ACTIVE STRUCTURES OF THE EMILIA-ROMAGNA

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1 M. Boccaletti (1,2), M. Bonini (1), G. Corti (2), P. Gasperini (3), L. Martelli (4), L. Piccardi (1), C. Tanini (2) and G. Vannucci (5) (1) CNR - Istituto di Geoscienze e Georisorse, Sezione di Firenze (2) Dipartimento di Scienze della Terra, Università di Firenze (3) Dipartimento di Fisica, Settore di Geofisica, Università di Bologna (4) Regione Emilia-Romagna - Servizio Geologico, Sismico e dei Suoli, Bologna (5) Istituto Nazionale di Geofisica e Vulcanologia, Bologna ACTIVE STRUCTURES OF THE EMILIA-ROMAGNA To improve the knowledge of the territory, and in particular to gain an insight into recent and ongoing evolution, the Emilia-Romagna Region deemed has launched a region-wide research project that sets out to collate and present seismological data and information relating to active structural elements identifiable on the basis of morphological and geological data. The results of this project are synthesized in the Seismotectonic map of the Emilia-Romagna Region (Boccaletti et al., 2004). This map correlates surface active structures with deep ones both in the Apennines and in the Po Plain, taking into account stress fields and, in relation to earthquakes, hypocentral distribution at depth. Specific geological and geomorphological studies were carried out (see also RER, 1999; Cerrina Feroni et al., 2002), and instrumental and historical seismological data was revised. Valuable information on the stratigraphy and the structures of the subsoil of the Apenninic margin and of the Po Plain was derived from seismic reflection surveys (courtesy of ENI S.p.A. Exploration & Production Division) and from RER and ENI-Agip (1998). The analysis was implemented by a revision of instrumental and historical seismological data for a comparison with the potentially active structures. From distribution and frequency of recognisable surface active structures, it emerges that these are not uniformly distributed throughout the region, but are clearly localized in 3 areas. 1. An almost continuous fault system can be identified along the apenninic watershed, coinciding approximately with the Cervarola and Falterona thrustfront. Indications of recent and ongoing activity along this front derive from morphostructural evidence, for example the morphological contrast along the fault scarp which brings the Macigno and glacial deposits together in the M. Orsaro area, the morphological contrast and the fluvial capture around the anticline of Castiglion de Pepoli. In the upper Apennines also strike-slip and minor extenxional faults are active. 2. Another active fault system coincides with the apenninic margin. Based on frequency and evidence of the structures, 3 quite separate sectors can be identified: a) the area W of the Taro valley; b) the area between the Taro valley and Bologna; c) the Romagna apenninic margin. a. The structures showing signs of recent activity, between Castel S. Giovanni and Salsomaggiore, coincide with the buried Emilia Folds, the SE continuation of the Broni-Stradella thrust (Benedetti et al., 2003). Between Salsomaggiore and the Taro valley, the apenninic margin coincide with an active front that is responsible for the formation of the anticline and the tectonic window of Salsomaggiore. The corresponding sector of the lower

2 and mid Apennines shows no evidence of surface active structures of particular importance, with the exception of the reactivated thrust faults which led to the formation of the Bobbio tectonic window. b. In this sector, a continuous active front can be traced, coinciding with the apenninic margin. The corresponding Apennines area also presents much evidence of active structures, both morphological and geological: middle Pleistocene deposits are faulted and the terraces of the upper Po Plain, with late Pleistocene deposits, are tilted. Throughout the area, seismic profiles of the subsoil reveal that middle Pleistocene sediments are folded and faulted. Reconstruction of the base of holocenic sediments shows that the greatest depth of this surface is in front of the apenninic margin, between Reggio Emilia and Bologna. In the Bologna hills there are also back-thrusts that link up in the subsoil with the pede-apenninic thrust. Through the Parma, Taro and Ceno valleys, the pede-apenninic structure divides and trends SW with splays typical of strike-slip structures. In the Apennines, the thickness of the Ligurian units is greatest between the Ceno and Nure valleys, while it is greatly reduced between the Taro and Secchia valleys; thus the area E of the Taro appears uplifted compared to the area to the W. Several active faults, transverse to the axis of the chain, have been identified in the Secchia valley. c. Between the Idice and Ronco-Bidente valleys, plio-quaternary marine deposits dip under the alluvial sediments to form a monocline and do not appear dislocated by active faults.the main signs of activity are in the Apennines and consist mainly of the Modigliana anticline, blind back-thrust localized at the base of the Pliocene sequence and in the Marnoso- Arenacea between the valleys of Lamone and Montone, and the Cesena high. The structural order of the Cesena hills is more complex, with thrusts and folds cut by an extensional anti-apenninic structure in the Savio valley. East of Cesena, post-evaporitic deposits again dipped under quaternary alluvium to form a monocline and do not appear dislocated by active faults. The only structure that present signs of activity is an anticline which deforms middle Pleistocene deposits between Rimini and Cattolica, probably the NW trending continuation of the Pesaro-Senigallia anticline described in Vannoli et al. (2004), at the top of the buried Rimini Ancona thrust (Adriatic folds in Pieri and Groppi, 1981). 3. Lastly, an important active fault system coincides with the Ferrara Folds and the Emilia Folds (Pieri and Groppi, 1981). In this case the principal manifestations of activity derive from seismic profiles. It also emerges from seismic profiles that the active and recent structures described above are connected at depth with compressive structures affecting the carbonate sequence and the basement. The carbonate sequence is thrust-faulted; these thrusts linking up with the active structures of the upper Apennines, the pede-apenninic front and with the Ferrara ridge. The structures affecting the basement (Argnani et al., 1997) also connect up with surface active thrust fronts, in particular with the pede-apenninic thrust and with the Ferrara ridge. The folds of the basement and of the carbonate sequence clearly explain the variations in thickness of the Ligurian allochthonous nappe and the formation of tectonic windows.

