Modeling of the Ionian basin formation: Deep processes and tectonic evolution
We present evolutionary models (both conceptual and quantitative) for the Ionian basin formation based on vast geological and geophysical datasets. The tectonic analysis of the region revealed three phases of rapid subsidence of southern Sicily and adjacent northwestern Ionian basin: in Middle-Late Triassic, Late Jurassic-Early Cretaceous, and Neogene. On the basis of recent seismostratigraphic data we assume that the basin started to form in the Late Jurassic due to extension of continental lithosphere. During the active rifting phase, accompanied by thinning of the crust and of the lower part of the lithosphere, the underlain asthenosphere moved upward. The mantle material rose and partially melted when the geotherm moved up crossing the solidus. A part of the magmatic melt moved upward, partially crystallized within the lower crust and partially reached the surface through active volcanism . We assume that the extension in the Late Jurassic did not lead to the formation of oceanic crust, but significantly attenuated the continental crust and the lithosphere. We also assume that another part of the melt remained at the lithosphere/asthenosphere boundary and formed a magmatic lens. The basaltic melt in the lens crystallized owing to changed temperature-pressure conditions and then turned into dense rocks (eclogites). Gravitational anomalies and increased $P$ wave velocities in this zone confirm the existence of anomalously dense bodies in the upper mantle. The eclogite lens sank in the asthenosphere causing a viscous flow that resulted in the evolution of the Ionian basin since the Early Cretaceous. We analyze numerical models of the flow induced by the uplift of the anomalous mantle and by the subsidence of the heavy lens. By combining results of subsidence analysis and of numerical modeling, we conclude that the processes in the upper mantle leading to the Ionian basin formation can be explained by the upwelling mantle stream, phase transition and sinking of the eclogite lens. It is believed that the extension in the Late Jurassic, the rapid basin subsidence in the Early Cretaceous, and the subsequent shortening in the Late Cretaceous and Tertiary constitute the tectonic expression of these processes.