Available online http://amq.aiqua.it ISSN (print): 2279-7327, ISSN (online): 2279-7335 Alpine and Mediterranean Quaternary, 27 (1), 2014, 5 - 28 QUATERNARY STUDIES AS A TOOL TO VALIDATE SEISMIC HAZARD POTENTIAL OF TECTONIC STRUCTURES: THE CASE OF THE MONFERRATO THRUST FRONT (VERCELLI PLAIN, NW ITALY) Carlo Giraudi ENEA C.R. Saluggia, Vercelli, Italy Corresponding author: C. Giraudi <[email protected]> ABSTRACT: This paper proposes the study of the Quaternary geological evolution of the Vercelli Plain (Piedmont, NW Italy) with the aim of validating the reliability of assumptions about seismic hazard of tectonic structures based on seismotectonic data obtained from stud- ies at regional scale. In particular, the Quaternary evolution is interpreted in order to verify and date the tectonic activity of the Monferrato thrust front, i.e., the westernmost arch of the buried northern Apennines front. In reference to the main purpose of the work, it is observed that only some stretches of the buried front (Lucedio and Cavourrina fault and, very likely, the flexures of Crescentino and Morano Po) were active between 870 and 400 ka BP. After 400 ka BP, near Crescentino and Trino, some structures transversal to the Monferrato front were probably activated, inducing the uplift of N-S elongated areas. The uplift affected both the northern Monferrato slope and the areas of the plain located south and north of the thrust front. The uplifted Trino area was limited to the east by the Salera Line. There are no indications of structures (fault or flexure) to the west of the Trino area or which form the limits of the uplifted Crescentino area, although their presence could be hypothe- sized. The Salera Line is the most important identified structure; it is very likely a complex-kinematics fault that was active from the Plio- cene to the Upper Pleistocene and the Holocene. It continues to the south inside the hill areas, for a length of about 20 km. Therefore, the recent tectonic movements are not associated with the Monferrato thrust front, as claimed by other authors, but rather with N-S structures which are transversal to the front. Although elements to evaluate the seismic hazard possibly associated with these structures are not available, it is unlikely that the seismicity alone of the easternmost Apennine fronts (Emilia and Ferrara Folds) can provide useful information to assess the seismic hazard of the Vercelli Plain and of Northern Monferrato. KEYWORDS: quaternary evolution, buried tectonic structures, climatic impact, climate and tectonic interaction, river diversions, Vercelli Plain, Monferrato hills. 1. INTRODUCTION Valley, led to the hypothesis that the Monferrato front may be seismically active, even if characterized by A detailed study of the Quaternary evolution of the earthquakes with long return periods. Both Bonadeo et Vercelli Plain (Northwestern Italy) is used here as a tool al. (2010) and Michetti et al. (2012), suggest that the to assess the reliability of previous assumptions based level of seismic activity of the Monferrato front could be on seismic hazard which were generally based on the similar to the two other Northern Apennine fronts. In- analysis of historical seismicity, on the evolution of the deed, according to Michetti et al. (2012), the Monferrato drainage network, on the presence of Quaternary sedi- front might be able to generate earthquakes of Mw 6. ments displaced by faults, and on GPS measurements. In the past, the presence of the Trino isolated ridge Data collected and presented in this paper are inter- (known as RIT in the literature), lying above the buried preted in order to verify and date the tectonic activity of Monferrato front, had already stimulated research by the the buried thrust front of this sector of the Apennines. As Working Group on the Po Valley Quaternary (Gruppo di Quaternary sediments of the Vercelli Plain bury the Studio del Quaternario Padano: GSQP, 1976) which Monferrato thrust front (i.e., the westernmost of the established that the area could have been affected by three major buried arches of the Northern Apennines; Middle Pleistocene tectonic activity. Moreover, geodetic Fig. 1A) the detailed geological and geomorphological data reported in Arca & Beretta (1985) provide informa- study of the plain provides a reliable tool for understand- tion on some aseismic deformations that occurred along ing the possible activity of the front. a section which crosses the SE portion of the area of In fact, according to several Authors (e.g. Costa, this study during the period between 1897 and 1957. 