Reconstructed Paleoelevation in the Bolivian Andes Using Hydrated Volcanic Glass
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Constraints on temporal and spatial variations in surface elevations are crucial in understanding the geodynamic processes responsible for large-magnitude uplift of orogenic plateaux. At ~4–6 km elevations, the Central Andean Plateau is the Earth’s largest orogenic plateau produced by ocean-continent convergence and inevitably serves as a natural laboratory to study surface uplift mechanisms. Recent paleoaltimetry studies using paleotemperature from sedimentary carbonates point to a Neogene non-uniform pulse of rapid surface uplift across and along the Peruvian Western Cordillera and Bolivian Altiplano, consistent with foundering of mantle lithosphere via Rayleigh-Taylor instability. In this study, we present the first quantitative paleoelevation reconstruction of the timing and magnitude of surface uplift of the Bolivian Western Cordillera. We reconstruct the isotopic compositions of surface water using hydrogen isotopic composition of hydrated volcanic glass. Glass was isolated fourteen ignimbrites and air-fall tuffs and chronology is based on U-Pb zircon of the same samples. Ten samples exhibit acceptable water content (>1 wt%) and were included in our paleoelevation reconstruction. Paleoelevation is reconstructed based on differences between modern low-elevation δ18O and high-elevation δ18O of meteoric water (Δδ18O) using a one-dimensional thermodynamic Raleigh distillation model. Our data indicates that surface elevation of the Bolivian Western Cordillera was at modern elevation since at least 22.9 Ma, much earlier than previous reconstruction suggested by foliar physiognomy (~ 10 Ma). Two samples show anomalously positive δD values, implying highly evaporative depositional environments. Thus further investigations incorporating measured sections and stratigraphy evaluations are necessary. However, our reconstruction strongly suggests modern elevation of the Bolivian Western Cordillera was established since 22.9 Ma, consistent with Miocene stratigraphic record and provenance data of the Altiplano.