GPS study of active tectonics in Bulgaria: results from 1996 to 1998

To determine the present day kinematics of the major tectonic elements in Bulgaria, part of the northern extent of the Aegean extensional system, we established and surveyed a regional GPS network in 1996, 1997 and 1998. Based on our estimates of the velocities of 17 stations surveyed at least twice, we identify two areas of significant deformation. Between the central Danubian Plain and a 200-km-wide area adjacent to Black Sea, we find 3±1 mm/year of shortening, consistent with compressional focal mechanisms for the Gorna Oriahovitsa fault zone and a NW-trending flexure between higher topography in northeast Bulgaria and the area to the west. South of the Danubian Plain, there is a broad zone of N-S to NNE-SSW extension encompassing the E-W trending Sub-Balkan graben system and Thracian basin of central Bulgaria and with a total differential motion of 3–5 mm/year. The zone of extension appears to continue into the Sofia graben and through a topographically low part of the Stara Planina mountains. This zone of extension marks the northern boundary of the Aegean extensional region, but its western continuation remains unclear. Velocities of 3–4 mm/year to the south and 3 mm/year to the ESE in south central Bulgaria express a clear transition to the Aegean extensional region farther south.