Flexural uplift of the Stara Planina range, central Bulgaria

Abstract

The Stara Planina is an E–W‐trending range within the Balkan belt in central 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 shortening associated with those events. However, uplift of this belt appears to be much younger than the age of thrusting and correlates instead with the age of Pliocene–Quaternary normal faulting along the southern 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 plate thickness of 12 km under the Moesian platform is reduced to about 3 km under the Stara Planina. This small value of elastic plate thickness under the Stara Planina is similar to values observed in the Basin and Range Province of the western United States, and suggests that weakening of the lithosphere is due to heating of the lithosphere during extension, perhaps to the point that large‐scale flow of material is possible within the lower crust. Because weakening is observed to affect the Moesian lithosphere for ≈10 km beyond (north of) the surface expression of extension, this study suggests that processes within the uppermost mantle, such as convection, play an active role in the extension process. The results of this study also suggest that much of the topographic relief in thrust belts where convergence is accompanied by coeval extension in the upper plate (or ‘back arc’), such 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.