Neotectonics and Recent Paleogeography

Abstract

The Northern Hellenides are characterized by slow convergence between the Apulian platform and the European margin with a rate of 8mm/year whereas the Southern Hellenides are characterized by a rapid oceanic subduction with a rate of 40–50mm/year. The Cephalonia right-lateral strike–slip fault forms a tear of the crust with 100km of horizontal slip. The extensional detachments are characterized by the structural omission of tectonic units or stratigraphic horizons between their footwall and hanging wall. Their presence in the Hellenic arc and the Aegean usually follows the Tertiary outcrops of the tectonic windows, especially in the metamorphic core complexes, where they separate the overlying non metamorphic units and syntectonic sediments from the underlying metamorphic and magmatic rocks. Two almost symmetric upper mantle convection cells dominate in the whole Mediterranean Basin: the downwellings are found in the center of the Mediterranean with opposite direction and are associated with the descent of the Tyrrhenian and Hellenic slabs. A return flow of the asthenosphere from the back arc regions is created toward the subduction zones, expressed in two upwellings beneath Anatolia and Eastern Iberia. The present geotectonic regime in the Hellenic arc and trench system is characterized by an asymmetry of motion, caused by the orthogonal geometry of the arc, with normal convergence at the Ionian Sea and sub-parallel strike slip motion at the Pliny and Strabo trenches. Numerous tilt motions of E-W fault blocks can be observed at a large scale, which systematically show a southward rotation, leading to the creation of tectonic dipoles. The northern boundary of the Aegean plate in the North Aegean is not evident in the crustal structure, since there is no difference in the crustal thickness. Major neotectonic faults represent marginal faults of the post-Alpine sedimentary basins with high seismicity. Three major neotectonic segments can be distinguished in the region of the Hellenic arc with different seismotectonic characteristics. Folds “in-the-making” are observed, in the marine Lower-Middle Pleistocene formations of Western Cephalonia. Fault plane solutions indicate the dominance of compressive structures at the front of the Hellenic arc and of extensional structures at the back arc area, with minor distinct strike slip zones. Paleo-seismological research has provided excellent results regarding the recurrence rate of the seismic activity of the active faults during the Holocene, with time periods ranging from a few hundred years to a few thousand years, thus greatly surpassing the uppermost limit of the historic seismicity recording in Greece. The volcanic outcrops of the modern Aegean volcanic arc are much larger, when the newly discovered submarine volcanic formations are also taken into account. The interplay between the eustatic sea-level fluctuations and the tectonic activity define both the geomorphic features such as notches and marine terraces as well as the depositional history. The extensional collapse of the Aegean region during Quaternary destroyed the previously continuous continental area of Aegeis and lateral migrations of mammals from Minor Asia to continental Greece gradually became impossible. Eustatic movement of the Holocene since the end of the last glacial period isolated many islands of the present geography of Greece and favored the development of specific adjustments to the plants and the animals of the region, resulting in the endemic phenomenon of the area. Sea-level changes may result in dramatic reduction of the islands during the high stands with immediate impact on their biodiversity. Recently discovered fossils of Homo Sapiens at the southernmost peninsula of Mani in continental Greece in the Apidima Cave were dated 210 Ka, the earliest found in Eurasia. Philosophical currents and science itself were developed in the Aegean region, through the efforts of man to understand and pursue the harmony of nature, and finally, the reasoning of its mechanisms. It is in this charismatic area that geomythology has been created by the Ancient Greeks.