Grand Canyon now the Grand Old Canyon?

Grand Canyon in Winter

Grand Canyon in Winter (credit: Wikipedia)

Among the best known and certainly the most visited topographic feature on the planet, the Grand Canyon resulted from erosion by the Colorado River keeping pace with uplift of the south-central United States. It is the archetype for what is known as antecedent drainage. Since that uplift is still going on, albeit slowly, the Grand Canyon has been assumed to be a relative young landform. By dating the first appearance of debris from the eastern end of the canyon in sediments at its western limit geomorphologists estimated that incision began around 6 Ma ago. Yet a range of other observations present puzzling contradictions. One means of settling the issue is to somehow to date the uplift radiometrically.

A long-used technique is to determine ‘cooling ages’ of crustal rocks exposed by uplift and erosion, exploiting the way in which rock temperature determines whether or not products of radioactive decay cab be preserved intact. One method uses the tracks of defects produced by electrons or helium nuclei from radioactive decay as they pass through various minerals that incorporate high amounts of elements such as uranium. Above a certain temperature the fission tracks anneal and disappear quickly, while below it they accumulate over time. Quantifying that build-up allows the date of cooling below the threshold temperature to be estimated. Similarly, gases produced by radioactive decay of some radioactive isotopes, such as argon from the decay of 40K or helium from uranium and thorium isotopes, can only stay in their host mineral if it remains cooler than a narrow range of temperatures. As rock rises towards the Earth’s surface, it starts out hot at depth but cools by conduction as it get closer to the surface. For the 1.8 km of uplift of the Grand Canyon and the relatively cool nature of the underlying crust, neither the fission-track nor the  40Ar/39Ar cooling-age methods give meaningful results. However, minerals lose helium at temperatures above about 70°C, so a method based on helium accumulation from uranium and thorium isotope decay is a possible means of assessing uplift timing. But there have been plenty of snags to overcome to make this approach reliable. In the case of the Grand Canyon analytical quality and careful sample collection has given a credible result (Flowers, R.M. & Farley, K.A. 2012. Apatite 4He/3He and (U-Th)He evidence for an ancient Grand Canyon. Science , doi 10.1126/science.1229390)

Flowers and Farley from the University of Colorado at Boulder and the California Institute of Technology, Pasadena, respectively, produced a result that completely overturns previous conceptions. The western end of the Canyon had been incised to within a few hundred metres of modern depths by 70 Ma ago; more than ten times earlier than previously thought. The eastern end has a more complex history that reveals cooling events in the Neogene as well as an end-Cretaceous initiation of uplift and erosion. Their data are consistent with early incision of the Grand Canyon by a Cretaceous river flowing eastward from the Western Cordillera, with a reversal of flow in the late-Tertiary as uplift of the Colorado Plateau began and western mountains subsided. Whether or not this fits with Cretaceous and later geological history of the SW US, is beyond my ken, but you can bet there will be a storm of comment from US geomorphologists once the paper appears in the print issue of Science.

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