Dating of lithospheric buckling: ⁴⁰Ar/³⁹Ar ages of syn-orocline strike-slip shear zones in northwestern Iberia

Orogenic curvature is a ubiquitous feature of mountain belts, and the plate tectonic and geodynamic setting responsible for the development of curved orogens is a subject of debate. In order to distinguish between different models of orocline formation it is necessary to tightly constrain the absolute timing of oroclinal development. However, determining the absolute (isotopic) timing of oroclinal bending is challenging. The most direct method available to constrain the time interval during which an orocline formed is to obtain absolute age constraints on fabrics generated within syn-orocline strike-slip shear zones that accommodated rotations around a vertical axis during the development of crustal or lithospheric scale orogenic curvature. Here we present a data set of ⁴⁰Ar/³⁹Ar ages obtained from five shear-zones, some of which display curved traces parallel to the Cantabrian Orocline structural grain in NW Iberia and are interpreted to have been generated coevally to it. The ⁴⁰Ar/³⁹Ar ages were obtained by laser-rastering induced step-heating on single muscovite crystals that grew synkinematically during shearing. All five samples yielded ages that cluster tightly at 308±3Ma providing direct evidence for the age of oroclinal bending. This age is consistent with the age constraints obtained from paleomagnetic and structural data that place the onset of oroclinal bending in Moscovian times. Our ⁴⁰Ar/³⁹Ar age determinations therefore provide a reliable absolute constraint on the age of buckling of the Variscan orogen around a vertical axis and provide further evidence that oroclinal bending is unrelated to Variscan convergence/collision or the subsequent extensional collapse of the mountain belt.