Drainage base-level fall history in North-western Apennines and implications on the Alessandria Basin tectonic activity

Drainage networks are particularly sensitive systems among all the topographic features in terms of their response to perturbations driven by active tectonics. Indeed, fluvial landscapes can record several information about different processes especially in geodynamically active areas, allowing to relate spatial-temporal variation in base-level fall and vertical incision of stream channels with certain morphometric features. This study focuses on the tectonic evolution of the Alessandria Basin, a syn-orogenic tectonic basin located at the junction between the Alps and the Apennines, that experienced progressive subsidence during the overthrusting of the Monferrato Arc (the westernmost outer arc of the Apennine belt) onto the Po Foreland Basin. Different studies carried out in this region have assessed the Neogene tectonic evolution at a regional scale, although Quaternary activity is still poorly understood in terms of both Alps/Apennines uplift and activity of the compressive front of the Monferrato Arc. In this study, we applied river linear inversions to reconstruct the baselevel-fall history of 6 catchments that drain into the Alessandria Basin. We used 9 10Be-derived basin-average denudation rates to calculate the erodibility parameter needed for inferring base-level fall rates from previously chi-transformed river profiles. The results describe the last ~ 3 Ma of tectonic activity, highlighting increases in baselevel-fall rate with an initial peak around 3 Ma, and a second around 2 Ma. While the first peak is coeval with the vertical uplift that affected most of the northern-central Apennine, the second one suggests an acceleration in subsidence of the Alessandria Basin concurrently with the uplift of the Monferrato Arc.