Геологические аспекты глубинного строения земной коры в Южной Болгарии

Abstract

The layered structure of the Earth's crust in South Bulgaria is a result of the complicated geologic development, and of changes in the physical properties of the rocks under high pressures and temperatures. The following layers are distinguished (from bottom to top): Layer "A" (dehydrated lower crust) with layer velocities from 6.4 to 6.7 km/s, and characterized by low-velocity ductile deformations: Layer "B" (hydrated upper crust) with layer velocities 5.9–6.0 km/s, and characterized by homogeneous strains, as well as by deformations by cataclastic flow; low-velocity layer which developed only partially in the west part of Rhodope Massif, and is deformed by superplastic flow; presumably, it compensated differently directed movements in adjacent layers during some deformational stages; granitoid-metamorphic crust characterized mainly by brittle deformations. This structure probably resulted from multiphase deformations during several tectonic cycles. The dynamic environment changed considerably, compression stages being replaced by extension stages; in some stages different crust layers behaved differently, e. g. with an extension environment in the granitoid-metamorphic crust, and compression – within the deeper levels. Crustal thickening is due to different mechanisms but has been bound mainly to continental collision. With reaching a crytical thickness, isostasy-bound uplift led to thinning by erosion. Other mechanisms of crustal thinning are bound to environments of generalized extension and rifting, homogeneous deformation, or non-homogeneous ductile deformation with necking. As a result of multiphase crustal thickening, Rhodope Massif formed as a peculiar crustal lensoid body. The collisional orogen on the Balkan Peninsula, at least during the Alpine cycle, had a transitional character – from Alpine towards the Himalayan-Tibethan type, the Rhodope Massif playing the part of plateau.