In Situ and Step‐Heating ⁴⁰Ar/³⁹Ar Dating of White Mica in Low‐Temperature Shear Zones (Tenda Massif, Alpine Corsica, France)

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.

40Ar/39Ar Age Constraints on HP/LT Metamorphism in Extensively Overprinted Units: The Example of the Alpujárride Subduction Complex (Betic Cordillera, Spain)

“…Most probably, such resetting was not purely thermal‐diffusion driven, however. In keeping with recent UV‐laser probe 40 Ar/ 39 Ar studies documenting subgrain‐scale 40 Ar disequilibrium patterns developed in dynamically exhumed and overprinted peak ‐ pressure phengites (Beaudoin et al., 2020; Laurent et al., 2021), our data show that resetting occurred in a way locally combining deformation along with thermal‐decompression effects regionally defining a major event at 20 Ma.…”

“…In terms of internal isotopic disturbance, we note a systematic trend of steadily increasing apparent ages as the degree of discordance and extent of degassing increase in these samples. The resulting staircase pattern is reminiscent of partial 40 Ar loss/retention or slow cooling (Beaudoin et al., 2020; Harrison & Lovera, 2014). Slow cooling in the Ar‐muscovite closure interval over more than 20 Myr (e.g., ALP1601h; Figure 13b) can be safely discarded given the documented P‐T paths and the tectonic context.…”

Lateral variations of pressure-temperature evolution in non-cylindrical orogens and 3-D subduction dynamics: the Betic-Rif Cordillera example

“…As discussed in depth elsewhere (e.g. Beaudoin et al, 2020), the significance of 40 Ar/ 39 Ar white mica ages in the context of deformation involves both the concept of closure-T (in the Dodsonian sense, Dodson, 1973) along with the concept of dynamic closure whereby thermally-induced Ar diffusion interacts with deformation-induced subdomains in the case of cold P/T paths <450-500°C, as recorded here. Ar retention/resetting can be dominated by recrystallization (over diffusion), locally producing an array of apparent ages at the scale of a single sample that do not conform with theoretical grain size trends in cooling ages but rather reflect the sequence of overprinting/ recrystallization steps recorded at the mineral scale.…”

“…These data, acquired with different methods applied to different minerals, are often interpreted solely based upon the closure-temperature concept but other parameters such as fluid-rock interactions or strain intensity may imply more complex interpretations (e.g. Rossetti et al, 2010;Sánchez-Navas et al, 2014;Beaudoin et al, 2020), which we discuss here.…”

Geodynamic evolution of a wide plate boundary in the Western Mediterranean, near-fieldversusfar-field interactions

“…In the Nevado-Filabride also, 40 Ma seems the age of the peak of pressure (Li and Massonne, 2018;Bessière et al, 2021). 34 Ma correspond to the very end of the compressional period in Alpine Corsica (Beaudoin et al, 2020) and inception of rifting in the Liguro-Provençal basin and it falls just before the drastic change in the shortening regime in the French Alps with the first involvement of the External Crystalline Massifs and the flysch-to-molasse transition, associated with a retreat of the European slab (Vignaroli et al, 2009). We thus see a relation between the change of subduction regime at about 35-32 Ma in the Western Mediterranean and the end of HP-LT metamorphism, especially the fast exhumation of the UHP units in the Alps and Edough Massif.…”