THE TOPOGRAPHY OF THE MODERN AND ANCIENT CENTRAL ANDEAN OROCLINE: A COMPARISON USING VOLCANIC GLASS PALEOALTIMETRY, SOUTHERN PERU

The South American Central Andes are the archetype for subduction-driven processes and serve as a modern analogue for the Jurassic to Paleogene North American Cordillera. Despite the inherent importance of the Andean orogen, the timing and nature of the surface uplift that led to its modern topography is poorly understood. Constraining this uplift history is critical for identifying geodynamic mechanisms that drive the growth of large orogens and the impact of surface uplift on atmospheric circulation.

To reconstruct past elevations of the northern Central Andes, we measured hydrogen isotope ratios of hydrated volcanic glass (δD_glass) from Eocene to recent (i.e., < 1 Ma) ignimbrites and ash deposits collected across the 300 km width of the Peruvian Central Andes. These volcanic glass samples (73 samples processed) provide new constraints for testing models of Andean surface uplift. From west to east across the Western Cordillera, δD_glass values of recent and < 6 Ma ash samples decrease from -28‰ to -194‰, similar to soil water δD values, which decrease from -20‰ at the Pacific coast to -150‰ in the Western Cordillera. However, ignimbrite samples older than 15-10 Ma across this same transect show a smaller range in δD values, decreasing from -17‰ to -107‰. We tested if there is a statistically significant difference in ancient δD_glass value gradients from the recent based on available data. The statistical analysis of δD_glass values indicate a significant change in topography and/or climate occurred before 10 Ma in the Western Cordillera and that δD gradients in the Peruvian Central Andes were close to modern gradients by 15-10 Ma. The timing of this isotopic shift is > 5 Myr earlier than hypothesized for rapid late Miocene surface uplift. These preliminary results support models of surface uplift during the Paleogene and early Neogene. This may indicate that the northern part of the Central Andes had a different uplift history than the southern Central Andes, and potentially different driving mechanisms of uplift. To quantify the precise timing and magnitude of surface elevation changes, future work will integrate high-precision geochronology and modeling of the influence of topography and atmospheric circulation on isotopic lapse rates.