Timing of syn-orogenic extension in the Western Alps revealed by calcite U-Pb and hematite (U-Th)/He dating

Abstract

Understanding fault activity over time provides valuable insights for reconstructing the tectonic history of an orogen, assessing seismological risks and understanding mineralization processes. In the Western Alps, one of the main controversies in existing tectonic models is the understanding of syn-orogenic extension. Seismological evidence shows widespread extensional deformation related to the reactivation of major lithospheric structures, such as the Penninic Frontal Thrust (PFT). However, the onset age and origin of extension are still debated due to the lack of suitable geochronological data. Fault hematite and calcite geochronology as well as clumped isotope data can be used to relate fluid regimes to fault activity. The analysis of calcite brecciae from extensional faults above the PFT shows that two distinct fluid regimes were present. The first regime, occurring before 2 Ma is associated with upwelling of deep fluids and is recorded by fault calcite at a temperature > 110°C. The second fluid regime is characterized by a meteoric signature and temperatures around 36 °C, representing crystallization since 2 Ma. This study presents a new model for the Miocene tectonic history of the Western Alps that combines (U-Th)/He and U-Pb geochronology on fault hematite (13.3 ± 0.8 to < 0.8 Ma) and calcite (5.3 ± 0.6 Ma). Results demonstrate a progression of extensional fault activity from east to west, from the Middle Miocene (ca. 13 Ma) to the Quaternary. The onset of extension in the inner part of the belt coincides with the development of the fold and thrust belt in the western Alpine foreland. Our new model proposes that extension occurs in the hanging wall of a large top-to-the-west thrust, known as the Alpine Frontal Thrust. This thrust, located to the west of the External Crystalline Massifs gives rise to their uplifting and extension at the rear.

Keywords

Western Alps / Briançonnais zone / Hematite (U-Th)/He / U-Pb calcite / Clumped isotopes / Syn-orogenic extension

CRediT authorship contribution statement

Antonin Bilau: Writing – review & editing, Writing – original draft, Project administration, Methodology, Investigation, Formal analysis, Data curation, Conceptualization. Yann Rolland: Writing – review & editing, Writing – original draft, Visualization, Validation, Supervision, Project administration, Investigation, Funding acquisition, Conceptualization. Stéphane Schwartz: Writing – review & editing, Visualization, Validation, Supervision, Investigation, Funding acquisition, Conceptualization. Cécile Gautheron: Writing – review & editing, Validation, Supervision, Methodology, Investigation, Formal analysis, Data curation. Thierry Dumont: Writing – review & editing, Visualization, Validation, Supervision, Investigation, Conceptualization. Dorian Bienveignant: Investigation. Benjamin Brigaud: Writing – review & editing, Validation, Supervision, Methodology, Data curation. Nicolas Godeau: Validation, Supervision, Methodology, Formal analysis, Data curation. Abel Guihou: Writing – review & editing, Validation, Supervision, Methodology, Data curation. Pierre Deschamps: Writing – review & editing, Funding acquisition. Xavier Mangenot: Formal analysis, Data curation. Marianna Corre: Formal analysis, Data curation. Rosella Pinna-Jamme: Formal analysis, Data curation. Nathaniel Findling: Formal analysis, Data curation.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

This work forms part of the first author’s Ph.D. project funded by the French BRGM in the frame of the RGF-Alps program (Référentiel Géologique de la France). The LA-ICP-MS hosted by the ENVITOP platform at CEREGE has received funding from the “Excellence Initiative” of the Aix Marseille University A*MIDEX – project DATCARB, a french “Investissement d’avenir” program. Special thanks to Fayçal Soufi and Pierre-Luigi Canepa for their preparation of polished thin-section. The authors thank the reviewers, Meinert Rahn and Martin Danišík, for their careful reading and insightful comments, and efficient editorial handling by Stijn Glorie (Associate Editor) and M. Santosh (Editorial Advisor), which greatly helped to improve this manuscript.

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