Slow Cenozoic uplift of the western Andean Cordillera indicated by cosmogenic ³He in alluvial boulders from the Pacific Planation Surface

To determine the mechanisms responsible for mountain belt growth, it is important to accurately establish the timing of surface uplift. Here we exploit the altitude control on the production rate of in situ cosmogenic nuclides to test the hypothesis that the Andes was uplifted in the late Miocene. High concentrations of in situ cosmogenic ³He (³He_cos) have previously been measured in alluvial boulders on the western flank of the Central Andes, northern Chile (Evenstar et al. 2009). These are consistent with deposition soon after formation of the surface (13-14 Ma). We have modeled the accumulation of cosmogenic ³He in several different surface uplift scenarios and compared them to the measured concentrations. The measured ³He_cos concentrations are too high to be produced by late Miocene uplift and imply that the western flank of the Andean Cordillera attained a substantial part of its current elevation prior to 14 Myr ago.