Morphostructural evidence of Late Quaternary tectonics at the Po Plain-Northern Apennines border (Lombardy, Italy)

The landscape of foreland basins adjacent to active mountain ranges evolves under the control of basinward tectonic propagation of the structural fronts, in competition with climate dynamics. The resulting palimpsest landscapes record the sequence of geomorphic evolutionary steps, and the spatial-temporal relations between the active geological processes. However, a precise evolutionary sequence is hard to decipher from low-relief settings, like the wide alluvial plains worldwide. About these topics we investigate the sequence of tectonic- and climate-driven processes which shaped the palimpsest landscape of a large Quaternary foreland basin, the Po Plain in Northern Italy. Above the average glacio-fluvial and alluvial plain of the basin, Late Quaternary intra-basin reliefs emerge, owing to syndepositional ramp folding and uplift driven by N-wards propagation of the outermost thrust fronts of the active N-Apennine chain. The incremental tectono-morphological and depositional history of the region permits to describe the propagation of the structural front as a polyphase process, involving uplift, wrenching and late collapse of the reliefs. This history is documented by: relicts of uplifted planation surfaces covered by latest Pleistocene weathered loess units; polygonal facets, landslides and slope wedges along faults delineating the slopes of the largest among the intra-basin reliefs (San Colombano hill); hydrographic anomalies on the relief network (river diversions, piracy and perching of valleys, paleovalley fills), river diversions and cross-cut relations on the adjacent alluvial plain. These features testify the thrust-fold-related, pre-LGM uplift of the Late Pleistocene alluvial stratigraphy, the subsequent segmentation of the relief in differently uplifting blocks along Riedel faults, owing to activation of a transfer fault zone, and the LGM extensional collapse of the relief, along inherited fault systems. A late, post-glacial outwards propagation of the Apennine thrusts was buttressed by the opponent Alpine thrust-belt front and induced the entrenchment of the river network. River diversions on the plain were controlled by the orientation of the San Colombano hill fault systems. The distribution of historical earthquakes and the present-day geodetic data are coherent with this evolution. The study documents how, in foreland basins, the syndepositional propagation of “blind” orogenic fronts shapes the palimpsest landscape with superimposed morphological, tectonic, and stratigraphic features of paleoseismic significance, and permits comparisons with the other low-relief basin settings worldwide.