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

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 (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- analysedthe 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 Apennines thrust fronts along the Bologna-Ferrara transect, obtained integrating the different results.