et al., 2001, 2007; The purpose of this study is (1) to provide absolute chrono- Ceriani and Schmid, 2004). However, the PFT reactivation as logical constraints on the structural inversion of the PFT and a normal fault remains unconstrained. The onset of PFT ex- (2) to give insights into the scale and nature of fluid circula- tensional activity has been proposed to have occurred in the tions along this major fault using stable isotope analysis of late Miocene (∼ 12 to 5 Ma), based on indirect AFT ages in carbon and oxygen. the Pelvoux external crystalline massif (Tricart et al., 2001, 2007), which record a cooling episode related to relief cre- ation and erosion. The current seismicity (e.g. Rothé, 1942; 2 Geological setting Sue et al., 1999, 2007) and observed GPS motions (Walpers- dorf et al., 2018; Mathey et al., 2020) all along the so-called The western Alpine collisional belt results from the con- High Durance fault system (HDFS) highlight the fact that ex- vergence and collision of the European and Apulian plates, tensional and minor strike-slip deformations along the PFT which culminated with top-to-the-west displacement on the are still ongoing. This seismicity mostly occurs at shallow PFT acting as the major Alpine tectonic structure in the depths, less than 10 km, and mainly at 3 to 8 km, where the Late Eocene to Oligocene times (e.g. Dumont et al., 2012; HDFS is structurally connected to the PFT (Sue and Tricart, Bellahsen et al., 2014). This lithospheric-scale structure ac- 2003; Thouvenot et al., 2006; Sue et al., 2007). commodated westward thrusting of highly metamorphosed The study area is focused on a portion of the PFT located “internal zone” units over slightly metamorphosed “external in the southeast of the Pelvoux external crystalline massif in zone” units (Fig. 1; Schmid and Kissling 2000; Lardeaux et the Western Alps (France) (Figs. 1–2). Here, the PFT rests al., 2006; Simon-Labric et al., 2009; Malusà et al., 2017). on late Eocene (Priabonian) autochthonous nummulitic fly- The external zone is composed of the non-metamorphosed sch, so-called “Champsaur sandstone” (Fig. 2), which lies European Mesozoic and Palaeozoic sedimentary cover and unconformably on the Pelvoux crystalline basement. In the its Palaeozoic basement corresponding to the external crys- southern part, the PFT lies on the Cretaceous helminthoid fly- talline massifs (ECMs). sch nappes (Fig. 2). These two flysch units are intensely de- The internal zone corresponds to a high-pressure meta- formed by top-to-the-west PFT compressional deformation. morphic wedge formed by the stacking of the palaeo-distal The PFT hanging wall corresponds to the Briançonnais zone European margin of the Briançonnais zone, comprising the composed of Mesozoic and Palaeozoic sedimentary units, internal crystalline massifs and their sedimentary cover, with which underwent high-pressure metamorphism (Lanari et al., the oceanic-derived units of the Piedmont zone. These units 2012, 2014). The Briançonnais zone is composed of the Bri- were incorporated and juxtaposed in the subduction accre- ançonnais zone houillère, which consists of Carboniferous tionary prism from the early Late Cretaceous until the late sediments overlying a crystalline basement, stratigraphically Eocene (e.g.