87Sr/86Sr of calcium sulfate in ancient soils of hyperarid settings as a paleoaltitude proxy: Pliocene to Quaternary constraints for northern Chile (19.5-21.7°S)

Several lines of geomorphic and geophysical reasoning suggest that the western fore arc of northern Chile has undergone kilometer-scale surface uplift relative to sea level during the late Neogene. We have developed a new paleoaltimeter based on the ⁸⁷Sr/⁸⁶Sr ratio of ancient gypsic soils in the hyperarid Atacama Desert and used it to investigate the uplift history of the Andean fore-arc coastal mountain range (Coastal Cordillera) and of the Andean fore-arc nonmarine basin surface (Central Depression). Sampled sites span 330 km strike-parallel distance and elevations between 450 and 1850 m above sea level (asl). A minority of the sites place firm constraints on minimum or maximum vertical movements in an absolute framework. For the majority of locations, the data determine a maximum permissible magnitude of uplift. In all cases the magnitudes of paleo-elevation changes are small compared to the elevation of the study area relative to sea level. We conclude that more than 45% of the 1000 m asl average elevation of the Central Depression main axis and that more than 70% of the 900 m asl average elevation of the westernmost Coastal Cordillera in the study area were achieved by preearly Pliocene regional-scale tectonic processes. These results refute the hypothesis of kilometer-scale surface uplift of the western nonmarine Andean fore arc during the late Neogene.