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Geomorfologia strutturale e domini di frattura dei fondali marini pericostieri dell’Isola di Procida (Campi Flegrei insulari, Italia meridionale)

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Collocazione:
Il Quaternario Italian Journal of Quaternary Sciences, 23(2), 229-242
Autore/i:
Maria Luisa PUTIGNANO & Marcello SCHIATTARELLA
Abstract:

I fondali marini che orlano l’Isola di Procida nel Golfo di Napoli sono stati oggetto di uno studio morfostrutturale basato sull’investigazione diretta, fino alla profondità di circa 30 m, e sulla interpretazione di profili sismici ed ecografici che hanno portato alla redazione di una carta geomorfologica. Sono stati in tal modo cartografati depositi, superfici erosionali e morfolineamenti di origine tettonica che hanno contribuito a definire il quadro morfoevolutivo della fascia pericostiera procidana. La buona corrispondenza tra il sistema di frattura rilevato sull’isola e ai Campi Flegrei con il pattern dei morfolineamenti sottomarini induce a ritenere non dissimile la storia strutturale tardo-quaternaria di queste aree. In aggiunta a ciò, la definizione di diversi paleolivelli di base dell’erosione consente di dettagliare meglio l’interazione tra oscillazione eustatica e sollevamento o subsidenza indotti da vulcanesimo e tettonica. In particolare, l’attribuzione cronologica dei prodotti vulcanici recenti e le loro relazioni morfostratigrafiche con le superfici di abrasione sommerse hanno permesso di discriminare le tappe morfoevolutive dell’area di studio.

Sea floors surrounding Procida Island in the Bay of Naples have been studied from a morpho-structural point of view. Such a study has been based on both direct survey of the submarine environment (up to the depth of 30 m) and seismic lines and echographic data interpretation, and allowed us to realize an accurate geomorphological map of the pericoastal areas. Marine recent deposits, tectonic morpho-lineaments, submarine erosional terraces, submerged notches and other submerged sea level markers have been mapped, so contributing to define the Holocene morpho-evolutionary picture of the Phlegraean offshore. The good fit between the submarine fracture system and the inland fault and fracture pattern suggests that the investigated coastal marine areas underwent the same late- Quaternary structural history of the nearby continental region. In addition, the recognition of several base-palaeolevels of the erosion allows to better define the interplay among sea-level changes, tectonics, and volcanogenic vertical movements. Archaeological data, ages of the volcanic products, their morpho-stratigraphic relationships with the submarine erosional terraces and other geomorphological constraints permit now a suitable chronological attribution to the different orders of land surfaces, making more reliable the calculation of the subsidence rates and helping the comprehension of the morpho-evolutionary trend of the study area. The planimetric geometry of the strain field has been achieved by both submarine survey and morphostructural analysis: it clearly shows en echelon style, consisting of three dimensionally similar fault-bounded sectors (Canale di Procida, Isola di Procida p.p. and Promontorio di Solchiaro - Isola di Vivara - Canale di Ischia). From a physiographic viewpoint, such sectors result separated by E-O to N120°-striking faults producing an apparent dextral offset. It is likely that they may represent transfer structures due to extensional deformation by NNE-SSO-striking master faults and, therefore, featured by a “passive” left-lateral slip. The fault set displacing the Neapolitan Yellow Tuff in the Capo Posillipo area and absent in the Procida Island area is N50-60° oriented. Such a trend also characterizes joint sets affecting submarine outcrops associated to hydrothermal phenomena. Then, one can conclude that this trend represent very recent structures, cut only by erosional surfaces located at a depth of 14/-15 m and attributed to the Middle Bronze Age. The subsidence rate calculated for five different orders of submarine erosional surfaces is apparently constant, showing always values around 4-5 mm/y and so suggesting a regular evolution trough the time of the studied pericoastal system, characterized by the submersion of portions of coastal sectors by continuous and invariable volcano-tectonic subsidence. Yet, using the Post-Holocene sealevel rising curve to set the eustatic correction, it is possible to obtain a more articulated picture of the morpho-evolutive sequence, characterized by alternating periods of different subsidence rates. This scenario seems to be more realistic in an area of complex interplay between tectonics and sea-level changes such as the Phlegraean Fields.

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