CLUMPED ISOTOPE PALEOTHERMOMETRY OF SOUTHERN ALTIPLANO PALEOSOLS: IMPLICATIONS FOR SURFACE UPLIFT OF THE ANDEAN PLATEAU

Spatially extensive paleoelevation records of the Bolivian Altiplano are key to determining the geodynamic mechanisms that formed this high elevation plateau. Previous paleothermometry studies based on stable isotope compositions of soil and lake carbonates in the northern Altiplano show a rapid surface uplift of 2.5 ± 1 km between 10 and 6 Ma. Whether this is an accurate representation of the entire Altiplano remained unknown. Our present study employs clumped isotope paleothermometry of middle to late Miocene paleosol carbonates from the southern Altiplano to obtain improved spatial resolution of the plateau's surface uplift history. A surface temperature decrease of 14°C between 16 and 8 Ma indicates that southern Altiplano/Eastern Cordillera surface elevations increased by 1.6 ± 0.5 km between 16 and 13 Ma and an additional 0.5 ± 0.5 km between 13 and 8 Ma. This record shows an earlier onset of surface uplift in the south than in the northern Altiplano. Recent studies of the surface uplift and incision of the Altiplano and adjacent Eastern and Western cordilleras have argued that the removal of eclogitic lower crust and mantle lithosphere is required to accommodate the regional scale and rapid rate of Miocene surface uplift. This is supported by geophysical studies that reveal thin or missing dense lower lithosphere, geochemical studies that identify mantle contributions to magmas and gases, and thermomechanical modeling of evolution of the Andean lithosphere. Crustal shortening causes mafic lower crust to obtain a thickness that generates eclogite, which may lead to gravitational instability, eventually resulting in loss of the lower lithosphere. In addition, rapid changes in the distribution of surface topography may cause flow of weak crustal material from regions of thicker and/or more elevated crust to regions of thinner/lower elevation crust. This study shows rapid surface uplift in the southern Altiplano that is diachronous with the north, therefore necessitating piecemeal removal of mantle lithosphere and possibly the flow of weak crustal material from south to north.