Time constraints for low-angle shear zones in the Central Rhodopes (Bulgaria) and their significance for the exhumation of high-pressure rocks

Abstract

In the Central Rhodopes of southern Bulgaria, an eclogite-bearing rock sheet belonging to the Middle Allochthon (Starcevo Unit) is over- and underlain by eclogite-free, amphibolite-facies rock units along low-angle shear zones, the Borovica Shear Zone at the top and the Starcevo-Ardino Shear Zone at the base. The age of these shear zones is determined by U–Pb zircon dating of pre-, syn- and posttectonic magmatic rocks, mostly pegmatite veins, using LA–SF–ICP–MS. Zircons from pre- to syntectonic pegmatites within the Borovica Shear Zone yielded ages of ca. 45–43 Ma, indicating that the shear zone was active at that time, and zircons from a pretectonic pegmatite and a posttectonic granitoid body within the Starcevo-Ardino Shear Zone yielded ages of ca. 45 and ca. 36 Ma, respectively, giving a time frame for the activity of that shear zone which probably rather postdated the activity of the Borovica Shear Zone. By combining the ages with the kinematics of the shear zones and the metamorphic history of the rock units, the following scenario is sketched: Soon after the Starcevo Unit reached peak pressure (eclogite facies), it was exhumed to a mid-crustal level by top-to-the-north-west, extensional unroofing along the Borovica Shear Zone, in a kinematic framework of orogen-parallel extension. Beginning at ca. 40 Ma, the partly exhumed Starcevo Unit was underthrust from the south-west by continental crust of the foreland (Apulia), forming the Lower Allochthon of the Rhodopes, along the Starcevo-Ardino Shear Zone. These results underline the significance of orogen-parallel extension for the exhumation of high-pressure rocks. With respect to regional geology of the Hellenides and the Aegean, it is found that the tectonic architecture of the Rhodopes is essentially of Tertiary age. Cretaceous syn-metamorphic shear zones do exist but are largely restricted to higher levels of the nappe stack (Upper Allochthon). The Rhodopes do not represent an older essentially Mesozoic core of the Hellenides but are formed by the internal, higher-metamorphic portions of the same major nappe systems as occur in the Hellenides.