Shallow Asthenospheric Volumes in the Circum-Mediterranean and their Relation to Intraplate Volcanism and Topography

Based on a high-resolution Rayleigh-wave tomography of the Mediterranean region, shallow asthenospheric volumes are identified characterized by low S-wave velocities between about 70 km and 250 km depth. We distinguish between five major shallow asthenospheric volumes in the Circum-Mediterranean: the Middle East, the Anatolian-Aegean, the Pannonian, the Central European, and the Western Mediterranean Asthenospheres. Remarkably, they form an almost closed circular belt of asthenospheres interrupted only by thick Triassic oceanic lithosphere in the eastern Mediterranean. Shallow asthenosphere beneath the Rhone and Rhine Grabens connects the Western Mediterranean with the Central European Asthenosphere. Beneath the Serbian and Rhodope Mountains shallow asthenosphere forms a link between the Pannonian and the Anatolian-Aegean Asthenosphere.

Cenozoic intraplate volcanic fields are found above all areas underlain by shallow asthenosphere, and is absent in areas of thick lithosphere. Thus, anorogenic intraplate volcanism in the circum-Mediterranean appears to be associated with shallow asthenospheric volumes. Specifically, this applies to volcanic fields in the central Apennines and Sicily underlain by the Western Mediterranean Asthenosphere. Regions without significant tectonic extension above shallow asthenosphere are characterized by elevated topography. Examples are the Anatolian Plateau, the Western Carpathians, the Atlas Mountains and the low mountain ranges in Central Europe and Iberia. In back-arc regions like the Aegean and Pannonian Basins, and the western Mediterranean, strong tectonic extension leads to low topography above shallow asthenosphere. High continental shoulders are present in transition regions towards thinner lithosphere. Examples are the Levantine coast, the Moesian platform and the Bohemian Massif or the southern Atlas Mountains. We further note that in regions of past volcanism continental lithosphere is thickening by long-term cooling. The showcase for this is the North-German Basin where sedimentation and an about 100 km thick lithosphere developed after extensive Permian volcanism. These observations hint at considerable  variability in time of the continental lithosphere-asthenosphere boundary: Thinning of continental lithosphere may be caused by extension and/or thermal erosion whereas cooling may lead to thickening of continental lithosphere as is evident from the Mesozoic evolution of continental lithosphere in central Europe.