Sediment budgets by detrital apatite fission track dating (Rivers Dora Baltea and Arc, Western Alps)

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 mineral fertility in bedrock determined from detrital samples, in order to infer reliable erosion patterns on short-term timescales. 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 experienced 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 indicates that 29% of the total river load derives 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 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/yr, irrespective of their position inside the drainage, and a westward migration of erosional foci through time along the Western Alps transect.