Tethyan evolution of the Black Sea region since the Paleozoic: a paleomagnetic approach. Geologica Ultraiectina (319)

Abstract

In the early Jurassic, Africa plus Arabia and Eurasia were separated by a ~2000 km wide domain containing oceanic lithosphere – including continental fragments that rifted off Africa - known as the Tethys ocean(s). In the northern part, the Paleo-Tethys ocean was subducting, while in the south, the Neo-Tethys ocean was spreading between the continental fragments and the African plate. The subsequent break-up of the supercontinent Pangea led to convergence between Africa and Eurasia, resulting in the disappearance of the Tethyan oceanic domains, and collision of the Tethyan continental fragments with Eurasia. Paleolatitude analysis and the timing of the collision of these continental terranes help to constrain this history. Here, the results of a paleomagnetic study of the circum-Black Sea region since the Paleozoic are presented, focusing on the geological history of Ukraine and Turkey. New paleomagnetic results from Carboniferous and lower Permian sediments exposed in the Donbas Foldbelt of Ukraine - corrected for inclination error and hence paleolatitude error - improved the Eurasian apparent polar wander (APW) path. This APW path was earlier not well constrained for the Carboniferous, and the new results were discussed in the light of the Pangea A versus B controversy. The upper Jurassic results from the European plate (Pontides, Turkey and Crimea, Ukraine) show much lower paleolatitudes than predicted by the APW paths. Similar results from an earlier study on Adria, as part of the African plate, have been interpreted as fast southward motion to low latitudes in the late Jurassic, followed by a northward motion. However, in the late Jurassic, the African and Eurasian plates were separated by subduction zones. For the Black Sea region we could prove northward subduction in the middle Jurassic to earliest Cretaceous by conducting isotopic 40Ar/39Ar dating and geochemical analysis on Crimean volcanics. Because subducting slabs function as an anchor in the mantle, we find it unlikely that the low paleolatitudes were the result of rapid motion of entire plates. We therefore argue that this cusp in the APW paths represents motion of the entire mantle and crust with respect to the Earth’s spin axis, a process known as True Polar Wander. We also studied Cretaceous to Eocene rocks from the central and eastern Pontides in north-central Turkey and were able to prove that this peculiar northward arc-shaped orogen is the result of latest Cretaceous to early Paleocene oroclinal bending. We suggest that this orocline resulted from the collision and indentation of the Anatolide-Tauride Block and its promontory, the metamorphic Central Anatolian Crystalline Complex. Our paleomagnetic data from Carboniferous to Eocene rocks from the central Taurides in south-central Turkey were aimed at reconstructing the paleolatitude history. It appeared, however, that a remagnetization event affected all sampled sections. We provide evidence for remagnetization by a novel end-member modeling approach of rock magnetic properties (IRM acquisition curves). Our new data and previously published results bracket the remagnetization event between ~85 and 20 Ma (late Cretaceous to early Miocene). We discuss the implications of our findings for the dimensions, timing and amount of vertical axis rotations in this region.