www.nature.com/scientificreports OPEN The Crotone Megalandslide, southern Italy: Architecture, timing and tectonic control Received: 2 March 2018 Massimo Zecchin1, Flavio Accaino1, Silvia Ceramicola1, Dario Civile1, Salvatore Critelli2, Accepted: 1 May 2018 Cristina Da Lio3, Giacomo Mangano1, Giacomo Prosser4, Pietro Teatini3,5 & Luigi Tosi3 Published: xx xx xxxx Large-scale submarine gravitational land movements involving even more than 1,000 m thick sedimentary successions are known as megalandslides. We prove the existence of large-scale gravitational phenomena of the Crotone Basin, a forearc basin located on the Ionian side of Calabria (southern Italy), by seismic, morpho-bathymetric and well data. Our study reveals that the Crotone Megalandslide started moving between Late Zanclean and Early Piacenzian and was triggered by a contractional tectonic event leading to the basin inversion. Seaward gliding of the megalandslide continued until roughly Late Gelasian, and then resumed since Middle Pleistocene with a modest rate. Interestingly, the onshore part of the basin does not show a gravity-driven deformation comparable to that observed in the marine area, and this peculiar evidence allows some speculations on the origin of the megalandslide. Kilometer- to tens of km-scale submarine gravitational collapses, here referred to as megalandslides, consist of land movements involving up to 1 km (or even more) thick sedimentary successions sliding on a basal surface that may classify as ‘basal overpressured shale detachment’ or ‘salt detachment’1. Gravity gliding may be instanta- neous, associated with mass wasting and shallow detachment, or slow, in connection with a deep detachment and long-term geological processes such as high sedimentation rates or uplif in adjacent areas1. Tese large-scale phe- nomena typically produce an updip extensional domain and a downdip contractional domain, which are linked via a basal detachment surface2,3. Te interest for megalandslides has involved also the hydrocarbon industry, as they may be associated with structural traps consisting of large anticlines1. Several examples were reported worldwide, mostly but not exclusively on passive margins, such as in the Bight Basin, Australia4, Niger Delta5–7, West African margins8–10, Gulf of Mexico11–13, Brazil14, Antarctic Peninsula15, and Brunei16. European examples (see Canals, et al.15 and references therein) include the Storegga, Trænadjupet and Finneidford slides (Norwegian margin), BIG’95 Slide (of the Ebro river, Spain), Central Adriatic Deformation Belt (Adriatic margin, Italy), and Afen Slide (Faeroe-Shetland Channel). Te existence of a previously unknown, ca. 1,000 km2 megalandslide in the Neogene Crotone Basin, southern Italy (Fig. 1A,B), was recently suggested by Minelli, et al.17 on the basis of seismic, well and GPS data and litera- ture information. Te Crotone Basin is interpreted as a forearc basin located on the Ionian side of the Calabrian Arc (Southern Italy) (Fig. 1A); it is partly exposed in the Crotone area (Fig. 1B,C) and it is also widely doc- umented ofshore. Te Calabrian Arc is interpreted as a composite terrane that migrated to the SE since the Middle Miocene in response to the subduction of the Ionian oceanic crust18–22. Migration was facilitated by the formation of major NW-trending shear zones, which are particularly frequent in the NE sector of the Calabrian arc and represent the NE and the SW boundaries of the Crotone Basin23–25 (Fig. 1A,B). Te sedimentary infll of the basin consists of Serravallian to Middle Pleistocene marine, coastal and continental deposits, which record main tectonic events and glacio-eustasy26–31. Since the Middle Pleistocene, the basin has undergone rapid uplif with an average rate of ca. 1 mm/yr32. 1(OGS) Istituto Nazionale di Oceanografa e di Geofsica Sperimentale, Borgo Grotta Gigante, 42/c, 34010, Sgonico, Trieste, Italy. 2Dipartimento di Biologia, Ecologia e Scienze della Terra, Università della Calabria, 87036, Arcavacata di Rende, CS, Italy. 3Institute of Marine Sciences, National Research Council, Arsenale - Tesa 104, Castello 2737/F, 30122, Venezia, Italy. 4Dipartimento di Scienze, Università della Basilicata, Potenza, Italy. 