Thrust-fold activity at the mountain front of the Northern Apennines (Italy) from quantitative landscape analysis

Abstract

We document the active tectonics of growing anticlines embedded in the northern Apennine mountain front using a quantitative analysis of the topography. These structures appear to be growing at rates of 0.1–0.3 mm/year, more slowly than the long-term general rate of emergence of the Apennines from the Po plain. Our analysis is dedicated to extract the signal of this tectonic activity, recording it primarily by the channel steepness ( k sn ) of streams traversing the mountain front and by swath profiles and relief of the topography. Topographic swath profiles and k sn are consistent with field stratigraphic, structural mapping and seismic reflection profile relationships. In general, the spatial distribution of k sn values of small channels (with upstream drainage area < 20 km 2) appears to coincide with fold uplift and deformation, being the maxima located at the fold axes, towards the foreland or along strike as the tilting of a growing fold propagates frontally or laterally. Quantitatively, we use local development of relief as a proxy for rock uplift (measured as river incision) across the mountain front and anticlines to test the expected linear scaling between rock-uplift rates (relief) and k sn . In this low rock-uplift setting (0.1–0.3 mm/year), this predicted scaling relationship is not supported by k sn and topographic data, suggesting a condition of transient response of fluvial incision that generates the relief to the tectonic forcing, where k sn and relief have not achieved steady-state values. However, a weak linear scaling between rock uplift and k sn may hold for the local highest uplift settings where over 40 m of relief have been generated, thus establishing a lower threshold for where the channel steepness approach may be valid. In this slowly deforming landscape assuming a state of transient response of fluvial incision to tectonic forcing, the high k sn values can be explained as places where the channels are being actively deformed by tectonics, but have not yet fully responded by incising and generating relief. Therefore, in the absence of relief generation, the spatial distribution of k sn maximum values for small streams can hold for rock uplift and work as a model for indicating locations of strongest anticlinal growth. Collectively, the correspondence among the stratigraphic markers used to define the structures, topography and relief as quantified in the swath profiles, and the k sn values that indicate active channel adjustments to tectonic forcing support the evidence of continued shortening across the European–Adria plate boundary and the contemporary slow growth of the structures embedded in the emergent Apennines mountain front.