USING COSMOGENIC 3HE DATING OF ALLUVIAL BOULDERS AS A TOOL FOR CONSTRAINING UPLIFT HISTORY OF THE WESTERN ANDES

The timing of the uplift of the Andes is still poorly constrained. Scenarios range from gradual uplift from the start of the Miocene (e.g. Hartley et al. 2007) to rapid uplift of 2.5 to 3.5 km in the Late Miocene (e.g. Hoke et al. 2008). The extreme hyperarid climate (< 10 mm rainfall per year) of the Atacama Desert leads to planation surfaces with little to no significant erosion for 25 million years in some areas (Dunai et al. 2005). Cosmogenic nuclide production rates are a function of atmospheric depth and in the absence of erosion allows the concentration of cosmogenic nuclides be used directly to model surface uplift history.

The Aroma Quebrada of Northern Chile is a flat lying planation surface underlain by a 16.2 ± 0.7 Ma volcanic deposit (Farías et al. 2005). Boulders collected from the Aroma Quebrada have concentrations of cosmogenic He-3 that require the surface to have been at a low elevation for a substantial period of its history to be consistent with the underlying stratigraphy. This is incompatible with a rapid Late Miocene uplift of the Andes instead the exposure ages restrict the majority of surface uplift of the Western Cordillera to prior to the Late Miocene.

References

Dunai, T.J., Lopez, G.A.G., and Juez-Larre, J., 2005, Oligocene-Miocene age of aridity in the Atacama Desert revealed by exposure dating of erosion sensitive landforms: Geology, v. 33, p. 321–324, doi: 10.1130/G21184.1.

Farías, M., Charrier, R., Comte, D., Martinod, J., and Herail, G., 2005, Late Cenozoic deformation and uplift of the western flank of the Altiplano: Evidence from the depositional, tectonic, and geomorphologic evolution and shallow seismic activity (northern Chile at 19°30′S): Tectonics, v. 24, TC4001, doi: 10.1029/2004TC001667.

Hartley, A.J., Sempere, T., and Wörner G., 2007, A comment on "Rapid late Miocene rise of the Bolivian Altiplano: Evidence for removal of mantle lithosphere" by Garzione C.N. et al. [Earth Planet. Sci. Lett. 241 (2006) 543-556]": Earth & Planetary Science Letters, v. 207, p. 625-629.

Hoke, G. D., and Garzione, C. N., 2008, Paleosurfaces, paleoelevation, and the mechanisms for the late Miocene topographic development of the Altiplano plateau: Earth and Planetary Science Letters, v. 271, p. 192-201.