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muLtichanneL seismic study of the gondoLa fauLt system (adriatic sea) G. AielloCNR IAMC Sede di Napoli, Italy

Introduction. A seismic study of the Gondola fault system, located in the Southern Adriatic sea is herein presented based on multichannel seismic data (Zone D and Zone E) supplied by Italian Ministry of Industry, coupled with well lithostratigraphic data drilled in the same area and providing a good stratigraphic framework of the investigated area.

The Gondola fault system has been previously studied in several papers on the Adriatic area (De Dominicis and Mazzoldi, 1987; Colantoni et al., 1990; Aiello and de Alteriis, 1991; Aiello, 1992, 1993; de Alteriis and Aiello, 1993; Patacca and Scandone, 2004; Billi et al., 2007; Argnani et al., 2009; Di Bucci et al., 2009; Dalla Valle et al., 2015), showing its close relationships with fault systems located onshore in the Gargano Promontory (Billi et al., 2007). New insights have been recently furnished on the palaeo-seismological meaning of the Gondola fault system in recasting historical earthquakes in coastal areas of Gargano Promontory based on marine palaeo-seismology based on high resolution seismic data (Subbottom Chirp; Di Bucci et al., 2009; Fracassi et al., 2012). The historical earthquakes of the northern sector of the southern Adriatic area have been often related to the activity of faults located onshore, such as the Mattinata fault, E-W trending and having both regional meaning and extension. Earthquakes are suggested to occur in correspondence to the Gondola fault zone, representing the offshore prolongation of the Mattinata fault and probably triggering historical earthquakes of the area, such as the seismic event of 1893 (Di Bucci et al., 2009).

The Gondola fault system is herein studied based on a dense seismic network of multichannel profiles (Fig. 1) provided by the Italian Ministry of Industry to put it in a regional geological context of southern Apennines and Apulia-Gargano foreland of the chain (Mostardini and Merlini, 1986; Casero et al., 1988; Sella et al., 1988; Patacca and Scandone, 1989; Marsella et al., 1992; Ricchetti et al., 1992).

About 1500 km of multichannel seismic profiles have been interpreted pertaining to the Zone D and the Zone F. The Zone D survey has been acquired in the 1968 from the American company GSI, while the Zone F survey has been recorded in the 1976 from the French company CGG.

Main parameters of acquisition and processing are herein indicated and shown: Zone D; GSI survey; source: dynamite-airgun; acquisition frequency: 8-75 kHz; coverage:

600-2400%; processing: stack-deconvolution; Zone F: CGG survey; source: vaporchoc; acquisition frequency: 8-125 Hz; coverage:

4800%; processing: stack-deconvolution. The lithostratigraphic data and the electric logs of the following deep exploration wells have

been used, drilled in the Apulia offshore from AGIP and other geophysical companies: Gargano Est-Marine 1, Gondola 1bis, Cigno Mare 1, Jolly 1, Giuliana 1, Grazia 1, Grifone 1, Rovesti 1; Imago 1, Picchio 1, Rosaria Mare 1, Sabrina 1, Branzino 1, Simona 1, Cristina 1, Sonia 1, Stella 1, Famoso 1 and Eterno 1. Some data will be furnished in the section on results.

Geodynamical and geophysical setting. The whole geophysical data herein exposed (distribution of the Bouguer anomalies, thickness of the Pliocene cover in the Adriatic area, seismicity) have confirmed the occurrence to a regional scale of the Apulian block (the Apulian microplate), separating the system of Pliocene foredeeps of the Apennine, running from the Pescara offshore to the Bradanic foredeep and the Taranto Gulf from the Mio-Pliocenic foredeep, located in the southern Adriatic offshore of Puglia since the Jugoslavia and the Albania and linked to the Dinaric-Hellenic chain.

The gravimetric data interpretation has allowed to hypothesize for the Apulian area onshore average crustal thicknesses with a minimum of 25 km below the Gargano and a maximum of

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30-35 km below the Murge-Salento (Giese and Morelli, 1975). The seismicity of the Apulo-Adriatic region is mainly concentrated in the first 100 km of

lithosphere and is distributed in correspondence to the Apenninic and Dinaric chains. The study of focal mechanisms has allowed to distinguish a tectonic regime mainly extensional in the Apenninic chain from a mainly compressional one in the Dinaric-Hellenic chain (Anderson, 1987). The Adriatic-Ionian region constitutes then a relatively aseismic area, excluding epicenters of middle and shallow earthquakes offshore the Gargano and the Tremiti islands (Favali et al., 1990).

Fig. 1 – Location map of the seismic lines.

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This evidence has induced some geophysics to hypothesize the existence of an Adriatic microplate, or alternatively of a promontory of the African megaplate (African Promontory; Channell et al., 1979) located between the Neogene Apenninic and Dinaric-Hellenic thrust systems.

