QUANTIFYING PALEOELEVATIONS USING HYDRATED VOLCANIC GLASS

Tectonic reconstructions depend not only on the magnitude, but also on the spatial distribution and stability of past high elevations, necessitating quantitative, orogen-scale paleoelevation datasets. Felsic volcanic glasses record the hydrogen isotope compositions (δD) of meteoric water shortly after deposition, providing a proxy for ancient precipitation and thus past elevations. As with any stable isotope proxy, glasses must resist alteration on geologic timescales to be useful. Precipitation percolating through tephra deposits replaces mobile cations in the glass, followed by the development of an impermeable, high-density silicate gel layer near the glass surface that resists subsequent hydrogen exchange. To test this, we subjected natural glass samples to long-term DHO solution treatments, which did not effect glass δD values in comparison to untreated samples for all units >1 Ma. Samples of the 7.7 ky Mazama ash, however, show deuterium enrichment, suggesting that more time is necessary to completely hydrate glass and form a gel layer. Additionally, 45-23 Ma glasses record δD values that directly reflect their depositional environments as determined by stratigraphy: lacustrine-deposited glasses reflect D-enriched evaporative waters while fluvial-deposited glasses reflect highly depleted precipitation. Thus gel layers remain impermeable on geologic time-scales, allowing glass to faithfully record ancient water δD values. Ignimbrites are especially well suited for paleoaltimetry, as deposits also reflect relative topography. Many tectonic reconstructions of the North American Cordillera have suggested the presence of an Altiplano-like plateau in the location of the modern Basin and Range, with conflicting timing and mechanisms for the onset of surface-lowering extension and orogen collapse. Paleotopographic profiles based on δD values of Paleogene ignimbrites show a high mountain range with a distinct crest and continuous westward-draining slope extending across Nevada, which is consistent with our geologic reconstructions. This orogen maintained demonstrably higher than modern elevations, reaching >3500 meters in the late Oligocene. Surface-lowering extension did not occur until Farallon slab rollback and transform migration changed the external kinematic framework.