Kraishtides (Kraištiden) 70 years later: Myth or Reality?

Abstract

The notion of Kraishtides underwent considerable transformations during 70 years since its introduction in the tectonic nomenclature of the Balkan Peninsula. Initially it was introduced for the allegedly youngest (Early Miocene) fold and thrust (orogenic) zone (Krajštiden) situated obliquely to Balkanides and South Carpathians. This notion evolved into a fault belt composed of normal faults that were transformed into reverse faults (Krajštiden-Lineament), and in a lineament-geosynclinal zone (Kraishtid lineament-geosyncline) of repeated opening (in latest Jurassic, Palaeogene and Neogene times) of basins of rift character, and following (“Austrian”, “Pyrenean” and “Savian”) compression and closures. All these meanings of the term may be regarded as redundant now. They are inconsistent with the modern knowledge about the geological structure and evolution of the region, and do not fit the modern tectonic ideas. Only the term “Kraishtid (Strouma) Lineament” could be preserved as a synonym to “Strouma (Kraishtid) fault belt”. However, the word “Kraishtid (Krajštiden)” refers to an orogenic belt (zone), and cannot be preserved as a toponym of fault belt. “Kraishtides” are neither a structural zone with a definite characteristics. The structure of SW Bulgaria is a peculiar mosaic of fold zones and blocks. In the Late Jurassic – Early Cretaceous palaeogeodynamic setting, a part of this area was the westernmost part of the Nish-Troyan flysch trough whereas South of it, the largest area belonged to an uplifted “plateau”. In mid-Cretaceous times (Aptian – Albian), the “Austrian” orogenesis led to the closure of the relics of the flysch trough, and the formation of the complex Morava-Rhodope (Macedonian-Rhodope) zone (in its westernmost part composed of the Morava, Strouma, and Ograzhden units). In Late Cretaceous times, the whole Morava-Rhodope zone represented an area of thickened continental crust (elevated frontal arc, “plateau”) whereas North of it, the Srednogorie volcanic arc and see was opened. In Palaeogene times, the whole Morava-Rhodope zone was again a volcanic island arc with a WNW-ESE trend. Only in Palaeogene and Neogene times, the Strouma (Kraishtid) lineament played the role of a fault belt with considerable dextral strike-slip movements, and repeating rifting in transtensional conditions.