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Nemcok, Michal1, R. Eric Higgins2, Dian Vangelov3 
(1) Energy and Geoscience Institute, University of Utah, Salt Lake City, UT 
(2) Vintage Petroleum, Tulsa, OK 
(3) Sofia University "St. Kliment Ohridski", Sofia, Bulgaria

ABSTRACT: The Influence of Varying Resistance to Orogenic Advance on the Structural Styles of the Onshore and Offshore Balkanide Thrustbelts - Controls on Trap Development in the Western Black Sea Basin

Studies of the Bourgas Deep-Sea Block, offshore Bulgaria, were conducted using seismic, well and surface geologic data from the onshore Balkanides, and offshore seismic and well data. The curvilinear Balkan and Forebalkan orogenic zones are characterized by E-W strike onshore, rotating to nearly N-S strike offshore. Strike variation is controlled by differences in basal friction affecting the onshore and offshore thrustbelt segments, advancing onto the Moesian Platform and Black Sea Basin, respectively. 
Compression of the onshore segment resulted in Paleocene piggyback thrusting in the rear of the Balkans and inversion of Mesozoic rifts to the front, developing sedimentary basins between. These basins experienced Late Paleocene-Eocene infill and shortening, increasing thrustbelt taper. Increased taper resulted in Late Eocene shortening of the Forebalkans, forming five thrust sheets of NE vergence. 
The offshore orogenic belt did not climb onto the stable Moesian Platform, but advanced into the Western Black Sea Basin, supported by weaker transitional to oceanic crust. Encountering lower basal friction conditions than onshore, the thrustbelt taper was larger than necessary for further advance. Low basal friction resulted in combined ENE and WSW thrust vergencies and upright detachment folds. Thrustbelt taper was subsequently reduced and deformed by thin-skinned collapse operating under brittle conditions. 
Large ENE- and WSW-vergent anticlines deforming Paleogene strata in the deep- water, offshore Forebalkans offer potential traps charged from the foreland wedge or underlying Mesozoic strata. Neogene extension, accompanied by subsidence and passive margin sedimentation, resulted in the burial and preservation of these features under a thick sedimentary wedge.


AAPG Search and Discovery Article #90026©2004 AAPG Annual Meeting, Dallas, Texas, April 18-21, 2004.