Hydrogen isotopes test uplift hypotheses

When Cenozoic mountain belts and high plateaux began to rise and be eroded has become a disputed topic about most of them, such as the Himalaya and Tibetan Plateau, the North American Western Cordillera, the Andes and the Ethiopian Plateau. A host of techniques have been used, including plant-leaf stomatal indices, the bubbles in lavas, cosmogenic isotopes and various stratigraphic approaches. For the Tibetan Plateau (see When did Tibet rise? and Tibetan uplift: looking a gift horse in the mouth in the March and May 2006 issues of EPN) opinion is polarised: as soon as India collided with Asia, around 40-50 Ma ago, due to crustal thickening; as a result of a slab of lithosphere delaminating from the region as recently as the Late Miocene (8-10 Ma), when the crust rose gravitationally. There is support for both hypotheses. Much the same divergence of opinion applies to the Sierra Nevada of the Western USA, which may have been a major feature throughout the Cenozoic, or subject to delamination in the Pliocene (3 to 5 Ma ago). In its case, a new way of estimating topographic elevation in the past may resolve the disputation (Mulch, A. et al. 2006. Hydrogen isotopes in Eocene river gravels and paleoelevation of the Sierra Nevada. Science, v. 313, p. 87-89).

The Stanford University group led by Andreas Mulch have focused on the way in which the proportion of deuterium (2H) in rainwater changes as clouds rise over high areas. This is known quite precisely from modern hydrological studies. Of course, you cannot find ancient rainwater ponded in the place where it once fell, but that rain does find its way into clay minerals during weathering. The western flank of the Sierra Nevada is mantled by extensive Eocene graves deposited by rivers that drained the area now occupied by the mountains. Handily, those gravels were the main target for the 1849 California Gold Rush and subsequent alluvial gold mining, so they have been well exposed by the prospectors throughout the Sierra Nevada. At high elevations they are preserved in river terraces, so it is possible to trace roughly the ancient drainage courses and sample a range of modern elevations. The hydrogen isotope data from kaolinite in cobbles of weathered granitic rocks are extremely interesting. They show that the Eocene rainwater that entered kaolinite molecules at different modern heights had fallen at a very similar range some 50 Ma ago. There seems little doubt that the Sierra Nevada had risen more than 2 km before the Eocene

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