Large deep-seated slump structure off Ischia volcanic island,Eastern Tyrrhenian sea (Italy)

Ischia island is located over the Campania sector of EasternTyrrhenian margin and represents the sub-aerial section of alarger, E-W trending volcanic ridge including others submergedor buried volcanic edifices. The island itself result from thecoalescence of a multitude of small to medium scale eruptionsleading to the emplacement of domes, lava flow and pyroclasticdeposits and ignimbrites (VEZZOLI et al., 1988) ranging fromalkali basalts to trachytes. The oldest basement dates back to 150ky and crops out along the perimeter of the island especially tothe south. Latest eruption occurred in 1302 A.D. and togetherwith strong hydrothermal activity, ground uplift and seismicshaking indicates the presence of a still active magmatic reservoirat depth. Most recent (Holocene) magmatic activity with localvolcanic eruptions has clustered in the eastern island’s sector thewhile central sector is dominated by the Mt. Epomeo, consistingof an ignimbritic tuff (Green tuff Auct.) uplifted of 600-700 m inthe past 33ka.In the past decade the island’s offshore has been the object ofextensive hydrographic and marine geophysical surveys that haveshown the structural complexity of the undersea sections andhave overall shown the importance of gravity failures in island’sevolution. In particular a 1.5-3 km3 debris avalanche due to asubaerial and/or submarine flank collapse was emplaced alongthe steep and unbuttressed island’s flank during pre-historical oreven historical times (CHIOCCI & DE ALTERIIS, 2006; de Alteriiset al., 2010) whereas three other similar deposits of comparablevolumes were found over the continental shelf to the west and tothe north (VIOLANTE et al, 2004; DE ALTERIIS & VIOLANTE,2009).Here we report a previously unrecognized deep-seated slumpstructure and associated surficial mass wasting phenomena whichoccur off Ischia south-western flank. Recently acquiredhydrological and geophysical data lead to identify themorphological features and the internal organization of the failedsediments which spread along the continental slope. The extentof this deep-seated deformations and the deep structural levelsinvolved lead to investigate on the influence played by volcanicprocesses on slope failure.DATA AND METHODSOur dataset was acquired during the geophysical cruisePECOS 2010 carried out on R/V Urania (Consiglio Nazionaledelle Ricerche, CNR, Italy) between December 22th 2010 andJanuary 2nd 2011 in the frame of a project leaded by Istituto perl’Ambiente Marino Costiero, (IAMC-CNR), Naples-Italy withthe collaboration of Dipartimento di Scienze della Terra e delMare (Palermo University), Palermo-Italy regarding coastal andoffshore slope instability in the Bay of Napoli.The Ischia southern slope was explored through a multibeamsurvey and a single-channel seismic survey. Acquisition wascarried out between 400 and 1200 m. The bathymetric data werecollected using a hull mounted Reson 8160 multibeam sonar.Resolution resulted in a 20x20 m implemented with 50x50gridded size provided by a previously collected data. The seismicsurvey consisted of 6 dip-lines NNE-SSW run along the slopeand 5 cross lines parallel to the slope totalling 170 km. Averagespacing between diplines was slightly less than 1 km whilespacing between crosslines was variable from 1.2 to 2.5 km.NNE-SSW and WNW-ESE directions. The acoustic source usedwas a 1Kjoule high-energy power supply system with a multitips(400) sparker array, fired at 2s time interval.RESULTSThe collected data show that a wide submerged area of 350km2, between 400 to 1200 m depths is undergoing slow-movingdeformation and