Evoluzione pleistocenica del margine tirrenico dell’Italia centrale tra eustatismo, vulcanismo e tettonica

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Collocazione:
Il Quaternario Italian Journal of Quaternary Sciences, 17(2/2), 2004, 523-536
Autore/i:
Donatella DE RITA, Marina FABBRI & Corrado CIMARELLI
Abstract:

Viene presentata una sintesi dei dati ottenuti durante la realizzazione dei fogli geologici alla scala 1:50.000 riguardanti alcune areecostiere del Lazio. La possibilità di porre a confronto l’evoluzione di settori che presentano problematiche simili ma con situazionilocali specifiche ha permesso di evidenziare, a partire da circa 850 ka, una storia comune per la costa del Lazio settentrionale e centrale,diversa da quella che ha invece interessato nello stesso intervallo di tempo la costa del Lazio meridionale. La ricostruzione deglieventi a scala regionale ha permesso di distinguere gli effetti di una tettonica regionale (soprattutto di sollevamento isostatico lungo lacosta del Lazio settentrionale e centrale e di subsidenza per quello meridionale) rispetto a movimenti locali che potrebbero essereessenzialmente legati alle fasi vulcaniche che hanno contemporaneamente interessato il margine tirrenico del Lazio. Vengono inoltreevidenziate alcune problematiche che riguardano l’organizzazione cartografica nazionale ed in particolare la denominazione dei sintemia scala regionale e la necessità di introdurre una nomenclatura in grado di associare nel suo interno più sintemi non distinguibili sulterreno.

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 ofItaly, concerning some coastal areas of Latium. The possibility to compare the evolution of areas envolved in the same geodinamic contestbut having different local structural framework, has allowed us to evidence a common history, starting from 850 ka, for the coast ofnorthern and central Latium , different from that of the southern coast. The effect of local estensional tectonic related to the volcanismwhich interested central Italy in that period was distinguished. The units have been organized in Unconformity Bounded StratigraphicUnits (UBSU) on the base of the hierarchy order of the basal surfaces delimithing them. Synthemes have been limited by surfaces recognizedat regional scale caused by oscillation of the sea level. Each surface has been dated on the base of the geochronological dataand /or the faunistic content of the above sediments and then it has been correlated to an isotopic stage. The vulcanic units have beenorganized following the indication of Fisher and Schmincke (1984), by introducing the Eruption units. “An eruption unit is a deposit definedas a thickness of volcanic material deposited from an eruptive pulse, an eruptive phase or an eruption…it is a conceptual entity thatrelates volcanic activity and a rock stratigraphic unit”. This method allows to consider all the unconformities recognizable in volcanicareas which cannot be extended at regional scale but are important to reconstruct the main phases of the volcano evolution. It allowseven to consider the volcanic evolution in its regional contest. The volcanic deposits originated by the reworking of the primary unitshave been distinguished in syn-eruption and inter-eruption units (Smith, 1991; De Rita et alii, 2002 b; Giordano et alii, 2002 a). The geologicaland structural evolution of the island of Ponza, Zannone and Palmarola in the Pontine Archipelago, 30 km south east from thecentral coast of Latium, has been reconstructed. These three islands are the remnants of a submarine dome complex developed fromthe Upper Pliocene to almost 1.1 Ma. Three main ryolithic domes have been recognized in Ponza, developed along NE-trending lineamentsand showing different hyaloclastic facies reflecting the different geological contest in which they developed. The norhernmostPiana d’Incenso dome shows a prevailing hyaloclastic facies made of clast supported breccias and by flow banded lavas produced byits intrusive mode of emplacement, as a cryptodome. Similar facies characterize the Zannone dome in the Zannone island, where theintrusive nature is demonstrated by the structural relationship between the flow-banded lavas and the sedimentary units: these are tiltedand 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 Mesozoic-Cenozoic brittle rocks. The other domes of Ponza and Palmarola show different hyaloclastic facies, mainly made of thick hyaloclastitesshowing different degree of brecciation reflecting the different degree of magma-water interaction. We suggest that the extrusive modeof emplacement of these domes is because their emplacement was along regional extensional faults controlling the structural evolutionof the continental platform on which the volcanic dome complex developed. In the southern part of Ponza a subaerial trachytic domedeveloped almost 1.1 Ma, after that the rhyolitic volcanism ended. The geological gap between the rhyolitic and the trachytic phases ofvolcanism, is evidenced by a high relief erosive surface. On the three island, in the range between 80 and 110 m a.s.l., marine terracesare present. These terraces have been correlated on the base of their similar morphology and because the above sediments are lithologicallysimilar. They have to be formed after the end of the volcanism in the three islands. The uplifting of the surface occurred probablyduring a contemporaneous relative low standing of the sea level. We suggest to relate this event with the uplifting of the central coast ofLatium, that occurred between 0.9 and 0.8 Ma, during the isotopic stage 22, when the alkali-potasssic volcanism started. The differenceof the altitude at which the surface presently lies, is probably due to the effect of local extensional tectonic. Along the northern and thecentral coast of Latium, we have recognized three orders of terraces which coincide in the altitude and the age. In the northern coast thepresence of a small basin infilled by volcaniclastic sediments, was helpful in correlating stratigraphic succession of the Vulsini, Sabatiniand Vico volcanic districts to the coastal area successions. Here, the UBSU have been organized in 5 Synthemes whose basal surfaceswere produced during low standing of the sea level. On the base of geochronological data and /or the faunistic content of the abovesediments, the last three synthems have been ascribed to the 9,7 and 5 isotopic stages respectively (De Rita et alii 2002 b). In the centralcoast of Italy, in the area of Roma city, the UBSU have been organized in 7 synthemes (Giordano et alii 2003), the last three of whichhave the same altitude and age of those recognized along the northern coast. Along the southern coast no terraces can be recognized. Ithas 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 thevolcanism 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 associationof synthemes. In fact, in some cases it has not been possible to recognize all the discontinuities related to sea level oscillations, precludingthe possibility to distinguish single synthemes that have been unified together. They do not represent a supersynthem becauseunconformities of major hierachic order were not recognized.

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