RELATING SPATIAL PATTERNS OF EROSION TO TECTONICS IN THE APENNINES, ITALY

The spatial distribution of erosion rates within a watershed can be used to distinguish between different modes of landscape evolution. Landscapes can be in a state of dynamic equilibrium, where erosion rates are equal to regional uplift rates, or in a transient state where relief is adjusting to a rapid change in uplift or river incision rate. Cosmogenic nuclides in soil and sediment can be used to quantify the spatial pattern of erosion rates and therefore to discriminate between equilibrium and transient conditions. Here, we show contrasting patterns of erosion rates at two locations in the Apennines, Italy, with markedly different tectonic histories.

The Romagna Apennines in northern Italy expose a uniform sequence of Miocene turbidites that have been uplifted in response to subduction retreat. Cosmogenic ¹⁰Be concentrations in sediment from five drainage basins show no spatial pattern. Erosion rates vary by less than a factor of two, from 0.3-0.5 mm/yr. Paleo-erosion rates inferred from 10Be in nearby cave and marine deposits suggest that the erosion rate of quartz-rich lithologies has not changed significantly over the past million years.

In contrast, the Campanian Apennines in southern Italy expose a heterogeneous lithology, and are postulated to have likely been rapidly uplifted ~1 km over the past million years. Patches of a relict landscape preserved at elevations up to 1 km suggest that this region is being actively dissected in response this uplift. Cosmogenic ¹⁰Be concentrations in sediment from four drainage basins show strong variability. Erosion rates vary by over an order of magnitude, from 0.02 to >0.57 mm/yr. The highest erosion rates occur along the most recently active thrust front. The lowest erosion rates occur where the relict landscape is preserved.

We suggest that the contrasting patterns of erosion rates in these two areas are a result of their different tectonic histories. Erosion of the Romagna Apennines is ad response to relatively constant uplift rates due to ongoing subduction retreat, while erosion of the Campanian Apennines is responding to rapidly accelerated uplift in the Late Neogene. These examples illustrate the potential of cosmogenic nuclide methods to distinguish landscapes at steady-state from those in a transient state, whether from tectonics, climate, or escarpment retreat.