The sprawl of the External Hellenides: from post-Alpine collapse to present-day kinematics

During the Neogene, the Aegean domain underwent intense deformation, leading to a thinning by a factor of two or more of the Alpine orogenic prism. Today, tectonic velocity gradients are still among the fastest in Europe due to the Anatolian extrusion induced by the Arabian indentation and by the Hellenic slab retreat. The present-day deformation essentially localizes in the subduction backstop. With respect to the central Aegean, which is almost stable today, this still-thick buttress has remained at a much earlier and brittle deformation stage. This is particularly the case in the ~east–west-extending External Hellenides (Southern Greece), shaped by a series of major NNW–SSE-oriented normal faults.

• How has the crustal deformation been accommodated by the various fault systems present in the Peloponnese since the Paleogene?
• Which of those fault systems are still active today?
• To what extent can boundary forces such as the Hellenic slab pull be sufficient to explain this extension?

Thanks to a significant increase in the GNSS network density in the Peloponnese, we present an updated local strain field. The resulting strain confirms the ~east–west sprawl of the External Hellenides, with extension also, to a lesser extent, in the other directions. Through identifying low-angle detachments by field and satellite morpho-structural analysis, we show that this spreading has been occurring since the Pliocene, mostly by reusing décollement layers of the Alpine nappes as extensional structures. We suggest that the main high-angle normal faults existing in the Peloponnese correspond to a localization of the extension in the weakest azimuth dictated by the Alpine backbone. We propose that this surface sprawl results not only from the Hellenic slab retreat but also from the exhumation of the deep Peloponnesian stacked units, and the subsequent crustal gravity collapse.