Plio-Quaternary tectonic evolution of the Northern Apennines thrust fronts. along the Bologna-Ferrara section (Po Plain, Italy), based on geological

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1 Plio-Quaternary tectonic evolution of the Northern Apennines thrust fronts along the Bologna-Ferrara section (Po Plain, Italy), based on geological observations and analogue modelling: seismotectonic implications G. Toscani 1, P. Burrato 2, D. Di Bucci 3, S. Seno 1, G. Valensise Dipartimento di Scienze della Terra, Università di Pavia. Via Ferrata, Pavia, Italy Istituto Nazionale di Geofisica e Vulcanologia, Via di Vigna Murata, Roma, Italy Dipartimento della Protezione Civile. Via Vitorchiano, Roma, Italy Key words Fold-and-thrust belt, active tectonics, earthquake, Po Plain. Abstract The outermost thrust fronts of the Northern Apennines fold-and-thrust belt (Italy) are buried under a thick clastic cover that fills the Po Plain basin (Fig. 1), and have been studied by means of seismic sections and deep well logs acquired for oil exploration purposes (e.g. PIERI & GROPPI,1981). These data show a system of NE-verging blind thrusts and folds that controlled the deposition of very thick syntectonic sedimentary wedges, with the Plio-Quaternary sequence locally up to 7-8 km thick (e.g. BIGI et alii, 1990). The fast sedimentation (BARTOLINI et alii, 1996) hid the growing structures, and as a consequence there are few direct surface evidences of the possible ongoing activity of the thrusts. However, evidence for ongoing although weak tectonic activity is provided by seismicity and drainage anomalies, the latter represented by river diversion and channel pattern changes controlled by the growth of the buried anticlines 1

2 (CASTIGLIONI & PELLEGRINI, 2001; BURRATO et alii, 2003). The historical and instrumental Italian seismic catalogues show that the southern Po Plain is affected by low to moderate seismicity (CPTI WORKING GROUP, 2004; CASTELLO et alii, 2006), characterised by contactional focal mechanisms (PONDRELLI et alii, 2006). The borehole breakouts and the focal mecanisms both show Shmax oriented perpendicular to the trend of the buried thrust fronts (MONTONE et alii, 2004). GPS data suggest a weak SW-NE shortening at a rate of less than 1 mm/a (SERPELLONI et alii, 2005). In this framework, the goal of this paper is to study how the deformation is partitioned among the different Northern Apennines thrust fronts along the Bologna-Ferrara section (Fig. 1), and which of the active thrusts can be the source of damaging earthquakes. To do this, we carried out an integrated analysis of geological, structural (from the interpretation of deep well logs and reflection seismic lines) and morphotectonic data, and realised a N-S striking section at regional scale (Fig. 2). In order to highlight the activity of the buried thrust ramp anticlines, we: 1- analysed the morphotectonic setting of the study area, comparing the location of drainage anomalies with that of the buried structures, and 2- plotted seismicity on the section to compare its distribution (location, magnitude, depth, focal mechanisms) with the reconstructed structural setting and the tectonic deformation of the Quaternary deposits. We also developed and analyzed a set of sandbox models, which reproduce the Plio- Quaternary deformation along the same transect (Fig. 3), in order to investigate the thrust activity through time and its effects on the sedimentation. Finally, we propose a refined reconstruction of the Plio-Quaternary tectonic evolution of the Northern 2

3 Apennines thrust fronts along the Bologna-Ferrara transect, obtained integrating the different results. Acknowledgments. Seismic lines are courtesy of Eni S.p.A. References BARTOLINI C., CAPUTO, R. & PIERI, M. (1996) Pliocene-Quaternary sedimentation in the Northern Apennine Foredeep and related denudation. Geological Magazine 133/3, BIGI G., COSENTINO D., PAROTTO M., SARTORI R. & SCANDONE P. (1990) Structural model of Italy and gravity map (1: ). CNR, Prog. Final. Geodinamica, Quad. Ric. Sci., 114 (3), 6 sheets, S.EL.CA., Firenze. BOCCALETTI M. & MARTELLI L. (EDS.) (2004) Carta sismotettonica della Regione Emilia-Romagna. Scala 1: RER, Servizio Geologico, Sismico e dei Suoli. BURRATO P., CIUCCI, F. & VALENSISE G. (2003) An inventory of river anomalies in the Po Plain, Northern Italy: evidences for active blind thrust faulting. Ann. Geophys., 46/5, CASTELLO B., SELVAGGI, G., CHIARABBA, C. & AMATO, A. (2006) CSI Catalogo della sismicità italiana , versione 1.1. INGV-CNT, Roma CASTIGLIONI G.B. & PELLEGRINI G.B. (EDS.) (2001) Note illustrative della Carta Geomorfologica della Pianura Padanai. Geogr. Fis. Dinam. Quat., Suppl. to IV, pp

4 CPTI WORKING GROUP (2004) Catalogo Parametrico dei Terremoti Italiani, version 2004 (CPTI04). INGV, Milan, available from DISS WORKING GROUP (2007) Database of Individual Seismogenic Sources (DISS), Version 3.0.3: A compilation of potential sources for earthquakes larger than M 5.5 in Italy and surrounding areas. INGV 2005, MONTONE P., MARIUCCI M.T., PONDRELLI S. & AMATO A. (2004) An improved stress map for Italy and surrounding regions (central Mediterranean). J. Geophys. Res., 109, doi: /2003JB002703, PIERI M. & GROPPI, G. (1981) Subsurface geological structure of the Po plain (Italy). Pubbl. PFG-CNR, 414, PONDRELLI S., SALIMBENI S., EKSTRÖM G., MORELLI A., GASPERINI P. & VANNUCCI G. (2006) The Italian CMT dataset from 1977 to the present. Phys. Earth Planet. Int., 159, 3-4, , doi: /j.pepi SERPELLONI E., ANZIDEI, M., BALDI, P., CASULA, G. & GALVANI, A. (2005) Crustal velocity and strain-rate fields in Italy and surrounding regions: new results from the analysis of permanent and non-permanent GPS networks. Geophys. J. Int., 161, , doi: /j x x,

5 Figures Fig. 1. Location map of the study area in the southern Po Plain (Italy), showing the buried Northern Apennines thrust fronts (gray lines), historical and instrumental seismicity from CPTI and CSI catalogues (black squares and diamonds), drainage anomalies (white circles), and Shmin directions (white bars) from MONTONE et alii (2004). Cities: BO, Bologna; FO, Forlì; RA, Ravenna; VE, Venezia. GPS vectors in the inset from SERPELLONI et alii (2005). 5

6 Fig. 2. Schematic structural section across the study area. See Fig. 1 for location. Labels: Pls=Pleistocene; upl= upper Pliocene; mpl= middle Pliocene; lpl= lower Pliocene; ume= upper Messinian; Ol-lMe= Oligocene-lower Messinian; Ap-uCr= Aptian-upper Cretaceous; mls-uls= middle-upper Lias; uca-no= upper Carnian- Norian; upe-lca= upper Permian-lower Carboniferous. Fig. 3. Example of cross-section from one of the analogue models carried out for this study. Détachment layers within the sand models are realised by means of beds of glass microbeads (in white in the picture). Nominal scale of the models: 1:175,000. 6