Geochronological and structural constraints on the Cretaceous thermotectonic evolution of the Kraishte zone (Western Bulgaria)

In the Kraishte region, near the junction of the Balkanides and Dinarides, late Early Cretaceous compression led to the thrusting of the Morava unit over the Struma unit. Structural investigations, combined with zircon fission-track and 40Ar/39Ar analyses, have been used to reconstruct the geological history of the area and to clarify the original tectonic position of the main units before Cenozoic extension. The results show that Early Cretaceous lower amphibolite-facies metamorphism and deformation in the Osogovo-Lisets Metamorphic Complex were related to top-to-the-NE directed nappe stacking, whereas the deeper parts of the allochthonous Morava unit experienced low-grade metamorphic overprint at temperatures < ~260°C. The structurally intermediate Struma Diorites and their Mesozoic cover experienced temperatures between ~170°-300 °C. Thrusting of the Morava onto the Struma unit started probably in Valanginian times (140-136 Ma), soon after the cessation of pre- to syn-orogenic turbidite sedimentation on Struma. The metamorphic peak was reached before 112 Ma. Subsequent extension-related cooling of both units was probably accompanied by the formation of low-angle normal faults.