3 Seismicity linked to surface structures (hypocentres < 15 km) is widespread throughout the Apennines and localized above all next to the Ferrara ridge. Computed focal mechanisms are thrust and strike-slip faulting, indicating both NE- SW and NW-SE compressions. Distensive focal mechanisms are secondary. This is entirely consistent with findings from studies of recent and active stress fields. It transpires therefore that the kinematics of the principal surface active structures are transpressive in type. Crustal seismicity (hypocentres between 15 and 35 km) has a similar distribution pattern to more superficial seismicity, but with lower density. Again in this case the focal mechanisms are thrust and strike-slip faulting with both NW-SE and NE-SW compressions; distensive mechanisms are few and very much localized. The deepest seismicity (hypocentres > 35 km) is again widespread in the Apennines and above the apenninic margin, and significantly more limited in the plain. Deep earthquakes are almost all low in magnitude, consequently there are few computed focal mechanisms. Despite this, they are consistent with findings in upper structural levels. The Moho (Bigi et al., 1990; Argnani et al., 1997) is on average 5 km deeper in the plain and below the apenninic margin than in the Apennines. With this increase in depth, the hypocentres are clustered in a strongly S-dipping band. We also note an increased frequency of earthquakes S of this structure and the hypocentres are clustered along S-dipping bands, connected to the compressive structures of the surface. These bands could correspond to the main crustal faults. Lastly, we should also note the remarkable correspondence between the strike of the boxes (Gasperini et al., 1999) of the possible seismogenetic faults and the trending of the main recognised active structures. In particular the boxes along the margin and in the Apennines themselves mostly trend between NW-SE and E-W, approximately coinciding with the axial direction of the chain, while the boxes in the plain, between Reggio Emilia and Parma and between Bologna and Ferrara, present a more or less parallel orientation to that of the main buried active structures. As regards the Northern Apennines, there are no regional studies indicating areas with different uplift rates. The only study available (Bartolini et al., 1982), from the middle Pleistocene to Present, finds that the entire Emilia-Romagna Apennines are uplifted, but fails to distinguish areas of different growth rates. In contrast, in the Emilia-Romagna plain, which is generally subsiding, the Authors actually report minor subsidence in relation to the Ferrara ridge. Data collected allows us to identify areas which over the past 450,000 years (start of the Emilia-Romagna Upper Synthem, SERS) have been subject to different rates of uplift and subsidence. In order to quantify uplift, at least with regard to the margin and the lower Apennines, maps illustrating uplift from the start of the late Pleistocene have been compiled, based on the level of dated terraces. It emerges that along the apenninic margin, uplift is greatest between the Reno and Taro rivers and from the Bidente to the Savio rivers (> 1 mm/year). Least uplift (< 1 mm/year), meanwhile, is registered W of the Taro river and between the Santerno and Montone rivers. For the remaining areas, where little information can be gleaned from terraces, it is impossible to make a quantitative estimate regarding uplift. Nonetheless, it is possible, using morphostructural data, to recognise albeit approximately areas with different degrees of recent and ongoing uplift. The areas presenting greatest uplift, with tectonic windows, thinning of the Ligurian allochthonous nappe, presence of structural highs of the carbonate sequence, reactivated thrusts and the formation of out-of-sequence thrusts, are the upper high Emilia Apennines and the entire Romagna Apennines, above the basement thrust. As for the Po Plain, it transpires

