Deciphering Quaternary palimpsest landscapes to constrain 3D (4D) geological models: an example from the Po Plain-Apennines border (N-Italy)

Palimpsest landscapes represent important archives of the geological evolution of a basin. Deciphering the nature, timing and hierarchy of successive episodes of landscape-changing events in dynamic settings helps to constrain reliable evolutionary models and/or simulations, which incorporate the 4th dimension into 3D geological reconstructions. The landscape of the Quaternary Po Plain, in the foreland between the Alps and the Apennines, is the surface expression of active thrusting of the Apennines (South) and Alpine glacial cycles (North). Reliable models of this complex, geomorphological and tectono-stratigraphic setting, are of interest for both the Quaternary reconstructions of the basin and the relevant issues in land and resources management. Aiming to describe the Late Pleistocene-Holocene incremental landscape evolution of the basin, and to develop 3-4D models, we focus on a region close to the Po Plain-Apennines border, where three isolated low-relief hills emerge above the plain as a consequence of N-wards blind thrust-related uplift and wrenching/collapse. The hills expose stratigraphic boundaries and (paleo-) morphological surfaces otherwise buried in the adjacent plain sectors and show the involvement of the Quaternary, mostly alpine-sourced littoral, alluvial and glacio-fluvial succession in Apennine folding and faulting. The adopted multidisciplinary approach combines different scales of geomorphological, geopedological, stratigraphic and structural field observations corroborated by OSL and 14C age determinations to feed GIS-assisted, quantitative morphotectonic and stratigraphic analyses. The 3D forward models were processed by interfacing the GIS management with the 3DGeoModeller software. The results highlight how, where and when climate and tectonic-driven processes fingerprinted the evolution of the hills and surrounding plain landscape. The proto-hills, i.e. paleo-highs uplifted during active Apennine thrusting in the early Late Pleistocene, were the sites for polycyclic loess-soil aggradation. This suite of non-erosional surfaces, characterized by in situ paleosols, relates to sites of morphological stability. Valley incision, fluvial terracing, soil truncation and redeposition occurred along the faulted boundaries of the uplifting reliefs and in the intervening structural lows. Late Pleistocene transtensional faulting induced abrupt changes of the drainage network by shifting the base-levels and enhanced reworking of paleosols. These were redeposited in colluvial wedges on the fault-block hangingwalls, the latter highlighted by peculiar alignments of morphotectonic indicators. A latest uplift stage during the latest Pleistocene-Holocene is recorded by the regional entrenchment of the major drainage network of the southern Po Plain, which is affected by several fault-related diversions, river captures and terrace-slope intersections. These cross-cut relationships and the tectonic increments have been converted into modelling rules, which have been incorporated as new routines into the GeoModeller suite. The work shows that palimpsest landscapes represent the topographic and geomorphological constrains for modelling, where the modelled volumes, assembled by their hierarchy and formative increments, incorporate the evolutionary steps and drivers deciphered from field observations.