Active deformation in the frontal part of the Northern Apennines: insights from the lower Metauro River basin area (northern Marche, Italy) and adjacent Adriatic off-shore

An integration of seismological data with geological and geomorphological information aided by seismic interpretation was performed to characterise the Quaternary tectonic evolution of the Metauro River basin area (northern Marche) and adjacent off-shore sector of the external Northern Apennines. On-shore, along the Adriatic coast, the youngest age of thrusting and folding post-dates the Early-Middle Pliocene, while Pleistocene deposits appear to be, at least in part, not involved in the deformation. Recent (i.e. post-thrusting) tectonic structures have been recognised both in pre-Quaternary substratum rocks and in Upper Quaternary continental deposits (Upper Pleistocene terrace alluvium, Upper Pleistocene-Holocene slope deposits). These faults are all compatible with a WSW-ENE oriented extension. In the Metauro River basin area, preserved flights of stream terraces have been categorised according to the presence or absence of alluvial suites in relationship to each terrace level. Here, based on both the heights above the valley floor and the areal distribution of stream terraces, a generalised vertical tectonic uplift can be inferred, particularly during the Middle-Late Pleistocene. Moreover, the along-valley distribution of stream terraces provides further constraints on the age of thrusting and folding. In fact, the reconstructed terrace-levels are substantially parallel, and no evidence for any significant deformation by fold activity has been recognised. Local deformation displayed by both terrace surfaces and alluvial/ slope-waste deposits suggests, instead, the intervening of some minor differential movements associated with the generalised uplift and/or to Middle-Late Pleistocene normal faulting. Their occurrence appears anyhow to be unrelated with the pattern of folds and associated thrusts. The present-day seismic activity of the study area was considered by analysing 83 seismic events that occurred from 1987 to 2000. The epicentre distribution is very scattered, and depths are generally comprised within the first 20 km of the crust, clustering between 0 and 15 km. Most of the events have magnitude < 3.0, while only a few of them (12) have magnitude between 3.0 and 4.2. Focal mechanisms were obtained for eight events. The results are complex, showing fault plane solutions of "normal fault", "thrust fault" and "unknown" types, according to commonly used stress categories. The low level of seismicity and the inhomogeneity of fault plane solutions confirm that, in the study area, this kind of data cannot provide clear constraints with respect to the problem of the occurrence or lack of activity along the thrust front. Finally, the Quaternary marine succession was studied in the northern Adriatic off-shore. Lower Pleistocene deposits appear to be slightly involved in the deformation associated with the frontal thrust of the Apennine chain. In contrast, the upper part of the Quaternary succession is clearly undeformed, showing planar and horizontal reflectors, which seal the most external thrusts of the belt. These data suggest that thrusting at the front of the Northern Apennines ended in Early Pleistocene times. Our integrated results indicate that, since Middle Pleistocene times, the external front of the northern Marche Apennines has been inactive. A generalised uplift has taken place, with subordinate tectonic activity being characterised by ENE-WSW extension and NNW-SSE compression. These main features of the stress field, although with spatial permutations of the principal stress axes, also characterise most of the external Northern Apennines during the whole Middle Pleistocene-Holocene time span.