FLEXURAL UPLIFT OF THE STARA-PLANINA RANGE, CENTRAL BULGARIA

The Stara Planina is an E-W-trending range within the Balkan belt in c entral Bulgaria. This topographically high mountain range was the site of Mesozoic through early Cenozoic thrusting and convergence, and its high topography is generally thought to have resulted from crustal sh ortening associated with those events. However, uplift of this belt ap pears to be much younger than the age of thrusting and correlates inst ead with the age of Pliocene-Quaternary normal faulting along the sout hern side of the range. Flexural modelling indicates the morphology of the range is consistent with flexural uplift of footwall rocks during Pliocene-Quaternary displacement on S-dipping normal faults bounding the south side of the mountains, provided that the effective elastic p late thickness of 12 km under the Moesian platform is reduced to about 3 km under the Stara Planina. This small value of elastic plate thick ness under the Stara Planina is similar to values observed in the Basi n and Range Province of the western United States, and suggests that w eakening of the lithosphere is due to heating of the lithosphere durin g extension, perhaps to the point that large-scale flow of material is possible within the lower crust. Because weakening is observed to aff ect the Moesian lithosphere for approximate to 10 km beyond (north of) the surface expression of extension, this study suggests that process es within the uppermost mantle, such as convection, play an active rol e in the extension process. The results of this study also suggest tha t much of the topographic relief in thrust belts where convergence is accompanied by coeval extension in the upper plate (or 'back arc'), su ch as in the Apennines, may be a flexural response to unloading during normal faulting, rather than a direct response to crustal shortening in the thrust belt.