Large-magnitude continental extension in the northeastern Iranian Plateau: Insight from K-feldspar 40 Ar/ 39 Ar thermochronology from the Shotor Kuh–Biarjmand metamorphic core complex

Abstract

The Late Cretaceous–early Paleogene tectonic evolution of the Iranian Plateau is not well understood in comparison to its well-studied late Paleogene–Neogene evolution. Exhumation, metamorphism, and changing sedi¬mentary environments are documented away from the plateau margin for this time interval, however the nature and mechanism of deformation in the interior of the Iranian Plateau remains controversial to the point that both compressional and extensional mechanisms are proposed. We use K-feldspar and mica ^(40)Ar/^(39)Ar thermochronology to determine the thermal evolution of metamorphic rocks exposed in northeastern Great Kavir Basin on the Iranian Plateau. Multi-domain diffusion modeling of K-feldspar and field relationships reveal a stage of rapid cooling via tectonic exhumation starting in the Late Cretaceous and lasting until the early Eocene. We attribute this to continental extension as being accommodated on detachment faults that exhumed the Shotor Kuh–Biarjmand metamorphic core complex. The metamorphic rocks of this core complex underlie a southeastward-younging structural-stratigraphic sequence that includes at its top Late Cretaceous (Campanian–Maastrichtian) carbonates. We interpret this sequence as a crustal section that was exhumed by a NW-dipping master detachment fault. Based on the locations of the syn-extensional detrital rocks we propose that this system accommodated ∼100 km of NW-SE–oriented extension. Our results indicate that extensional deformation started in the Iranian Plateau much earlier (Late Cretaceous) than previously thought (Eocene–Oligocene).