The Balkan terranes: a missing link between the eastern and western segments of the Avalonian-Cadomian orogenic belt?

The Alpine-Himalayan collision zone involves a number of crustal fragments that originated in the Neoproterozoic to Cambrian Avalonian-Cadomian belt of northern Gondwana. We use the detrital zircon U-Pb geochronology to examine four of these lithotectonic units, now exposed in the Balkans in Bulgaria and Serbia. The obtained age spectra suggest that the Diabase-Phyllitoid Complex (the maximum depositional age, MDA, estimated at 540 + 5/-9 Ma) was presumably an accretionary wedge or a forearc basin sourced from a nearby volcanic arc, however, its palaeo-position remains uncertain. The Vlasina Complex (MDA of 577 + 5/-6 Ma) was the most 'westerly' terrane, adjacent to the Trans-Saharan belt, whereas the Sredna Gora and Stara Planina complexes (MDAs of 546 +/- 7 Ma and 579 + 4/-5, respectively) were positioned next to the Saharan Metacraton and Arabian-Nubian shield. To put the Balkan terranes into a broad context, we statistically compare the detrital zircon ages in other terranes from the Eastern Alps to Iran with igneous and metamorphic U-Pb zircon ages from North African source areas. The statistical comparison is done through multi-dimensional scaling (MDS), a more rigorous method than a visual comparison of age spectra, to examine the degree of inter-sample similarity. This information is then transferred to a tentative palaeogeographic map showing position of each terrane with respect to its most likely source region. As a result, we define a 'westerly' terrane assemblage, characterized by Mesoproterozoic ages and sourced from the West African craton and the Trans-Saharan belt and an 'easterly' assemblage formed next to the Saharan Metacraton and the Arabian-Nubian shield. The present-day position of some of these terranes implies significant dextral strike-slip displacement, perhaps due to movement on the Pangea megashear during the Carboniferous and Permian.

Automated summary

The study focuses on analyzing four lithotectonic units in the Balkans using detrital zircon U-Pb geochronology to understand their formation and origins. Through statistical comparison and multidimensional scaling, the Balkan terranes are compared with other terranes from the Eastern Alps to Iran, revealing their paleogeographic positions and significant dextral strike-slip displacements.