Tectonometamorphic evolution of the Rhodope orogen: Constraints from macro- and microstructures, petrology and geochronology

Tectono-stratigraphy, macro- and microstructures, petrology and geochronology have been combined into a comprehensive model for the tectono-metamorphic evolution of the Rhodope orogen from the Jurassic to the Early Paleogene. High-grade deformed and metamorphosed continental and mantle rocks in two study areas in the Central and Eastern Greek Rhodope are part of a suture zone (Rhodope Suture Zone: RSZ) which extends over at least 120 km from the W to the E. It is structurally underlain by a Lower Unit consisting of Pangeon gneisses with Variscan age and a metasedimentary cover of marbles with Triassic(?) age and overlain by the Upper Unit, a Late Jurassic sequence consisting of orthogneisses at the base, discordantly covered by (meta-)ophiolites (Circum Rhodope) on top. Considering our proposed model, the Lower Unit was derived from the Apulian plate during the Paleogene whereas the Upper Unit indicates calc-alkaline magmatism which took place during the early exhumation phase of the suture zone rocks. (Meta-) ophiolites are related to a rifting stage within the southern Neotethyan Ocean and have been obducted in north direction during Jurassic/Cretaceous time. The material from the investigated suture zone most likely originated as an extensional allochthon south of the European continent during Permo/Triassic time was subsequently subducted beneath Europe in the Early Jurassic (≥180 Ma). On the basis of comparable metamorphic ages and coherent structures but differences in metamorphic conditions and lithologies the rocks of the RSZ are subdivided into an Upper and a Lower Part. The P-T-d history of both parts differs due to the relative tectonic position within the exhuming wedge. Information for the prograde history is derived from subductionrelated structures within quartzites in the Lower Part. Metapelites marking the transition between both parts contain microdiamonds and indicate that central parts of the RSZ experienced UHP conditions before exhumation started. Exhumation was controlled by buoyancy-driven normal displacement with SW-shearing at the base (Lower Part) and NE-shearing on top (Upper Part) indicating an early activity of a deep crust shear zone (Nestos Shear Zone) from the Late Jurassic to the Late Cretaceous. An intervening stage of mineral re-crystallization and thermal re-equilibration from partial anatexis in the Upper Part probably decreased exhumation rates and rocks experienced long-lasting overprinting at mid-crustal levels. Because of slow exhumation rates, peak indicators of a probable UHP stage were almost totally obliterated. A common exhumation history of both parts at pressures lower than about 12 kbar (35 – 40 km depth) is proposed. During this stage, rocks of the RSZ experienced their main ductile overprint due to southwest-directed shearing and folding. Differences in the deformation overprint of both parts are indicated by plunge angles of the stretching lineation which are steeply dipping to the NE in the Lower Part and subhorizontally dipping to the NE in the Upper Part. The Lower Part experienced long-lasting retrogression due to SW-directed shearing with the lack of partial anatexis and thermal re-equilibration. Upper crustal structures evolved most likely due to slab retreat and point to tectonic erosion as a final exhumation mechanism for the units of the Upper Part. Late stage shear activity at the Nestos Shear Zone accompanied with magmatism dominated in the footwall units of the Lower Part. This late tectonics is considered as coevally with the formation of Variscan basement domes originating from the underthrusted Lower Unit, thus making the Rhodope orogen a classic core complex juxtaposed to former structurally higher units.