A High Water Mark

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Science  02 Jul 2004:
Vol. 305, Issue 5680, pp. 19
DOI: 10.1126/science.305.5680.19a

Many areas of the western United States and Canada harbor evidence of large amounts of a roughly east-west extension of the crust. It appears that during the past 50 million years, the crust seems to have been pulled apart along enigmatic shallow faults. In some areas, it is thought that the crust has been extended by much more than 50%, even though many of the extended areas are expressed as mountains today. One idea for the cause of this extension is that many of these regions represent even higher mountains or plateaus that subsequently collapsed.

Mulch et al. examined hydrogen isotopes in a hydrogen-bearing mineral, mica, that formed during extension in the Shuswap Complex in southwestern Canada. Hydrogen isotopes in part reflect elevation in precipitation, and the micas can be dated directly to the time of extension. The data imply that the Shuswap Complex once stood on average more than 4 km above sea level at the inception of its collapse, 45 to 50 million years ago, which is more than 1 km above its current elevation. Such high elevations may have characterized more of the western Cordillera at this time. — BH

Geology 32, 525 (2004)

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