Kinematics and time of emplacement of the Upper Allochthon of the Rhodope Metamorphic Complex: evidence from the Rila Mountains, Bulgaria

The Rhodope Metamorphic Complex is a nappe pile of four large allochthonous megaunits, referred to as Lower, Middle, Upper, and Uppermost allochthons. These were originally derived from different paleogeographic provinces and were stacked on top of each other in the course of convergent processes along the European continental margin between the Late Jurassic and the Late Eocene. The direction and age of thrusting as well as the degree and age of Alpine metamorphism vary between individual allochthons and are altogether controversial. The affiliation of individual tectonic units to allochthons is often unclear. However, for the area of the Eastern Rhodope Mountains, there is agreement that the thrusting of the Upper Allochthon onto the Middle Allochthon happened before ca. 68–70 Ma, since Upper Cretaceous magmatic bodies crosscut the metamorphic fabric of the rocks. For the northwestern parts of the Rhodope Metamorphic Complex (Rila Mountains), the age of thrusting was considered to predate the Late Cretaceous. Here, we present the results of mapping, structural observations, and LA-ICP-MS dating of zircons and monazites from the Northwest Rila Mountains and revise the tectonic architecture and history of the area. There the boundary between the Upper and Middle allochthons is the Dodov-vrah Shear Zone. This regional-scale structure represents a top-to-the-southeast amphibolite-facies thrust, along which numerous syn-kinematic intrusive bodies were emplaced. U–Pb LA-ICP-MS zircon and monazite dating of the latter, as well as dating of pre- and post-kinematic plutonic rocks from the study area showed that the time of activity of the Dodov-vrah Shear Zone can be bracketed between ca. 58 Ma and 50–48 Ma. Thus, at least for the northwestern parts of the Rhodope Metamorphic Complex, the thrusting of the Upper onto the Middle Allochthon happened after the latest Palaeocene and before the end of the Lower Eocene.