Comparing long term deformations and short term fluvial response at the hanging-wall of an active low angle normal fault in the Northern Apennines of Italy.

The Northern Apennines of Italy are characterized by the interaction of three main morphogenetic processes which affect the rivers incision: a regional uplift in the order of 0.5 mm/yr; climatic/eustatic changes and active tectonics. While the first two can be considered quite uniform at a local scale, the tectonics effects change depending whether we observe an uplifted or a lowered block. We investigate the inteference between uplift and extensional tectonics along the hanging-wall of a low-angle NE-dipping normal fault (the Altotiberina fault -ATF) in the Northern Apennines of Italy. The fault is active since the lower Quaternary. Microseismic surveys and geodetic measures have higlighted that the fault is presently active at 2.5 mm/yr rates. The long-term offset distribution indicates that the fault is continuous for 60 km and it is the detachment of smaller splays the youngest of which border the Quaternary Tiber valley. The ATF splays have driven the Quaternary evolution of three thresholds-separated continental basins (1000 m of infill). Within these basins we identify three depositional cycles from early to late Pleistocene evolving from clay-dominated sediments to conglomerates and sands, up to heterogenic deposits. The Quaternary deposits distributed along the Tiber valley are deformed by the ATF splays and are deeply incised by the Tiber river. To evaluate the incision distribution, we made a geological/geomorphological mapping of the river terraces through aerial-photo interpretation and field investigations. We find that the Tiber river behavior is not homogeneous and the rates of incision varies downstream. The northwestern basin is pretty flat with little or no incision, while the southeastern basins are characterized by two orders of well-recognizable fluvial terraces resting on top of the Quaternary deposits up to 250 m of elevation. One of these surfaces, containing a travertine has been dated through U/Th yielding an incision rate of 0.4 mm/yr. We interpret the along strike changes of Tiber river incision as the result of the interference between the regional uplift and the local extensional tectonics. Within the competition between these factors, the northwestern sub-basin appears to be dominated by local subsidence. This is due to the fact that the basin bounding normal faults are vigorously active and the hanging-wall subsidence overcomes the regional uplift. On the other hand within the southeastern sub-basins, uplift dominates on the foot-wall subsidence promoting river incision.