Did Adria rotate relative to Africa?

Abstract

Abstract. The first and foremost boundary condition for kinematic reconstructions of the Mediterranean region is the relative motion between Africa and Eurasia, constrained through reconstructions of the Atlantic Ocean. The Adria continental block is in a downgoing plate position relative to the strongly curved Central Mediterranean subduction-related orogens, and forms the foreland of the Apennines, Alps, Dinarides, and Albanides-Hellenides. It is connected to the African plate through the Ionian Basin, likely with lower Mesozoic oceanic lithosphere. If the relative motion of Adria vs. Africa is known, its position relative to Eurasia can be constrained through the plate circuit, and hard boundary conditions for the reconstruction of the complex kinematic history of the Mediterranean are obtained. Kinematic reconstructions for the Neogene motion of Adria vs. Africa interpreted from the Alps, and from Ionian Basin and its surroundings, however, lead to scenarios involving vertical axis rotation predictions ranging from ∼0 to 20° counterclockwise. Here, we provide six new paleomagnetic poles from Adria, derived from the Lower Cretaceous to Upper Miocene carbonatic units of the Apulian peninsula (southern Italy). These, in combination with published poles from the Po Plain (Italy), the Istria peninsula (Croatia), and the Gargano promontory (Italy), document a post-Eocene 9.5 ± 8.7° counterclockwise vertical axis rotation of Adria. This result provides no support for models invoking significant Africa–Adria rotation differences between the Early Cretaceous and Eocene. The Alpine and Ionian Basin end-member kinematic models are both permitted within the documented rotation range, yet are mutually exclusive. This apparent enigma can be solved only if one or more of the following conditions (requiring future research) are satisfied: (i) Neogene shortening in the western Alps has been significantly underestimated (by as much as 150 km); (ii) Neogene extension in the Ionian Basin has been significantly underestimated (by as much as 420 km); and/or (iii) a major sinistral strike-slip zone has decoupled North and South Adria in Neogene time. Here we present five alternative reconstructions of Adria at 20 Ma that highlight the enigma: they fit the inferred rotation pattern from this study or previously proposed kinematic reconstructions from the surrounding.