Quantifying Active Hinge Migration and Fault Propagation from a Fluvial Terrace Record, Apennine Mountain Front, Italy

Fluvial terraces have been recognized as kinematic indicators above growing fault-related folds, but precise relationships between terrace geometries and various folding mechanisms remain largely unexplored. The age and geometry of strath terraces preserved across the forelimb and in river valleys dissecting the Salsomaggiore anticline, northern Apennines, constrains the Pleistocene-Recent kinematics of the underlying fault-propagation fold. Preserved terrace deposits and older growth strata document a fixed anticlinal hinge, a rolling synclinal hinge, and along-strike variations in uplift and forelimb tilting. The uplifted intersection of ~620 ka and ~140 ka terrace deposits on the forelimb documents ~2 km of synclinal axial surface migration and underlying fault-tip propagation at a minimum average rate of 1.4 cm/yr since the Middle Pleistocene. Synthetic channel profiles of variable concavity generated using observed slope-length and slope-area relationships show that these terrace geometries cannot be fully accounted for by a long-term increase in channel concavity. Furthermore, the amount of inferred synclinal axial surface migration and fault-propagation correlates along strike with variations in uplift (incision) and quantitative geomorphic metrics including catchment hypsometry, mean anticlinal hinge elevation, and channel concavity. In contrast to recent growth dominated by limb lengthening, a synthesis of the Miocene-Early Pleistocene history reveals fold growth with significant limb rotation, and complicated by fault-bend folding above a deeper lateral ramp. These findings suggest that hinge migration and fault propagation can be recognized and directly quantified from the uplifted intersection of old fluvial terrace deposits above emergent fault-propagation folds.