In the last few years, a consensus has emerged according to which the Rhodope Metamorphic Complex (RMC) has started undergoing post-orogenic extension in the early Late Eocene or before. Hence, no significant compressional structure younger than the Middle Eocene should be observed in it. In the Bulgarian Central Rhodope, the lower part of the metamorphic pile is mostly made of migmatitic orthogneisses from which several zircon and monazite U-Pb ages around 36-37 Ma have been reported. This may suggest that the structures formed during migmatization and subsequent cooling of this part of the dominating in the literature.
Our analysis in the area of Chepelare documents the following. The metamorphic rocks are exposed as a ~5 km-thick north-dipping monocline defined by foliations and lithological contours. In the largest part of this pile, structures consistently document top-to-SW shearing developed during and subsequent to anatexis. The middle part of the section shows a more variegated rock assemblage that coincides with a ~1 km-thick zone of intense strain here termed the ‘Chepelare Shear Zone’ (CSZ). The CSZ has previously been interpreted as a synmetamorphic thrust of presumed Mesozoic age. From a strongly sheared synfolial pegmatite sampled within the CSZ, we obtained a monazite U-Pb age of 36.3 ± 0.4 Ma (weighted mean age upon 16 analyses) and a muscovite 39Ar/40Ar single-grain plateau age of 34.9 ± 0.2 Ma. These results are consistent with published ages for the broader area and indicate that the CSZ was active during the time interval from 36 to 35 Ma (at least). Higher levels of the monocline, above the CSZ, show a domain of less severe strain and lacking pronounced stretching lineations, then a domain of higher strain that includes ~1-3 m-thick shear zones with low dips toward the north. The shear zones bear N-S-trending stretching lineations and display top-to-N shear criteria. Some of them are underlined by a synkinematic pegmatitic or granitic vein running along their axis. The top of the monocline is defined by a north-dipping fault zone (with ultramylonitic marbles and thick cataclasites) that also displays top-to-N shear criteria. This well known low-dipping fault zone, initially described as a thrust, has later been reinterpreted as an extensional detachment. Together with the domain of top-to- N ductile shearing in its footwall, we refer to this fault zone as the ‘Mihalkovo-Drianovo Shear Zone’ (MDSZ). Within the MDSZ, from one shear zone bearing a syn-kinematic pegmatitic vein, we obtained a monazite U-Pb age of 38.0 ± 0.1 Ma (weighted mean age upon 58 analyses) and two muscovite 39Ar/40Ar single-grain plateau ages of 34.2 ± 1.2 Ma (large flake) and 32.3 ± 1.2 Ma (recrystallized flake). From another shear zone, we obtained two muscovite 39Ar/40Ar single-grain plateau ages of 34.4 ± 0.2 Ma (granitic vein) and 34.4 ± 0.4 Ma (host gneiss). These results indicate that the MDSZ was active during the time interval from 38 to 34 Ma (at least).
As a consequence, the MDSZ and the CSZ were synchronously active, at least during the period from 36 to 35 Ma. Because the two shear zones have opposite kinematics but fairly identical dips (the difference is 10° at most), this synchronism implies that one of them was initially a thrust and the other was normal-sense, whatever the amount of tilting the metamorphic pile may have undergone subsequently. Of the two solutions left, the one where the underlying CSZ was a thrust and the overlying MDSZ was normal-sense is, by far, the most likely. Consequently, our study documents synmetamorphic thrusting in the Bulgarian Central Rhodope during the Late Eocene. This is consistent with the picture arising from the Nestos Shear Zone, in Greece, and confirms that the onset of post-orogenic extension in the RMC occurred in post-Eocene times. In addition, syn-orogenic extension, so far suspected, is now well established and appears to have developed within the RMC while it was the hot core of the Alpine orogen.