Abstract Interpretation of the origin of Oligocene Flysch exposed in the Andaman–Nicobar Islands has been the subject of debate. Previous work on the provenance of the Andaman Flysch based on samples from South Andaman has indicated major contributions from Myanmar affected by the India–Asia collision, mixed with subordinate detritus from the nascent Himalayas. This study examines the provenance of a larger suite of samples that extend to North and Middle Andaman islands as well as Great Nicobar Island. Rather monotonous petrographic and heavy-mineral assemblages testify to strong diagenetic imprint, leading to a poorly constrained identification of the sediment source. U–Pb zircon ages provide more robust and diagnostic provenance discrimination between the Myanmar Arc and the growing Himalayan range. Combining petrographic and mineralogical data with detrital zircon U–Pb analyses, we find that most of the Andaman Flysch is dominated by a strong continental-crust signal with only a minor contribution from arc material. Statistical analyses of the data show that most of the samples have a provenance similar to Palaeogene Bengal Fan sediments, although the type section on South Andaman has a closer affinity to the provenance of the modern Irrawaddy. Supplementary material: Sample location (Table A1), the complete petrographic (Table A2), heavy mineral (Table A3) and U–Pb zircon-age datasets (Table A4) are all available at https://doi.org/10.6084/m9.figshare.c.3634328.v1

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.