LATE CENOZOIC EXTENSION IN SW BULGARIA: A SYNTHESIS

Abstract

Southwest Bulgaria forms the northern margin of the Aegean extensional province. Since the Early Pliocene (c. 4 Ma), this region has accommodated southward or SSE extension at several millimetres per year, superimposed on c. 400 m of post-Early Pliocene regional uplift. This sense of deformation superseded earlier extension, oriented ENE-WSW, which is estimated to have begun in the early Late Miocene (c. 10-9 Ma) and lasted until c. 4 Ma. The regional topography is dominated by NNW-SSE-striking grabens and normal fault escarpments, relics from this time. Normal faults that are now active cut across these older structures, although in some localities normal faults that were oriented obliquely to the earlier extension have remained active, also oblique to the modern extension sense. It is suggested that this present phase of extension relates to the modern sense of deformation throughout the Aegean region and to the modern geometry of the North Anatolian Fault Zone (NAFZ), which is independently inferred to have existed since c. 4 Ma. The earlier ENE-WSW extension is inferred to have involved two phases, the first predating the NAFZ and the second synkinematic with its initial phase of slip during c. 7-4 Ma, when its geometry and the overall sense of deformation in the Aegean region were different from at present. Some previous studies have inferred that SW Bulgaria experienced large-scale extension on low-angle normal faults in the Mid-Miocene or earlier. However, the limited evidence in support of this view is open to other interpretations, and after due consideration can be discounted. © The Geological Society of London 2006.