Late Alpine (Palaeogene) superimposed depressions in parts of Southeast Bulgaria

Abstract

The Late Alpine superimposed depressions in thе south-eastern part of the Balkan peninsula are structures of collisional-collapse type. They play a role of neoautochton which overlays a highly disintegrated Middle and Late Alpine orogen of collage-accretional character. It is represented by the Sredna Gora and Rhodope superunits. The superimposed depressions are of Palaeogene-Neogene age and are elements of a separate tectonic entity (Maritsa superimposed graben system) within the boundaries of the Balkanides-Anatolian segment of the Alpine mobile belt. The following three wide depressions on South Bulgarian territory are characterized in this paper: Upper Thrace Depression (UTD), East Rhodope Depression (ERD) and the East Thrace Depression (ETD). The Tertiary evolution of each depression is characterised by three up to five destructive stages. The Late Eocene and Oligocene stages are accompanied by an active polycyclic or monophase magmatism mostly represented by intermediate to acid volcanics. Ca-alkaline, subalkaline and alkaline magmas are distinguished. Basic volcanics of toleiitic, subalkaline to alkaline composition are rare. During the stages outlined, phenomena of compression or extension with exhumation are recorded. A number of important faults and fault zones formed during those stages, some of them being now represented by dike bundles. Essential overthrustings took place only along some faults of late Laramian and Savian age.