The history of the Bachkovo-Dobralak metamorphic unit of the Upper Rhodopian terrane, Bulgaria

Abstract

The Rhodope massif is located in southern Bulgaria and northern Greece, north of the Aegean Sea. The Rhodope massif was affected by different stages of deformation and metamorphism during Mesozoic-Cenozoic times. It is situated in the transitional area between the late Jurassic – late Cretaceous shortening recorded in the Dinaric – Hellenic orogen by the N-ward subduction and collision of the Neotethys Ocean and the Cretaceous – Eocene shortening recorded in the Carpatho-Balkanides orogen by the S-ward subduction and collision of the Ceahlau-Severin Ocean, an eastern branch of the Alpine Tethys. Partly coeval and post-dating this orogenic build-up, the Rhodope was affected by the formation of a large core-complex starting with the Middle Eocene times, the driving extension being associated with the formation of a number of well-known top-S to SW detachments, such as Kerdilion and Strymon. The northern boundary of the core-complex in Bulgaria is marked by a significant number of kinematic indicators showing top-N sense of shear. It is rather unclear if this stretching took place in response to the earlier nappe stack kinematics or formed as result of local detachments dipping N-wards, mechanically accommodating the exhumation of the large dome. They could have accompanied the formation of extensional basins at the northern boundary of the core-complex starting in late Eocene times, such as the Plovdiv Basin. We have studied the balance between orogenic nappe stacking and subsequent extensional collapse in part of the Upper and Uppermost units of the northern Rhodope, by a field kinematic study followed by microstructural observations and laboratory analysis. The results demonstrate that a top NW-NNW shearing event documented in the upper Rhodope was associated with burial and prograde metamorphism during nappe stacking. This event was subsequently followed by one other tectonic deformation, characterized by pervasive top – ENE to NE senses of shear and retrograde metamorphism. This deformation is interpreted as an extensional detachment exhuming the deeper Rhodope units previously buried by the nappe stack. The deep exhumation of the Rhodope core-complex has probably reached a situation in which the extension was not accommodated anymore by top-S sense of movement and led to the formation of a number of local top-N detachments sustaining the continuation of footwall uplift. Local top-N detachment resulted in the unloading of originally structurally deeper rocks that are thought to have been subject to rapid retrograde metamorphism (partly) overprinting earlier prograde metamorphic characteristics.