Quaternary alluvial terraces and their response to climatic, lithologic and tectonic controls in low-rate tectonically active regions: a case study from the Marche Apennines (central Italy)

Understanding the continental record and the geomorphic dynamics of river basins is essential in the reconstruction of the landscape's history, even more over the Quaternary, when significant climatic oscillations took place. Fluvial terraces, coupled with the geochronology of their deposits, can open an archive that traces landscape evolution. They can be used to extract the rates of processes that encompass the geomorphic response of the landscape to the interactions among tectonics, eustasy and climate. To identify the role of these influencing factors, the plano-altimetric configuration of the terrace remnants, their absolute chronology and the fluvial stratigraphy need to be studied thoroughly, considering their variability both in space and time, along with historic and paleoenvironmental data. The selected case study is the well-preserved staircase of alluvial terraces located in the Adriatic piedmont of the Umbria-Marche Apennines (central Italy), in the foredeep domain of the Apennine orogenic system. This terrace staircase is widely recognized as being generated by the cyclical alternation of fluvial aggradation and incision over the Middle and Late Pleistocene, in response to the combination of low-rate tectonic uplift and climate changes. The uplift favoured river terrace formation, affecting the piedmont area of the Apennines with a rate of 0.3 – 0.5 mm yr⁻¹ during the last 1 My. The first step of the analysis was a preliminary study of the river catchments of the study area, using remote sensing techniques, with the aim of recognizing the remnants of relict surfaces of Pleistocene-Holocene fluvial terraces and analysing the hydrographic systems. The high-resolution Digital Terrain Model (DTM) obtained by topographic surveying with LiDAR technology has been converted in a Relative Elevation Model (REM), where the baseline elevation is detrended to follow the water surface of the streams. The DTMs have been subjected to a topographic analysis consisting in the semi-automatic extraction, using MATLAB© based TopoToolbox, of sub-planar, gently inclined topographic surfaces hanging along valley-sides, coinciding with the tread surfaces. The obtained surfaces were intersected with the REMs elevation, to obtain a preliminary subdivision into terrace levels. This analysis has been instrumental for the field work, where the deposits have been delimited and sampled. For the chronological characterization of the sampled sediments Luminescence dating techniques have been selected due to their sensitivity and accuracy and documented use in the literature. The obtained ages have then been integrated with the already existing ages available in the area. Numerical dating of the terrace deposits highlighted how fluvial sedimentation of the fill terrace deposits occurred during the late Quaternary main cold phases. To calculate the long-term bedrock incision rate, interpreted as a regional uplift rate, the model of Pederson et al. (2006) was applied to the dated terrace levels, following the now verified assumption that the sedimentation occurred during cold climate phases. The resulting uplift rates are in agreement with those already available for the area (0.3-0.5 mm yr⁻¹), but show internal variation. This result reflects the presence of local uplift variations acting in the study area.