Downstream changes of Alpine zircon fission-track ages in the Rhône and Rhine rivers.
Résumé
Zircons from 13 sediment samples from the Rhône and Rhine drainages were dated by the fission-track method to study downstream changes in detrital fissiontrack grain-age distributions in large river systems draining the European Alps. The orogen-parallel Rhône River shows a zircon fission-track grain-age distribution similar to that in its Alpine source areas. This signal is well preserved because there are Alpine sources along its full length and input from non-Alpine sources is small. In contrast, only the headwaters of the north-flowing Rhine River are located in the Alps. As a consequence, Alpine-derived zircons, which are distinguished by young fission-track ages, become progressively diluted downstream by older ages from zircon sources external to the Alps. Nevertheless, the Alpine component is persistent and can easily be detected in sediments more than 1000 km downstream at the Rhine delta. These results demonstrate that fission-track dating of detrital zircons can provide useful information about orogenic processes, even where sediments have been transported hundreds of kilometers from the orogenic source, crossing ephemeral lakes and subsiding basins. Deposits along the lower reaches of the river appear to have a short-residence time (< 1 Myr), and thus many of these deposits serve to smooth out variations in the supply of sediment from fast-eroding heterogeneous sources in the Alpine headwaters of these drainages. For the Rhône and the Rhine rivers, we show that fast erosion in the Alps accounts for most of the sediment load. This finding supports a widespread observation that the sediment in most large continental drainages is usually derived from a small part of the drainage, where uplift, relief, and erosion rates are greatest.
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