Detrital geochronological analyses, combined with information on river sediment load, are widely employed to constrain erosion patterns in orogenic belts. Major assumptions in most detrital studies are that detrital samples are fully representative of eroding bedrock, and variation in original mineral concentration, often referred to as fertility, is negligible. Nevertheless, hydraulic sorting effects during transport may strongly affect sediment composition, and mineral fertility strongly depends on bedrock lithology. In this detrital geochronology study, we illustrate how hydraulic sorting effects can be properly evaluated, and how mineral fertility in bedrock can be determined from detrital samples, in order to infer reliable erosion patterns on short-term time scales. Fission-track, bulk-petrography, and geochemical analyses were carried out on modern sands of Rivers Dora Baltea and Arc in the Western Alps. These rivers drain in opposite directions two major fault-bounded blocks (Eastern and Western Blocks) that have undergone contrasting exhumation paths since the Miocene. Samples were collected from different sites along the river trunk, in order to investigate how the detrital signal evolves when detritus from different sub-basins is progressively added to the system. In the Dora Baltea catchment, petrographic data indicate that 29% of the total river load was derived from the Western Block, whereas the Eastern Block contributes the remaining 71%. Petrographic signatures in the modern Arc sands are more homogeneous, thus preventing a precise discrimination of the sources. Apatite fission-track data from the Dora Baltea River show that the Western Block yields 43% of the total apatite load, and the Eastern Block the remaining 57%. In the Arc catchment, apatite contribution is 29% from the Eastern Block, 14% from the Houiller-Subbriançonnais units, and 57% from the Belledonne-Dauphinois units. We assessed apatite fertility in source rocks by measuring apatite content in processed sediments, after checking for anomalous hydraulic concentrations by geochemical analyses. Apatite flux from each sub-basin was converted into a specific sediment yield to infer the short-term erosion pattern in the drainage. The annual sediment load measured along the trunk was then partitioned between sub-basins, in order to calculate erosion rates during the late- to post-glacial time interval. Results document focused erosion in the External Massifs, at rates of 0.4–0.5 mm/a, irrespective of their position inside the drainage, and a westward migration of erosional foci through time along the Western Alps transect.

Abstract Fission-track dating on detrital apatites from modern sands of the Po Delta is used for a provenance study of sediments in the Po River basin. Analysed samples show a fission-track grain-age distribution characterized by two prominent peaks at 7.7 Ma and 17 Ma. The youngest peak accounts for 46% of the total population of dated grains. This young component in the grain-age distribution is consistent with bedrock cooling ages observed in the Western Alps between the External Massifs and the Houiller unit, as well as in the Lepontine dome of the Central Alps and in the Miocene foredeep units of the Apennines, that overall represent only 12% of the orogenic source area. Results suggest that most of the sediment load in the last 10 2 –10 5 years was supplied by focused erosion of relatively small areas that experienced short-term erosion rates one order of magnitude higher than in the rest of the belt.