It must be specified that the alternative concepts of microplate and African Promontory (Channell et al., 1979) were born and were developed in the palaeogeographic and palaeotectonic setting of the Mediterranean area and then pertain to analytic methodologies and consequent interpretations, which not always are comparable among them. This is more true if the considered seismicity is concentrated in the first 50 km of lithosphere and then furnishes reliable information on the plate boundaries. Perhaps, when the authors refer to the present situation not always is clear how much it can be compared with that one of the pre-Neogene and pre-Cretaceous geologic past.

It is worth to precise that the foreland areas should not to be considered as the stable and undeformed part located in front of a chain. In fact, due to the load induced from the lithospheric shortening of the chain, foredeep basins tend to form coupled with lithospheric bulges (Cloetingh, 1988). The stress field induced by the formation of a thrust belt may propagate from the collisional zones since to several hundreds of kilometers from the foreland.

In conclusion it is worth recalling that for the area of Adria it has been observed based on gravimetric data a segmentation due to a sector evolution of the Apenninic chain, which has provoked a depocentral migration of the Plio-Quaternary foredeep (Royden et al., 1987). This segmentation may explain the strike-slip structures, seismically active in the central-southern Adriatic Sea (Tremiti and Mattinata faults) apart from the plate margins.

Mongelli and Ricchetti (1979) have considered the geodynamic roles of foreland and foredeep of the Apulian carbonate platform during the thrusting of southern Apennines. These authors have noted that the geodynamic role of the oceanic plates in the general schemes of global tectonics is developed by a continental microplate defined as the Apulian microplate. An analysis of the gravimetric and elastic characteristics of the Apulian microplate has been carried out in correspondence to the Bradanic foredeep and the Apulian foreland. The microplate curved with an elastic behavior, downthrowing in the Bradanic foredeep and uplifting in correspondence to the Murge, due to the loading produced from the accumulation of Neogene-Quaternary sediments and for contemporaneous tangential shifts referred to the Apenninic thrusting. A geological-geophysical profile has been constructed (Fig. 2), crossing the Bradanic foredeep, the Murge foreland and the Adriatic sea, in order to show the geological structure of foredeep-foreland system related to southern Apennines.

Fig. 2 – Geological - geophysical profile.

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The Po Plain and the Adriatic sea represents only one microplate separated by the African plate (Finetti et al., 1987). Based on geophysical constraints these authors have suggested that the Adriatic plate is completely separated from the African one. The first rifting phase may be happened during the Middle-Late Trias with the formation of sea-channels in correspondence to the Ionian sea, but the first geodynamic phase happened during the Middle Jurassic. Between the Late Mesozoic and the Early Paleogene the Adriatic plate margins started to join with the European plate. The compressional movements continued since the Tortonian. At the end of the Oligocene the extensional phase started, leading to the opening of the western Mediterranean, while on the Tyrrhenian margin of the Adriatic plate the Apenninic thrusting began. This deformation was explicated along strike-slip and normal faults in the Ligurian sector, in correspondence to the junction among the Alps and the Apennines, while along the Apenninic belt the subduction of the Adriatic margin started.

Results. Well lithostratigraphic data have been analyzed to establish the stratigraphy in correspondence to the Gondola fault system (Gondola 1bis well; Agip, 1970). The well has drilled, in correspondence to a palaeo-structural high, a Mesozoic carbonate succession, ranging in age from the Late Triassic and the Middle-Late Jurassic for an average thickness of 3.4 km. Upper Trias to Lias time interval is characterized by the occurrence of shallow water limestones, indicating the transition from an evaporitic area to a platform area. Shelf sedimentation continued up the Early Jurassic, with the deposition of dolomites. Middle-Late Jurassic was characterized by slope carbonate successions, indicating a relative deepening of depositional environments, with the establishment of slopes on the flanks of the carbonate palaeo-structural high of Gondola. Marly limestones unconformably overlie Late Jurassic slope limestones during the Early Cretaceous, indicating the deposition of Maiolica Formation, pertaining to the Adriatic Basin.

Fig. 3 – Seismic line D438.

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The geological interpretation of the D438 seismic line has been calibrated through the Gondola 1 well lithostratigraphic data (Fig. 3). The Gondola high is crossed by the interpreted seismic line in its western sector and appears as an asymmetrical horst, with the north-eastern flank bounded by a wide normal fault (Fig. 3). This fault puts in lateral contact Mesozoic-Tertiary deposits cropping out in the palaeo-structural high with Oligo-Miocene to Plio-Quaternary siliciclastic deposits, pertaining to the Southern Adriatic foredeep (Fig. 3). The correlation with field geological data suggests that this fault represents the offshore elongation of the Mattinata strike-slip fault, cropping out in the Gargano Promontory. ReferencesAiello G. (1992) Analisi sismostratigrafica del margine apulo nell’offshore delle Murge settentrionali. Giornale di

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Geology, University of Naples “Federico II”, 230 pp.Aiello G., de Alteriis G. (1991) Il margine adriatico della Puglia: fisiografia ed evoluzione terziaria. Mem. Soc. Geol.

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Patacca E., Scandone P. (2004) The 1627 Gargano earthquake (Southern Italy): identification and characterization of the causative fault. Journal of Seismology, 8 (2), 259-273.

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