Discussion
The structural setting of the study area can be schematised as a superposition of three roughly subhorizontal layers (Fig. 6). The shallow (allochthonous Apenninic Platform and Lagonegro Basin units) and the deeper ones (Apulian Platform unit) are characterised by a brittle behaviour. However, the intermediate layer (the mŽlange) diverges from this behaviour. In fact the data exposed above indicate that, from a mechanical point of view, it is most probably similar to another water-saturated, clay-rich dŽcollement level, well known from the southern Apennines (Roure et al., 1990): the so-called ÒArgille varicoloriÓ (varicoloured shales) formation (Sgrosso, 1988; Patacca et al., 1992b; Di Bucci et al., 1996; Corrado et al., 1998). Due to fluid overpressure and circulation, these deposits show a ductile behaviour that not only controls the development of one of the most important detachment levels in the Apennines (Patacca et al., 1990; 1992a; Corrado et al., 1997; Mazzoli et al., 2000) but, occasionally, also generates mud diapirs (Roure et al., 1990).
The behaviour of brittle-ductile-brittle multilayers has been extensively analysed by analogue modelling (e.g. Richard, 1990; Allemand and Brun, 1991; Richard and Krantz, 1991; Mandl, 2000) as well as in real cases (e.g. Nalpas and Brun, 1993). When the ductile unit shows a sufficient thickness, normal faults do not directly propagate across it from the deep to the shallow brittle beds. Deep-seated extension propagates toward the surface through the ductile layer, which flows both toward the footwall and the hanging wall of underlying faults. In this way, the ductile layer acts as a decoupling and accommodating horizon. Its presence allows the surface fault pattern to be different from the subsurface pattern, as already outlined by Vendeville et al. (1987). Surface evidence of extension is then given by a wide, asymmetric pattern of normal faults that are more numerous, unconnected and shifted with respect to the deep parent faults.
All these features can be recognised in the model of Figs. 2-6, and provide an interpretative key for the development of the extensional system of the high Agri River valley. At surface, the valley is asymmetric, showing the most relevant Middle-Upper Pleistocene normal faults, as well as the maximum thickness of the coeval alluvial deposits (Morandi and Ceragioli, 2000; 2002), along its north-northeastern side. Locally, minor antithetic faults generate secondary grabens. All these faults develop only in the shallow allochthonous units, made up of carbonate platform and pelagic basin rocks and characterised by brittle behaviour.
At depth these faults do not crosscut the mŽlange zone, nor involve the top of the Apulian unit, which remains uninterrupted in correspondence with the hypothetical continuation of the main surface faults.
An important normal fault cuts, instead, the top of the Apulian unit just below the central part of the Quaternary plain, and does not continue in the tectonic units above. As previously mentioned, this fault extends for about 20 km into a WNW-ESE direction and is SSW-dipping; it follows that the most developed faults at surface are those synthetic with respect to it. The deep fault shows a vertical displacement of about 350 m, and post-dates thrusting within the Apulian domain that, in this area, is Late Pliocene-Early Pleistocene in age (Cello and Mazzoli, 1999; Mazzoli et al., 2001, and references therein). Therefore, this fault acted between Middle Pleistocene and present-day. Being conservative, a vertical displacement of 350 m in ca. 750 ka gives a vertical separation rate of ca. 0.47 mm/y, corresponding to a slip rate of about 0.54 mm/y. This value is comparable with the slip rates calculated, based on surface data alone, for many other active extensional faults in the Apennines (Galadini and Galli, 2000, and references therein).
According to the model presented in this work and also considering age, displacement and location of this fault, it appears to be the most probable deep structure responsible for the development of the active fault system and the related basin of the high Agri River valley; as a consequence, it is likely to be the source of strong earthquakes such as the 1857 event. This is also compatible with a seismic sequence which occurred in 1996 at a depth between 4 and 7 km below the SW-side of the high Agri River valley (Ciaccio et al., 2001), roughly in correspondence with the most likely location of the south-eastern termination of this fault in the core of the Apulian unit.
The occurrence of a buried ductile mŽlange zone intercalated within the Apennine thrust structure appears to exert a major control on the active extensional fault system which developed in the high Agri River valley of southern Italy. As shown by the 3D structural model presented in this paper, there is no direct brittle connection, across such a mŽlange zone, between the deep (seismogenic) structure and the active faults exposed at surface. Therefore, a Ôsoft-linkageÕ can be inferred to occur across the mŽlange zone between the deep master fault and the surface fault strands.
 |
| Fig. 10. The animation starts looking at the high Agri River valley from the South. As the model is rotating clockwise, our point of view moves to the SE. As we go down and closer to the model, we can appreciate the lack of continuity between normal faults in the shallow layer (i.e. the allochthon, overlying the green surface) and those in the deep brittle domain (bounded on top by the dark blue surface). As the model is further rotatedclockwise, our point of view progressively moves to the north and then to the NW. Also from this side, the relationships between shallow and deep faults can be observed. As the topographic surface (DEM) is removed, the top of the continental Quaternary deposits - faulted along the northeastern side of the valley - can be observed. The model is further rotated clockwise to reach the starting position (i.e. view from the South).Two formats are available: .AVI <click to view> (10.3 Mb) better resolution, Lotus Screencam Player; downloadable from http://www.lotus.com/products/screencam.nsf) zipped. Using Lotus Screencam you can download LotusMediaV2 plugin for Internet Explorer (version 4.0 or later). |
Our
study emphasises the fundamental importance of detailed 3D reconstructions
of the tectonic setting of the Apennine chain in order to achieve a
better understanding of the active fault systems in the Italian peninsula.
For some of these systems, in fact, models based on surface data alone
do not offer a univocal solution. Moreover, seismic events like the
1980 Irpinia (Italy) earthquake, caused by the activation of at least
three main faults both NE- and SW-dipping (Boschi et al., 1993),
outline the real complexity of the seismic sources. From this point
of view, 3D models of active fault systems in seismic regions represent
a possible upgrade with respect to the simplified single fault models
usually adopted in seismic hazard and scenario studies, providing a
more realistic source for destructive earthquakes.
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