Tectonically induced base-level change in the Quaternary revealed by alluvial fan shifts and rivers re-organization. A case study in the Northern Apennines of Italy

In active continental extensional basins, subsidence induced by normal faults activity at their hanging-wall is the main factor in the creation of an accommodation space where rivers deliver their sediments. The sediments ages, lithology and geometry provide the timing and rate of faults activity. The migration of active extensional tectonics through time produces a correspondent shift in the position of the active depocenter. This process produces features like wind gaps, abandoned valleys, streams captures and drainage inversions which provide hints on timing and rate of active tectonics migration. In this work, we address the issue of how the investigation of the recent geological history of a fossil alluvial fan can provide new insights on the spatial migration of active tectonics through time and the consequent re-organization of the rivers network. We investigate a fossil alluvial fan originally deposited into a lower Pleistocence depocenter, which has been uplifted about 200 m at a normal fault foot-wall above the present-day alluvial plain. The area, comprised between Perugia and Spoleto in central Italy, is presently active as indicated by both GPS measures, historical and present-day seismicity. Based on detailed mapping of the Quaternary deposits, information on the age of the mapped deposits, paleo-currents indicators, geomorphological investigations, and high resolution seismic reflection profiles we reconstruct a peculiar geological history of the area: the eastern part of a lower Pleistocene depocenter in which an alluvial fan was deposited underwent a sudden increase in the activity of a NE-dipping normal fault system. Consequently, the alluvial fan was cut and the western part of the depocenter was de-activated, undergoing incision. On the other hand the increase of subsidence in the eastern part of the basin promoted the deposition of a thick sequence of alluvials and headward erosion of the rivers which suddenly enlarged their drainage areas producing rivers captures and inversions. This process occurred in the last 0.8 Myr. By analyzing the present-day alluvial fans areas vs their contributing areas we discuss the variability of the data distribution in the reconstructed geological history of the study area. We stress that a widely multidisciplinary investigation of rivers anomalies and basins subsidence distribution can provide extremely useful hints in the understanding of the steadiness/unsteadiness of faults behavior.