In Situ and Step‐Heating 40Ar/39Ar Dating of White Mica in Low‐Temperature Shear Zones (Tenda Massif, Alpine Corsica, France). Issue 12 (26th November 2020)

• Record Type: Journal Article
• Authors: Beaudoin, Alexandre; Scaillet, Stéphane; Mora, Nicolas; Jolivet, Laurent; Augier, Romain
• Is Part Of: Tectonics. Volume 39:Issue 12(2020)
• Page Start: n/a
• Page End: n/a
• Publication Date: 2020-11-26
• Subjects: 40Ar/39Ar dating -- in situ and step‐heating combined technics -- white mica -- shear zone -- Tenda massif -- Alpine Corsica; Geology, Structural -- Periodicals; 551.8
• DOI: 10.1029/2020TC006246
• Languages: English
• ISSNs: 0278-7407

Abstract

Abstract: In order to clarify the link between 40 Ar/ 39 Ar record in white mica and deformation, we performed in situ and bulkwise 40 Ar/ 39 Ar 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. 40 Ar/ 39 Ar 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. Key Points: Data are interpretable in terms of age‐deformation relationships and capture a broad‐scale localization of strain in time and space At smaller scale, relating age with structures is more difficult because of greater complexity in 40 Ar/ 39 Ar systematics The shear zone has a duration of ~12 Myr during exhumation and deformation ended in the upper part of the viscous/brittle transition