Persistent synmetamorphic thrusting in the Rhodope until 33 Ma: evidence from the Nestos Shear Zone and implications for Aegean geodynamics

The Nestos Shear Zone (NSZ), mostly on the Greek territory, is recognized as one of the major structures of the Rhodope Metamorphic Complex (RMC). It consists of a thick NNE-dipping pile of mylonites with top-to-SW kinematics encompassing the contact of the ‘Sidironero Unit’ (SU) onto the ‘Pangaeon Unit’ (PU, the lowest exposed unit of the RMC). For most authors, the top-to-SW shear fabric of the NSZ reflects synmetamorphic thrusting.
   We carried out a structural, petrological and geochronological (U-Pb and ³⁹Ar-⁴⁰Ar) study of the NSZ. Inverted metamorphism is confirmed and is found to be coeval with top-to-SW shearing. The whole SU (including its base, overlapping with the NSZ) experienced the conditions of advanced partial melting at T > 650°C. Leucosomes that locally crosscut the main fabric crystallized between ≥ 50 and ca. 40 Ma (U-Pb zircon and monazite ages), just before cooling of the metamorphic pile. This shows that at least part of the migmatization is not an old event but is part of the syn-shearing metamorphic evolution. In contrast, rocks of the PU right beneath the SU do not show any evidence that they ever reached the conditions of anatexis. In orthogneisses, microstructures document amphibolite facies shearing. Although relatively rare, metabasites and Grt-bearing micaschists keep the record of a prograde metamorphic path culminating at T ≤ 620°C (at P ~ 8-10 kbar). Thus, higher-grade rocks were emplaced onto lower-grade rocks during top-to-SW shearing, attesting for synmetamorphic thrusting along the NSZ. Hornblende ³⁹Ar-⁴⁰Ar single-grain plateau ages from the NSZ are between 39 and 37 Ma, which we interpret as dating amphibolite facies shearing. Later strain increments have produced greenschist facies mylonites and ultramylonites subconcordant with the earlier fabric and with identical kinematics. White mica ³⁹Ar-⁴⁰Ar single-grain plateau ages from these rocks are between 36 and 33 Ma, which we interpret as dating mylonitization. With respect to peak conditions in the PU, this deformation occurred at lower grade conditions, therefore inverted metamorphism cannot be invoked in this case. Nevertheless, several lines of evidence indicate that this deformation reflects thrusting as well. Consequently, our study documents persistent synmetamorphic thrusting along the NSZ as late as ca. 33 Ma. This is consistent with results obtained from the Chepelare Shear Zone, in the Bulgarian Central Rhodope (Gerdjikov et al., this volume), and contradicts the view that post-orogenic extension was already active in pre-Oligocene times in the northern Aegean. Our analysis of the RMC further indicates that post-orogenic extension
did not start before ca. 27 Ma. Hence, it started at about the same time than it did further south in the Cyclades and Menderes region, at variance with the statement in some recent geodynamic syntheses. The picture arising from the RMC is consistent with a change in the geodynamic setting of the whole Mediterranean at around 30 Ma, from strongly compressional (i.e. Alpine collision) to a situation dominated by trench retreat and postorogenic extension.