THE SUB-BALKAN GRABEN SYSTEM OF CENTRAL BULGARIA

The Sub-Balkan graben system in central Bulgaria forms the present nor thern boundary of the Aegean extensional region. This east-trending gr aben system lies along the southern flank of the Stara Planina range a nd consists mainly of half-grabens. The sedimentary fill in the graben s ranges in age from late Miocene to Recent and records the initiation and evolution of the graben system. The sedimentary fill in the grabe ns is oldest in the central graben and becomes progressively younger t o the west and east, indicating a diachronous development of the grabe ns. Grabens are formed in the hangingwalls of south-dipping low-angle normal faults which have been displaced by younger higher angle normal faults along the foot of the Stara Planina. Hangingwall rocks have be en complexly faulted and rotated such that some graben fill has been r otated down-to-the-north. The Sredna Gora range south of the grabens i s part of a complexly faulted and rotated hangingwall block, bounded o n the south by south-dipping normal faults forming the northern bounda ry of the Thracian Basin. The Stara Planina range has been formed by u plift and rotation due to footwall unloading along the low-angle norma l faults and forms the northern margin of the graben system. Most of t he topography of Bulgaria south of the Sub-Balkan graben system is the result of late Miocene to Recent extensional processes linked to the Aegean region that have;been superposed on convergent features and ear lier extensional features that extend back to late Eocene time.