4 that it is widely subject to ongoing negative vertical movements; nonetheless even in this area we can identify areas with different levels of subsidence. In particular, subsidence is greatest (1 mm/year) in the Modena-Bologna-Ravenna syncline and along the NW coast, E of the Ferrara ridge. Slighter subsidence (< 1 mm/year) is meanwhile found above the Ferrara ridge, where the effect of subsidence is mitigated by the uplift of the ridge. Comparative analysis of seismicity, active structures and the isobaths of the 2 main and most recent unconformities (SERS and Holocene base), has made it possible to obtain a neotectonic and seismotectonic framework of the Emilia- Romagna, which takes into account all of these aspects. It emerges that the areas of greatest uplift (> 1 mm/year) are in the upper Apennines, corresponding to the area above the main crustal doubling. The area of intermediate uplift (> 1 mm/year) corresponds to the area just behind the surface structure of the margin, between Bologna and the Taro valley. The areas of least uplift (< 1 mm/year) are immediately in front of the crustal structure. It is interesting to note that the area of least uplift in the mid Apennines of Modena is between the crustal structure and the pede-apenninic structure, rhomboid in shape. This framework is typical of a left-lateral transpressive system and the area of lowest elevation seems a thrust-top basin. In the Po Plain too, the areas displaying least subsidence (< 1 mm/year) correspond to the areas affected by active structures. The distribution of active structures is also in accord to the distribution pattern of areas with the highest incidence of landslides, hydrocarbon emissions and hot mineral springs. REFERENCES Argnani A., Bernini M., Di Dio G., Papani G. e Rogledi S.; 1997: Stratigraphic record of crustal-scale tectonics in the quaternary of the Northern Apennines (Italy). Il Quaternario, 10 (2), Bartolini C., Bernini M., Carloni G.C., Costantini A., Federici P.R., Gasperi G.F., Lazzarotto A., Marchetti G., Mazzanti R., Papani G., Pranzini G., Rau A., Sandrelli F., Vercesi P.L., Castaldini D. e Francavilla F.; 1982: Carta Neotettonica dell Appennino Settentrionale. Con note illustrative. Boll. Soc. Geol. It., 101, Bendkik, A., M. Boccaletti, M. Bonini, C. Poccianti e Sani F.; 1994: Structural evolution of the outer Apennine chain. Mem. Soc. Geol. Ital., 48, Benedetti L., Tapponnier P., Gaudemer Y., Manighetti I. e Van der Woerd J.; 2003: Geomorphic evidence for an emergent active thrust along the edge of the Po Plain: The Broni-Stradella fault. J. Geoph. Res., 108, 2238, doi: /2001jb Bigi G., Cosentino D., Parotto M., Sartori R. e Scandone P.; 1990: Structural Model of Italy, 1: CNR, Prog. Fin. Geodinamica, Quad. Ric. Sci., N. 114, Vol. 3. S.EL.CA., Firenze. Boccaletti M., Bonini M., Corti G., Gasperini P., Martelli L., Piccardi L., Tanini C. e Vannucci G.; 2004: Carta sismotettonica della Regione Emilia-Romagna, scala 1: Con note illustrative. Regione Emilia-Romagna, SGSS CNR, IGG (Firenze). S.EL.CA., Firenze. Cerrina Feroni A., Martelli L., Martinelli P. e Ottria G.; 2002: Carta geologico-strutturale dell Appennino emiliano-romagnolo in scala 1: Con note illustrative. Regione Emilia-Romagna, SGSS CNR, IGG (Pisa). S.EL.CA., Firenze. Gasperini P., F. Bernardini, G. Valensise e Boschi E.; 1999: Defining Seismogenic Sources from Historical Earthquake Felt Reports. Bull. Seism. Soc Am. Vol. 89, No. 1, Pieri M. e Groppi G.; 1981: Subsurface geological structure of the Po Plain (Italy). C.N.R., Prog. Fin. Geod., n. 414, RER; 1999: Carta geologica di pianura dell Emilia-Romagna. A cura di D. Preti. Regione Emilia- Romagna, Servizio Cartografico e Geologico. S.EL.CA., Firenze. RER & ENI-Agip; 1998: Riserve idriche sotterranee della Regione Emilia-Romagna. A cura di G.M. Di Dio. Regione Emilia-Romagna, Servizio Cartografico e Geologico ENI, divisione Agip. S.EL.CA., Firenze.

5 Vannoli P., Basili R. e Valensise G.; 2004: New geomorphic evidence for anticlinal growth driven by blind-thrust faulting along the northern Marche coastal belt (central Italy). J. of Seismology, 8,