5Department of Civil, Environmental and Architectural Engineering, University of Padua, via Marzolo 9, 35121, Padova, PD, Italy. Correspondence and requests for materials should be addressed to M.Z. (email: [email protected]) or L.T. (email: [email protected]) SCIENTIFIC REPORTS | (2018) 8:7778 | DOI:10.1038/s41598-018-26266-y 1 www.nature.com/scientificreports/ Figure 1. (A) Structural map of the Calabrian Arc, which is comprised between the southern Apennines and Sicily (modifed from Van Dijk & Okkes24). Te main structural elements and the Ionian and Tyrrhenian basins are highlighted. (B) DTM map showing the onshore and ofshore parts of the Crotone Basin (see Fig. 1A for location). Te selected seismic profles and wells are shown. Te illustrated faults are reported from various authors27,30,31,38. Abbreviations: FU and MS – Fosso Umbro and Marcedusa-Steccato faults. (C) Tree- dimensional view of the Crotone basin highlighting the Crotone Swell, the extent of the recognized scar (surface SC, see text), and the thrust front showing SE-ward vergence (v. ex. 7×). In (B) and (C), bathymetric data were acquired by OGS in the frame of the MaGIC (Marine Geohazards along the Italian Coasts) project; public DTM data are available from https://www2.jpl.nasa.gov/srtm/. Te interpretation of Minelli, et al.17 indicates that most part of the Messinian to Plio-Pleistocene succession of the Crotone Basin (both onshore and ofshore) is gliding toward the Ionian Sea above a Messinian halite layer. Evidence of large-scale gravity tectonics afecting Pliocene strata was already reported in previous studies33. Te megalandslide would be characterized by a downdip contractional domain located ofshore and corresponding to the prominent Crotone Swell (Fig. 1B,C), and an onshore updip domain, represented by some seaward-dipping normal faults found in the northern part of the basin17 (Fig. 1B). However, many uncertainties remain about the origin, timing, extent and existence itself of the large-scale gravitational phenomenon involving the Crotone Basin. Tis generalized uncertainty is highlighted by the dif- ferent interpretation provided by other authors about the nature of the Crotone Swell34, and by the modest defor- mation and overall good preservation of the Messinian to Plio-Pleistocene succession in the onshore part of the basin, which even contains stratotypes27,28. Much more evidence is therefore necessary to address this question. SCIENTIFIC REPORTS | (2018) 8:7778 | DOI:10.1038/s41598-018-26266-y 2 www.nature.com/scientificreports/ Figure 2. (A,B,C) Interpreted seismic profles that cross-cut the Crotone Swell (Fig. 1B for location), interpreted to as a megalandslide. SW-verging tectonic structures (in black) are truncated by SE-verging, inferred gravity-driven structures (in red), which are dominated by a basal detachment surface and a thrust front that reaches the seafoor. Te gravity-driven phenomenon involves Late Miocene and Plio-Pleistocene deposits, which in seismic profles correspond to seismic units (Units MES and PL1-3) separated by unconformities. Te ages of the seismic units are derived from the available wells (Fig. 5). A minor landslide of inferred mid-Pliocene age is nested in the megalandslide and is bounded at the base by a scar (surface SC). Te timing of the gravity-driven process (see areas with diferent color) is discussed in the text. Seismic data from “Visibility of Petroleum Exploration Data in Italy” (ViDEPI Project) (http://unmig.sviluppoeconomico.gov.it/ videpi/). Uninterpreted seismic profles are available in Supplementary Material Fig. S1. Moreover, the existence of a still active large-scale gravitational collapse involving the onshore part of the basin would have social impacts, as it would raise security concerns about the population of this area, where the city of Crotone (64.000 inhabitants) is located. Te present study is aimed at resolving the existence, areal extent and timing of the Crotone Megalandslide. Tis has been achieved by integrating on- and ofshore datasets acquired across the Crotone Swell and in the onshore part of the basin, including seismic, morpho-bathymetric, sediment cores, outcrops and land movements. Results The ofshore area. South of the Crotone peninsula, the Crotone Swell, a morphological high 16 km long and 30 km wide, is characterized by a prominent lobate morphology with an undulated longitudinal profle (Fig. 1B,C). Te high includes a portion of continental shelf from 5 to 10 km wide. Te continental slope is sub- divided in an upper slope characterized by a fat, NE-SW elongated intraslope basin (at ca. 600 m water depth), and a rather steep (up to 11°) lower slope from 750 m to 1350 m water depth, which is incised by several single and multiple scars up to 5 km wide showing ‘fresh’ morphologies. Te Crotone Swell is bounded by two well incised and elongated canyon systems: the Botricello, a quite short (40 km) NW-trending system and the Esaro, a NS-trending asymmetric system developing along the eastern side of the morphological high (Fig. 1B,C). Te Crotone Swell exhibits a complicated structure, consisting of an ofshore (SE-ward) verging thrust, locally passing updip, via a seaward-dipping detachment surface inclined ca. 3° to 4°, into SE-ward-dipping exten- sional faults (Figs 1B,C, 2 and 3A). Orientation of tectonic