TZANKOV, T., Bulgarian Academy of Sciences, Sofia, Bulgaria; and B. C. BURCHFIEL,* Massachusetts Institute of Technology, Cambridge, MA

ABSTRACT: Alpine Tectonic Evolution of Bulgaria

The Balkan segment of the Alpine orogenic system developed along the southern margin of the Moesian continental fragment. Evidence for Alpine oceanic sutures within Bulgaria are generally lacking. Paleotethys and Neotethys oceanic regions existed south of the Moesian continental fragment, and sutures developed during their closure are present in Turkey, Greece, and Yugoslavia. Only fragments of far-traveled nappes derived from those oceanic areas are preserved in Bulgaria. The Alpine Balkan orogene developed by a sequence of superposed shortening and extensional deformational events that affected the mobile southern margin of the Moesian continental fragment. Because of the disruption of older structures by the superposition of numerous younger tectonic events, the continuity of defor ational belts, age of deformation for particular structures, and the magnitude of shortening or extension for a given event are difficult to determine.

From the oldest to youngest, the major tectonic events are as follows. (1) Extension of the Moesian continental margin in the Late Triassic and Early Jurassic. Probably older and contemporaneous arc volcanism in the Paleotethyan ocean south of the continental margin. (2) Emplacement of oceanic rocks onto the Moesian continental margin during closure of the Paleotethys in a poorly defined event between the Middle Jurassic and the early Late Cretaceous. (3) Formation by uncertain origin of a Late Jurassic-Early Cretaceous flysch basin in the northern Balkan region. (4) Major north-vergent thrusting during the middle and Late Cretaceous in at least two and possibly three separate events. (5) Development of a volcanic arc and extensional back-arc basin along the southern margin of the Moe ian continental margin in the Late Cretaceous. (6) Back-arc antithetic north-vergent thrusting of latest Cretaceous-early Cenozoic age. (7) At least two and possibly three periods of north-vergent thrusting during the Eocene, Oligocene, and possibly early Miocene, partly contemporaneous with extensive volcanism and extension in south Bulgaria. (8) Late Miocene to Holocene extension throughout Bulgaria.

Most of these events are recognized in the northern and southwestern parts of Balkan orogene, but the pre-Eocene Alpine evolution of the Rhodopian region remains poorly understood because of extensive metamorphism and lack of dated Mesozoic rocks. The marginal fold of the thrust belt of the Balkan orogene is unique because of the superposition of several shortening events and lack of progressive northward younging of deformation. Little recognized are the effects of major Neogene extension that probably are responsible for all the present topographic features of Bulgaria and the major disruption of pre-Neogene structural patterns.

AAPG Search and Discovery Article #90990©1993 AAPG International Conference and Exhibition, The Hague, Netherlands, October 17-20, 1993.