Lower Paleozoic low-grade metamorphic units from the Central Balkan Zone, Bulgaria: tectonic relationships, framework and geodynamic significance

The Central Balkan Zone belongs to the north-vergent fold-and-thrust belt of the Alpine Balkan orogen. In this zone, pre-Permian low-grade rocks, metamorphosed during the Variscan orogeny, were later reworked at two main stages of the Alpine compressional tectonics. Several tectono-stratigraphic subdivisions of these metamorphic rocks were previously presented, based on a purely stratigraphic approach in the absence of detailed structural studies and, most importantly, of reliable paleontological and geochronological records. In this study, we propose a new framework of the low-grade metamorphic rocks from the Central Balkan Zone, applying a critical analysis of the existing data combined with new geochronological data and detailed lithological and structural observations. Based on the structural relationships and geochronological constraints, several new entities, such as the Korduna, Bilo and Zvezdets units, were established together with a reassessment of the previously recognized Diabase-Phyllitoid Complex (DPC). Both DPC and the Bilo Unit are now defined as Cambrian–Lower Ordovician (?) mélange complexes part of an accretionary wedge and/or forearc basin formed along the north Gondwanan margin. In the Bilo, Murgash and Etropole mountains, a tectonically uninterrupted Cambrian–Upper Ordovician section, characterized by a normal metamorphic gradient from low-grade to nonmetamorphosed terrigenous rocks, is recognized. The deposition of these sediments is related to the subduction of the Prototethys and the early evolution of the Rheic Ocean. An inverted metamorphic gradient recognized along the southern slopes of the Etropole and Zlatitsa-Teteven mountains through the Korduna and Zvezdets units is related to Variscan syn-metamorphic deformation in the Stargel-Bulovanya Tectonic Zone.