87 Sr/ 86 Sr 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 87 Sr/ 86 Sr 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.