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In situ and step‐heating 40 Ar/ 39 Ar dating of white mica in low‐temperature shear zones (Tenda Massif, alpine Corsica, France)

Alexandre Beaudoin 1, * Stéphane Scaillet 1 Nicolas Mora 1 Laurent Jolivet 2 Romain Augier 1
* Corresponding author
1 Géodynamique - UMR7327
BRGM - Bureau de Recherches Géologiques et Minières (BRGM), ISTO - Institut des Sciences de la Terre d'Orléans - UMR7327 : UMR7327, INSU - CNRS - Institut national des sciences de l'Univers, UO - Université d'Orléans : UMR7327
Abstract : In order to clarify the link between 40Ar/39Ar record in white mica and deformation, we performed in situ and bulkwise 40Ar/39Ar dating over the East Tenda Shear Zone (Tenda massif, alpine Corsica). White micas from 11 samples were selected and extensively analyzed using in situ techniques across nested scales of strain‐intensity gradients developed at the expense of a Late‐Variscan protolith. 40Ar/39Ar systematics are unaffected by inherited Ar and directly linked to deformation with little or no Ar lattice (volume) diffusion. Extensive sampling allows constraining the end of deformation related to burial and exhumation respectively at ~34 and ~22 Ma, bracketing the duration of regional extensional shear to ~12 Myr. Results also highlight a regional strain localization toward the upper contact of the unit with smaller‐scale localization in specific lithologies, notably meta‐aplites. Second‐order complications exist, such as local ill‐defined correlations between ages and finite‐strain microstructures. Thus, the use of a strain gradient as a proxy for strain localization in time is regionally valid but sometimes locally too complex to track or resolve strain partitioning/localization trends at the meter (outcrop) scale and below. Age mixing and incomplete isotopic homogenization by dissolution/precipitation are identified as the main causes of local discrepancies that complicate the link between age and microstructure and the derivation of strain localization rates. Tracking temporal trends in shear distribution across regional‐scale deformation gradients in such settings is possible but requires a multi‐scale approach as implemented here to reveal younging patterns associated to strain localization.
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Alexandre Beaudoin, Stéphane Scaillet, Nicolas Mora, Laurent Jolivet, Romain Augier. In situ and step‐heating 40 Ar/ 39 Ar dating of white mica in low‐temperature shear zones (Tenda Massif, alpine Corsica, France). Tectonics, American Geophysical Union (AGU), 2020, 39 (12), pp.e2020TC006246. ⟨10.1029/2020TC006246⟩. ⟨insu-02985219⟩



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