Crust and upper-mantle inhomogeneities beneath the central part of the Balkan region

Abstract

Crustal and upper-mantle velocity inhomogeneities are studied by the interpretation of the P-wave travel times from local explosions and teleseismic earthquakes recorded in 30 stations on the territory of Bulgaria and some neighbouring regions. Different methods of the solution of inverse 2-D and 3-D kinematic problems are used. The established velocity inhomogeneities are discussed in connection with the major characteristics of some geophysical and seismotectonic fields. It is noted that the velocity patterns of the crust reflect the configuration of Alpine tectonic structures. The subcrustal inhomogeneities are discordant to the main features of the near-surface structures. However, these inhomogeneities are concordant to some seismic lineaments inexplicable from the viewpoint of the first-order tectonic structures. The observed relationship between the seismicity and deep velocity structures show that these lineaments are controlled by the uppermost mantle heterogeneities. It is also noted that the high-velocity zone in the deeper upper mantle indicates deep-seated inhomogeneities beneath the Rhodopean Massif. These inhomogeneities most probably represent palaeo-subduction of a lithosphere slab penetrating deeper than 250 km into the asthenosphere.