EVOLUZIONE PLEISTOCENICA DEL MARGINE TIRRENICO DELL’ITALIA CENTRALE TRA EUSTATISMO, VULCANISMO E TETTONICA

Abstract

De Rita D. et al., The evolution of the Tyrrhenian margin of Central Italy: the interplay between volcanism, eustatism and
tectonciv events). (IT ISSN 0394-3356, 2004).
We present a synthesis of the data collected during the field work devoted to the realization of the geological map at the 1:50.000 of
Italy, concerning some coastal areas of Latium. The possibility to compare the evolution of areas envolved in the same geodinamic contest but having different local structural framework, has allowed us to evidence a common history, starting from 850 ka, for the coast of northern and central Latium , different from that of the southern coast. The effect of local estensional tectonic related to the volcanism which interested central Italy in that period was distinguished. The units have been organized in Unconformity Bounded Stratigraphic Units (UBSU) on the base of the hierarchy order of the basal surfaces delimithing them. Synthemes have been limited by surfaces recognized at regional scale caused by oscillation of the sea level. Each surface has been dated on the base of the geochronological data and /or the faunistic content of the above sediments and then it has been correlated to an isotopic stage. The vulcanic units have been organized following the indication of Fisher and Schmincke (1984), by introducing the Eruption units. “An eruption unit is a deposit defined as a thickness of volcanic material deposited from an eruptive pulse, an eruptive phase or an eruption…it is a conceptual entity that relates volcanic activity and a rock stratigraphic unit”. This method allows to consider all the unconformities recognizable in volcanic areas which cannot be extended at regional scale but are important to reconstruct the main phases of the volcano evolution. It allows even to consider the volcanic evolution in its regional contest. The volcanic deposits originated by the reworking of the primary units have been distinguished in syn-eruption and inter-eruption units (Smith, 1991; De Rita et alii, 2002 b; Giordano et alii, 2002 a). The geological and structural evolution of the island of Ponza, Zannone and Palmarola in the Pontine Archipelago, 30 km south east from the central coast of Latium, has been reconstructed. These three islands are the remnants of a submarine dome complex developed from the Upper Pliocene to almost 1.1 Ma. Three main ryolithic domes have been recognized in Ponza, developed along NE-trending lineaments and showing different hyaloclastic facies reflecting the different geological contest in which they developed. The norhernmost Piana d’Incenso dome shows a prevailing hyaloclastic facies made of clast supported breccias and by flow banded lavas produced by its intrusive mode of emplacement, as a cryptodome. Similar facies characterize the Zannone dome in the Zannone island, where the intrusive nature is demonstrated by the structural relationship between the flow-banded lavas and the sedimentary units: these are tilted and deformed along low angle normal faults at the contact with the vulcanites. We have interpreted that the domes emplaced as cryptodome, because they were intruded almost at the center of a structural high of the sedimentary substratum made of the MesozoicCenozoic brittle rocks. The other domes of Ponza and Palmarola show different hyaloclastic facies, mainly made of thick hyaloclastites showing different degree of brecciation reflecting the different degree of magma-water interaction. We suggest that the extrusive mode of emplacement of these domes is because their emplacement was along regional extensional faults controlling the structural evolution of the continental platform on which the volcanic dome complex developed. In the southern part of Ponza a subaerial trachytic dome developed almost 1.1 Ma, after that the rhyolitic volcanism ended. The geological gap between the rhyolitic and the trachytic phases of volcanism, is evidenced by a high relief erosive surface. On the three island, in the range between 80 and 110 m a.s.l., marine terraces are present. These terraces have been correlated on the base of their similar morphology and because the above sediments are lithologically similar. They have to be formed after the end of the volcanism in the three islands. The uplifting of the surface occurred probably during a contemporaneous relative low standing of the sea level. We suggest to relate this event with the uplifting of the central coast of Latium, that occurred between 0.9 and 0.8 Ma, during the isotopic stage 22, when the alkali-potasssic volcanism started. The difference of the altitude at which the surface presently lies, is probably due to the effect of local extensional tectonic. Along the northern and the central coast of Latium, we have recognized three orders of terraces which coincide in the altitude and the age. In the northern coast the presence of a small basin infilled by volcaniclastic sediments, was helpful in correlating stratigraphic succession of the Vulsini, Sabatini and Vico volcanic districts to the coastal area successions. Here, the UBSU have been organized in 5 Synthemes whose basal surfaces were produced during low standing of the sea level. On the base of geochronological data and /or the faunistic content of the above sediments, the last three synthems have been ascribed to the 9,7 and 5 isotopic stages respectively (De Rita et alii 2002 b). In the central coast of Italy, in the area of Roma city, the UBSU have been organized in 7 synthemes (Giordano et alii 2003), the last three of which have the same altitude and age of those recognized along the northern coast. Along the southern coast no terraces can be recognized. It has been then possible to establish that the coast of central Italy was subjected to a regional uplift evaluated in the order of 0,2 mm/y, whereas the southern coast was subjected to a continuous subsidence active from Pliocene. Local extensional tectonic related to the volcanism has been recognized in the roman area. Some problems related to the organization of the national cartography are evidenced.
They concern the nomenclature of synthem at regional scale and the opportunity to introduce a specific hierarchy to indicate an association of synthemes. In fact, in some cases it has not been possible to recognize all the discontinuities related to sea level oscillations, precluding the possibility to distinguish single synthemes that have been unified together. They do not represent a supersynthem because unconformities of major hierachic order were not recognized