The northern Marche area of central Italy is the locus of a moderate - yet significant - tectonic activity, as also witnessed by historical and instrumental seismicity [maximum intensity = IX MCS; maximum magnitude = 6; Gruppo di Lavoro CPTI 1999; Frepoli and Amato 2000]. Not only are the seismogenic sources of these earthquakes still unknown [Galadini et al. 2000], but also the tectonic regime and the possible existence and size of further, presently silent, sources is a matter of debate [Valensise and Pantosti 2001, and references therein]. As an example, Basili et al. [2002] and Vannoli et al. [2004] recently re-proposed the idea that blind thrust faults, accompanied by anticlinal growth, would characterise the active tectonics of the study area. These Authors also suggested that the earthquake potential of this area may be substantially different (i.e. higher) from that currently estimated, thus posing new concerns on the seismic hazard of this highly populated region. According to this view, thrust ramps would host the sources of the largest earthquakes in the northern Marche region [Vannoli et al. 2002]. More in general, according to Meletti et al. [2000] the outer Northern Apennines would be characterised by low/medium energy compressional earthquakes related to active frontal and lateral thrust ramps. However, different interpretations from that of active thrusting have also been put forward to explain present-day tectonics of the outer Northern Apennines [e.g. Bertotti et al. 1997; Carminati et al. 1999; Di Bucci and Mazzoli 2002; Argnani et al. 2003].
The variability of geological interpretations and seismotectonic models for the outer Northern Apennines most probably arise by the fact that this part of the chain is characterised by the coexistence of strongly contrasting data. For instance:
focal plane solutions are not only compressional, but also strike-slip and extensional, with variably oriented P and T axes [e.g. Gasparini et al. 1985; Zollo et al. 1995; Frepoli and Amato 1997; Mariucci et al. 1999; Frepoli and Amato 2000; Selvaggi et al. 2001];
mesostructural data outline a middle-late Pleistocene stress field not compatible with thrust belt-related compression [e.g. Bertotti et al. 1997; Ghiselli and Martelli 1997; Morelli and Costa 1997; Piccardi et al. 1997; Borraccini et al. 2002];
thrusts buried beneath the Po Plain and the Adriatic Sea appear to be sealed by middle-upper Pleistocene deposits [e.g. Bally et al. 1986; Argnani et al. 1997; Bertotti et al. 1997; Picotti et al. 1997; Coward et al. 1999; Di Bucci and Mazzoli 2002; Di Bucci et al. 2003]; and
well breakout data mostly suggest active N-S compression [Montone and Mariucci 1999].
The aim of this paper is to examine the controversial active tectonic setting of the northern Marche region within the larger framework of the outer zones of the Northern Apennines by the integration of structural, geomorphological and seismological data.