2003; Dela Pierre et al. 2003b; Galadini et al., 2012) the Thus, the conflicting interpretation of tectonic activity activity of the frontal thrust of Monferrato ended in the and seismic hazard, and the presence of aseismic defor- early Pleistocene while, further to the East, the Emilian mations, require detailed study of the Quaternary geo- and Ferrara fronts are still active. This is testified also by logical evolution to determine which deformations were historical and instrumental seismicity, which is totally recorded by sediments and landforms, and thus which absent in the Monferrato front, and increases toward the structures are likely to be active. Emilian folds (Galli, 2005), reaching a maximum in the In this paper, the evaluation of the deformation of Ferrara Folds (Galli et al., 2012; Burrato et al., 2012a). Quaternary sediments, including those dated as the last On the other hand, the results of other seismotec- 2-3 ka, suggests the activity of tectonic structures which tonic research, conducted at the scale of the whole Po only partially match those hypothesized in the literature. 6 Giraudi C. To assess adequately the tectonic evolution of this area, this study takes into account the interaction between sedimentary and erosive phases driven by climate and those probably influenced by tectonics, starting from the up- per Lower Pleistocene. 2. STUDY AREA The Vercelli plain lies between the river Dora Baltea and the Ivrea morainic amphi- theatre to the west, the river Po and the Mon- ferrato hills to the south, the river Sesia to the east and the rivers Elvo and Cervo to the north (Fig. 2A). The plain is mainly formed by Plio-Pleistocene sediments, but in its south- ern part a thin Quaternary sedimentary cover lies on marine sediments. The Tertiary ma- rine sediments, outcropping in the Po river- bed, are folded and heavily deformed by the activity of the northernmost Monferrato thrust front (SGd’I, 1969a; Dela Pierre et al., 2003a). A large part of the Vercelli plain is in the Sesia River catchment whereas only the western and southern portions of the plain are in the catchments of the rivers Dora Bal- tea and Po (Fig. 2B). It should be noted than the Po catchment, which includes a large area of the western Alps and the northern slope of the Torino and Monferrato hills, before receiving the Stura di Monferrato stream, narrows to 1.5-2.5 km (Fig. 2B). In the Monferrato hills the divide of the small catchments that drain toward the Po migrates abruptly towards the north and re- mains very close to the plain down to the confluence with the Stura stream. Starting from the area of Gabiano, the bed of the Po is always very close to the hill slope and erodes it. The narrowing of the Po catchment in the southern Vercelli plain and in the northern Monferrato slope is an apparent anomaly compared to the westernmost areas. The Po, which is the main watercourse in the study area, during the Quaternary was mostly flowing south of the Monferrato hills. It migrated north of the hills only after a diver- sion that took place during the Late Pleisto- Fig. 1 - Apennine thrust fronts buried below the Quaternary sediments of the Po cene (Carraro, 1976; Carraro et al.,1980; Plain, and main structures of the Monferrato thrust front. 1A: general view of the 1995). Apennine buried fronts. 1B: the Monferrato-Torino Hills thrust front according to Before the diversion, the rivers Sesia and Costa (2003). 1C: the Monferrato thrust fronts below the Quaternary sediments of the Vercelli Plain, according to the Geological Map of Italy F. Vercelli (1969) and Dora Baltea flowed into a river, which from ENEL (1985). 1D: the Monferrato thrust fronts below the Quaternary sediments of now on will be referred to as the DROS, the Vercelli Plain according to (MSdI = Bigi et al., 1990) and (CGT = Dela Pierre formed by the confluence of the rivers Dora et al., 2003a). Riparia, Orco, Stura and other smaller streams, whose basin was in the north-western part of presented in order to assess the chronology of the tec- the Alps. tonic phases and to verify whether the most recent tec- tonic activity is actually connected with the Monferrato 3. METHODS thrust front or with other structures. Topographic obser- vations are based on the trend of the contour lines re- In this paper a detailed discussion of the topog- ported (Ajassa et al., 1990) on the Elevation Map of the raphical, geomorphological and geological elements is Piedmont Plain (Carta altimetrica dell'alta pianura Quaternary studies as a tool to validate assumptions about seismic hazard ... 7 Fig. 2 - Location of the studied area. 2A: contour map, slope of the plain, topographic anomalies and main geological structures buried below Quaternary sediments. 2B: the river Po catchment basin west of the confluence of the Stura stream. piemontese in Italian) at the scale of 1:250,000. The century often erased morphological features that, in data used to draw the topographical profiles are re- turn, could be relevant for the interpretation of the evolu- ported on the Carta Tecnica della Regione Piemonte tion of the area. (the official regional topographic cartography) at a scale The lithology of the sediment forming the surface of of 1:10,000.