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https://doi.org/10.5194/se-2020-119
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.
https://doi.org/10.5194/se-2020-119
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.

  02 Sep 2020

02 Sep 2020

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This preprint is currently under review for the journal SE.

Extensional reactivation of the Penninic Frontal Thrust 3 Ma 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 Antonin Bilau et al.
  • 1EDYTEM, Université Savoie Mont Blanc, CNRS, UMR 5204, 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, Aix en Provence, France
  • 4GEOPS, CNRS, Université Paris-Saclay, 91405 Orsay, France

Abstract. In the Western Alps, the Penninic Frontal Thrust (PFT) is the main crustal-scale tectonic structure of the belt. This thrust transported the high-pressure metamorphosed internal units over the un-metamorphosed European margin during the Oligocene (34–29 Ma). The PFT was later reactivated as an extensional detachment associated with the development of the High-Durance extensional fault system. This inversion of tectonic motions on a major tectonic structure has been widely emphasized as an example of extensional collapse of a thickened collisional orogen. However, the inception age of the extensional inversion remains unconstrained. Here, for the first time, we provide chronological constraints on the extensional motion of an exhumed zone of the PFT by applying U-Pb dating on secondary calcites from a fault zone cataclasite. The calcite cement of the cataclasite, formed after the main fault slip event, is dated at 3.4 ± 1.5 Ma. Cross-cutting calcite veins featuring the last fault motions are dated at 2.6 ± 0.3 and 2.3 ± 0.3 Ma. δ13C and δ18O fluid signatures derived from these secondary calcites suggest fluid percolation from deep-seated reservoir at the scale of the Western Alps. Our data give evidence that the PFT extensional reactivation is active since at least ~ 3 Ma, at the crustal scale and results from the westward propagation of the compressional deformation related to the exhumation of External Crystalline Massifs. In this context, the exhumation of the dated extensional fault is associated with the eastward translation of the seismogenic zone controlled by the deep rooting of the High-Durance fault system.

Antonin Bilau et al.

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Antonin Bilau et al.

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Short summary
As a result of the collision between the European and Apulian plates, the Alps have experienced several evolutionary stages. The Penninic Frontal Thrust (PFT) (major thrust) was associated to compression and now seismic studies show ongoing extensional activity. Calcite mineralisations associated to extension and compresionnal strucures were sampled. Last deformation stage is dated by U-Pb at 4.9–2.3 Ma and isotopes analysis evidence deep crustal fluid's mobilisation.
As a result of the collision between the European and Apulian plates, the Alps have experienced...
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